Unit 6: EcologyEcology is the study of the interactions between living organisms and their environment. As you can imagine, the study of these interactions can be both complex and very subtle. To truly appreciate ecology, the macro-level of biology, you must have a firm understanding of the micro-level of biology, biochemistry and cells, as ecology is primarily governed by the flow of energy through an ecosystem. In this unit we will explore the various layers of the biosphere, the sum of all life on Earth as we lay the foundation to study how organisms take on specific roles or niches in their environment and interact through complex but predictable symbioses. We will define a species and examine the various boundaries that distinguish one species from another. And finally we will take a look at how disturbances interrupt ecological stability and how ecosystems respond to disturbances through a process called ecological succession.
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The Biosphere
Organization
In its most general sense, ecology is the scientific discipline that investigates the interactions between species. But the Earth is an enormous place and ecologists study different levels of organization when observing the interactions between species. At its broadest level, ecology investigates the biosphere, the sum of all life on Earth. You should remember that the biopshere is one of the 4 major spheres the earth can be divided up into including the hydropshere, lithosphere (aka. the geosphere) and atmosphere. The biosphere is a closed system meaning that matter is neither added or lost from it, but rather recycled endlessly (these can be seen with the nutrient cycle: Carbon Cycle, Water Cycle, Nitrogen Cycle, Phosphorus Cycle, etc.). The one exception to this rule would be |
energy, which is primarily supplied by the sun. Because the biosphere contains all of the biomass on Earth, it is actually one of 4 spheres that comprise the Earth. The biosphere can be divided up into biomes- continent spanning regions with similar abiotic conditions and flora and fauna communities. Multiple, specific ecosystems are included under the various biome types: the Amazon and Daintree rainforests both fall under the tropical rainforest biome category Biomes can be terrestrial or aquatic and while different sources divide up the biomes in different ways, for our purposes we will consider 9 major types of terrestrial biomes and 8 major types of aquatic biomes.
Moving on from biomes, we have specific ecosystems. An ecosystem includes a community of living organisms from a multitude of species interacting with their non-living (abiotic), physical environment, including soil and climate. The ecosystem is the most important and fundamental unit of structure to an ecologist. Ecosystems come in a variety of sizes and forms but they are generally limited to a particular amount of space. All ecosystems are comprised on a community of living organisms, all the different living species in an ecosystem. And a community is composed of various populations, all of the organisms of the same species living in an area. Below is an example of all the various levels of organization we see in an ecosystem.
Biopshere: Earth Biome: Grassland Ecosystem: Praire Community: Hawk, snake, bison, praire dog, grass Population: Bison Herd Organism: a specific bison |
Natural Cycles
Water Cycle
As you know, matter cannot be created or destroyed, instead matter continually changes form. A great example of this would be the water cycle which follows the movement of water through the hydrosphere. There are a number of processes that occur within the water cycle that move water both in terms of its physical position and its state of matter. For example, when water goes through evaporation, liquid water is converted to water vapor using thermal energy. That same water vapor can return to liquid water through the process of condensation once thermal energy or heat is removed from the water vapor. Below you will find a list of additional processes that occur in the water cycle and help water move through the hydrosphere.
As you know, matter cannot be created or destroyed, instead matter continually changes form. A great example of this would be the water cycle which follows the movement of water through the hydrosphere. There are a number of processes that occur within the water cycle that move water both in terms of its physical position and its state of matter. For example, when water goes through evaporation, liquid water is converted to water vapor using thermal energy. That same water vapor can return to liquid water through the process of condensation once thermal energy or heat is removed from the water vapor. Below you will find a list of additional processes that occur in the water cycle and help water move through the hydrosphere.
Key Processes in the Water Cycle
- Evaporation: conversion of liquid water to water vapor that requires thermal energy (endothermic)
- Transpiration: release of water from plant leaves through stomata
- Evapotranspiration: sum total of transpiration and evaporation
- Condensation: conversation of water vapor (gas) to liquid water following heat loss (exothermic). This process forms clouds.
- Precipitation: The fall of water from the atmosphere to the ground as rain, snow, sleet, etc.
- Runoff: flow of excess water (rainwater, snowmelt, etc.) across the surface of the Earth
- Sublimation: The conversion of ice to water vapor (no liquid water).
- Percolation: the seeping of water through soil into an underground reservoir. This process purifies water.
- Infiltration: process of water entering into the soil. This is the precursor to percolation.
Water, as with so many other nutrients, is fundamentally important to life- life could not exist without water. Lucky for us, our environment constantly recycles water through the Hydrologic (Water) Cycle. The actual process is complicated and detailed, but overall water moves from one reservoir (storage form) to another through processes like evaporation, precipitation and sublimation. For example, water moves from the oceans and other bodies of water to the atmosphere via evaporation. Similarly, water can also be added to the atmosphere through transpiration as plants conduct water from their roots to their leaves for photosynthesis. All of this water (water vapor) in the atmosphere condenses (reverts from a gas to a liquid) to form clouds. Precipitation then adds water back to the Earth. This is just one of the many pathways water can take through the Water Cycle.
The Carbon Cycle
The Carbon Cycle is not so different from the Water Cycle- once again a nutrient (in this case the element Carbon) is being recycled and made available for organisms. Carbon is considered the backbone of life because it can form up to 4 covalent bonds even with other carbon atoms. These 2 properties make carbon the center of all organic molecules (carbohydrates, lipids, proteins and nucleic acids), making it crucial for life. Ironically, carbon can not be produced by life- it is forged in stars through the process of nuclear fusion.
Carbon follows a delicate homeostatic system on Earth, shuffling carbon through ecosystems and maintaining balance. As you know, the Earth can be divided into four major systems: the biosphere, hydrosphere, lithosphere and atmosphere. The hydrosphere refers to
The Carbon Cycle is not so different from the Water Cycle- once again a nutrient (in this case the element Carbon) is being recycled and made available for organisms. Carbon is considered the backbone of life because it can form up to 4 covalent bonds even with other carbon atoms. These 2 properties make carbon the center of all organic molecules (carbohydrates, lipids, proteins and nucleic acids), making it crucial for life. Ironically, carbon can not be produced by life- it is forged in stars through the process of nuclear fusion.
Carbon follows a delicate homeostatic system on Earth, shuffling carbon through ecosystems and maintaining balance. As you know, the Earth can be divided into four major systems: the biosphere, hydrosphere, lithosphere and atmosphere. The hydrosphere refers to
all bodies of water including the oceans, rivers and glaciers, while the biosphere includes all forms of life. The atmosphere, as you might imagine, contains all the air and gases of our planet. Finally, the lithosphere refers to the solid, outermost region of Earth and contains all the soil and rocks. These systems are not concrete and can and do overlap. Carbon moves through all four of these systems as it progresses through the carbon cycle. You will notice that the diagram above is divided into the 4 "spheres".
The carbon cycle consist of 2 major parts: the slow carbon cycle and the fast carbon cycle. The slow carbon cycle follows the path of carbon through nonliving (abiotic) components of ecosystems as carbon cycles through air, water, rocks and soils. On the other hand, the fast carbon cycle follows the movement of carbon through living (biotic) components of an ecosystem. This occurs faster because life moves more quickly than geologic processes. The fast carbon cycle is probably the one your are most familiar with already and focuses primarily on the biosphere, while the slow carbon cycle is more involved with the hydrosphere, lithosphere and atmosphere. |
Fast Carbon Cycle
The fast carbon cycle is governed by a variety of forces implemented by both living and nonliving constituents. Atmospheric carbon dioxide is converted into organic sugars (also known as "fixed") by photosynthesis. There are four ways the carbon taken up by plants can be returned to the atmosphere: the plant can metabolize the sugars itself through cellular respiration, an animal can consume the plant and perform cellular respiration, the plant can be burned, releasing carbon dioxide or the plant can die and be decomposed by bacteria and other decomposers, releasing carbon dioxide again via cellular respiration. Of course, sometimes organisms can die but instead of being broken down by decomposers, they are rapidly buried and compressed. The carbon in their bodies remains trapped, it does not return to the atmosphere as carbon dioxide and is instead converted into fossil fuels such as coal or oil. This carbon is trapped, isolated from the rest of the carbon cycle for millions of years.
For a long time, these photosynthesis and cellular respiration held atmospheric carbon dioxide in balance, but when when people began to burn fossil fuels, more carbon was added to the atmosphere, pushing it out of balance. Over the eons life has dominated the planet, carbon in the form of dead organisms has been buried deep in the Earth where it formed coal, gas and oil. This carbon was "locked up" and unable to return to the atmosphere, but when the industrial revolution began and humans consumed these fuels we rapidly reintroduced all of this carbon back to the atmosphere which has supported the Global Climate change crisis as carbon dioxide is a greenhouse gas.
The fast carbon cycle is governed by a variety of forces implemented by both living and nonliving constituents. Atmospheric carbon dioxide is converted into organic sugars (also known as "fixed") by photosynthesis. There are four ways the carbon taken up by plants can be returned to the atmosphere: the plant can metabolize the sugars itself through cellular respiration, an animal can consume the plant and perform cellular respiration, the plant can be burned, releasing carbon dioxide or the plant can die and be decomposed by bacteria and other decomposers, releasing carbon dioxide again via cellular respiration. Of course, sometimes organisms can die but instead of being broken down by decomposers, they are rapidly buried and compressed. The carbon in their bodies remains trapped, it does not return to the atmosphere as carbon dioxide and is instead converted into fossil fuels such as coal or oil. This carbon is trapped, isolated from the rest of the carbon cycle for millions of years.
For a long time, these photosynthesis and cellular respiration held atmospheric carbon dioxide in balance, but when when people began to burn fossil fuels, more carbon was added to the atmosphere, pushing it out of balance. Over the eons life has dominated the planet, carbon in the form of dead organisms has been buried deep in the Earth where it formed coal, gas and oil. This carbon was "locked up" and unable to return to the atmosphere, but when the industrial revolution began and humans consumed these fuels we rapidly reintroduced all of this carbon back to the atmosphere which has supported the Global Climate change crisis as carbon dioxide is a greenhouse gas.
Slow Carbon Cycle
Carbon moves through the slow Carbon Cycle on the order 100-200 million years. About 10-100 metric tons of carbon cycle through the slow carbon cycle each year compared to the 1,000 metric tons of anthropogenic CO2 released into the atmosphere each year and the 10,000-100,000 metric tons of carbon that move through the fast carbon cycle annually. When it rains, carbon is moved from the atmosphere to the lithosphere. Carbon dioxide (CO2) combines with water (H20) to form carbonic acid (H2CO3). Carbonic acid is a weak acid that dissolves rocks through chemical weathering, releasing calcium, magnesium, potassium, or sodium ions. These ions are then |
swept into the oceans by rivers. In the oceans, carbonic acid breaks down into H+ and bicarbonate and that bicarbonate can be further broken down into carbonate. This carbonate ion can combine with calcium ions to form calcium carbonate. Much of this calcium carbonate is used by organisms such as corals, planktons and a variety of mollusks and invertebrates to build their shells.
As more CO2 enters the atmosphere, more carbonic acid forms in the oceans. This increases the concentration of H+, lowering pH. In addition, the excess H+ ions actually reduce the availability of the carbonate ion, a derivative of bicarbonate that is used to build sea shells. In this way, excess CO2 emissions have a number of complex effects on the Earth's oceans.
As more CO2 enters the atmosphere, more carbonic acid forms in the oceans. This increases the concentration of H+, lowering pH. In addition, the excess H+ ions actually reduce the availability of the carbonate ion, a derivative of bicarbonate that is used to build sea shells. In this way, excess CO2 emissions have a number of complex effects on the Earth's oceans.
When those organisms die, they become shells that litter the ocean floor and with time, they are converted to limestone which represents 80% of carbon containing rocks. The remaining 20% comes from organic material being compressed to form sedimentary rock like shale and in some cases oil, coal and other fossil fuels. When the limestone is eventually destroyed due to tectonic activity, the carbon is contains will be released through volcanic activity. This return the carbon from calcium carbonate to the atmosphere again as carbon dioxide. At present, volcanoes emit between 130 and 380 million metric tons of carbon dioxide per year. For comparison, humans emit about 30 billion tons of carbon dioxide per year—100–300 times more than volcanoes—by burning fossil fuels.
Atmospheric carbon dioxide can also dissolve directly in and out of the oceans. The atmospheric carbon dioxide reacts with water to form carbonic acid which decomposes into H+ ions and bicarbonate, lowering the pH of the oceans. These H+ ions will then combine with carbonate released from rocks during chemical weathering to form more bicarbonate. As people burn more fossil fuels to release more and more carbon dioxide is dissolved into the oceans. This has a profound effect on the organisms living there, especially corals which require a narrow pH range to live in. It is predicted that most coral reef ecosystems will be lost and replaced over the coming decades by algae based systems, all because of human activity.
Atmospheric carbon dioxide can also dissolve directly in and out of the oceans. The atmospheric carbon dioxide reacts with water to form carbonic acid which decomposes into H+ ions and bicarbonate, lowering the pH of the oceans. These H+ ions will then combine with carbonate released from rocks during chemical weathering to form more bicarbonate. As people burn more fossil fuels to release more and more carbon dioxide is dissolved into the oceans. This has a profound effect on the organisms living there, especially corals which require a narrow pH range to live in. It is predicted that most coral reef ecosystems will be lost and replaced over the coming decades by algae based systems, all because of human activity.
Carbon Sinks
A sink is a natural or artificial reservoir that stores a resource. All elements have different sinks, not just carbon, but carbon sinks have gotten a lot of attention recently because of global climate change and fossil fuel use. A number of different carbon sinks exist in all 4 spheres of the Earth, but the largest include the soil, fossil fuels and the oceans. The graphic to the left demonstrates the relative size of these different carbon sinks. As you can see, the deep ocean is the largest sink based on the graphic with a relative size of 37,000. Fossil fuels likes oil, coal and natural gas also comprise a huge amount of stored carbon with a relative size of 10,000. As people burn those fossil fuels for energy, they converted the carbon in the fossil fuels into carbon dioxide, which can enter the atmosphere. From there, the CO2 could be used by plants in photosynthesis or dissolve into the oceans or other surface waters. |
Nitrogen Cycle
Yet another element that is fundamentally crucial to living organisms is nitrogen, which is found in proteins and nucleic acids such as DNA. The largest reservoir of nitrogen is the atmosphere which stores nitrogen as nitrogen gas (N2). Unfortunately, nitrogen in this form is not useable by living organisms. The process of nitrogen fixation is carried out by bacteria or in some cases lightning strikes or combustion reactions and converts N2 into ammonia (NH3) and subsequently ammonium (NH4+). Ammonium can be used by producers and assimilated into body tissues. Ammonium can be converted into nitrite (NO2-) and then nitrate (NO3-) through a process known as nitrification which is accomplished by specialized bacteria. Nitrates can also be assimilated by producers, but some nitrates will be converted back into N2 gas by bacteria through a process called denitrification. Finally, decomposers such as fungi perform a process called mineralization that break down organic tissues into inorganic compounds. In the case of the nitrogen cycle, this is known as ammonification because organic nitrogen is restructured into ammonium. |
Eutrophication
Nitrogen is a limiting compound for many photosynthetic organisms. As such, most fertilizers contain nitrogen in the form of nitrate and ammonium, two forms of nitrogen that can easily be assimilated by producers. However, the use of such fertilizers can lead to unintended consequences. When the fertilizer leaks into aquatic ecosystems such as streams, ponds or lakes (typically as runoff), the sudden increase in available nitrogen stimulates a rapid expansion of photosynthetic algae populations in a process known |
as eutrophication. The extra rates of photosynthesis temporarily stimulates excess oxygen production, raising the amount of dissolved oxygen (DO) in the water. Dissolved oxygen is a measure of water health as most aquatic organisms require oxygen to survive. This value plays opposite of biochemical oxygen demand (BOD), which measures how much oxygen is being used by living organisms in the water. As the algae begin to die off, there is now a sudden spike in food resources for decomposing bacteria which begin to proliferate. As they break down the dead algae, the bacteria rapidly consume oxygen, severely reducing the DO levels as BOD spikes. The decomposers exhaust the available oxygen in the water, leading to hypoxic conditions in which there simply is not enough oxygen available to sustain aquatic life. Additionally, the algal bloom itself can increase the turbidity or cloudiness of the water, blocking out light and inhibiting photosynthesis. In this manner, eutrophication leads to hypoxia and essentially starves organisms including fish of the dissolved oxygen they need to survive leading to so called fish kill events following the introduction of artificial fertilizers in aquatic ecosystems.
Terrestrial Biomes
As you know, biomes are globally repeated regions that share similar biotic and abiotic components. In general, there are two major types of biomes: terrestrial and aquatic. For this section we will focus on the 9 major types of terrestrial biomes. Each biome has its own distinctive combination of
abiotic and biotic characterisitics that help to identify it. Unfortunately, many biomes share overlapping traits. For example, the temperate rainforest, temperature seasonal forest, shrubland and grassland biomes all share nearly identical temperature ranges. To differentiate between these biomes, we need a wider variety of data including precipitation and indicator species including plants and animals. For our purposes, you should be familiar with the major characteristics of each biome including the data summarized in the table to the right, but also a brief description of each biome. These are outlined for you below.
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Climatograms
An important tool for identifying biomes is the climatogram, a graph that summarizes important climate information for a biome including precipitation and temperature fluctuations. Ecologists can use climatograms to make productionson the productivity of these large scale ecosystems. The example to the left summarizes how to read and interpret a climatogram. Notice that there are two y-axes being used, the left hand one if for average temperature in a given month and the right hand one is for average precipitation in a given month. Of course, |
time in months is plotted on the x-axid. You may also note that some, but not all of the months in the example have been shaded grey. This is done to highlight the months when the plants of the biome are able to grow, since plant growth can only occur at temperatures above freezing. Finally, the example climatogram notes that when temperature line is above the precipitation line, plant growth is limited by temperature. Just as we saw in photosynthesis, limiting factors control photosynthesis and there are multiple abiotic conditions that can limit plant growth including temperature and precipitation (which effectively controls water availability).
Tundra
The tundra is a cold, treeless biome dominated by low growing, hearty vegetation and sometimes referred to as the cold desert. Globally, tundra can be found in the northern-most Northern Hemisphere as well as the fringes of Antarctica. At lower latitudes, alpine tundra can be found at high altitudes where high winds and low temperatures inhibit tree growth. The growing season of tundra is short- only about 4 months. This means only the topmost soil ever thaws with deeper soils remaining frozen as permafrost. |
Boreal Forest
Coniferous evergreen trees such as pine and spruce dominate boreal forests sometimes referred to as taiga. Evergreen conifers, unlike broad-leafed deciduous trees, remain green year round and do not drop all their leaves. Boreal forests tend to form around 50-60 degrees North latitude, where temperates remain cold, but there is more rainfall than at the tundra due to global air circulation patterns called convection currents. This means that temperate is the limiting factor for plant growth rather than precipitation. |
Temperate Rainforest
The term temperate is one you will see throughout our discussion of the biomes. In this context temperate means moderate temperatures as opposed to more tropical ones. And of course rainforests experience high amounts of precipitation. Therefore, temperate rainforests are coastal biomes that experience mild temperatures and high annual rainfall, which supports the growth of tall, coniferous trees such as fir, cedar, hemlock and even redwoods (top image). Such forests are characteristic of the Pacific Northwest, southern Chile and Eastern Australia. |
Temperate Seasonal Forest
Much like temperate, seasonal is another term we will see applied in biome descriptions. In this context, seasonal means that the biome experiences distinct seasons. These seasons can be marked by changes in temperature or precipitation. For temperate seasonal forests, seasonal refers to the presence of distinct cold and warm seasons; warm summers and harsh winters with rainfall more consistent, as evidenced in the climatogram. Temperate seasonal forests are pretty typical of the Eastern U.S and are dominated by broad-leafed deciduous trees such as oak and maple. |
Shrubland
Much like temperate seasonal forests, shrublands experience distinct hot and dry seasons; hot summers and freezing winters. The major difference between a scrubland and a temperate seasonal forest is the lack of precipitation, which limits the growth of plants and encourages frequent, but low intensity wildfires. These wildfires represent ecological disturbance, which interrupt ecological succession, topics we will discuss in greater detail later in the unit. As their name suggests, scrublands are dominated primarily by short, woody plants such as scrub oaks. |
Grassland
Hot, dry summers and cold, harsh winters can also be observed in grasslands. Like shrublands, grasslands frequently experience low intensity fires. While that may sounds like a problem, vegetation in both biomes have evolved to become fire dependent- the plants have special adaptations to not only survive, but in some cases actually promote fires. However, unlike shrublands, grasslands like the Great Plains in the U.S. are dominated by grasses and flowers which are fire tolerant and can withstand grazing from animals likes bison and other herbivores. |
Tropical Rainforest
In this context, the term tropical refers to temperature. Much like how temperate indicated a more moderate climate, tropical highlights the high temperates of the biome. So tropical rainforests are warm and wet. Tropical rainforests are some of the most productive ecosystems on the planet and are generally found between 20 degrees North and South. In addition, tropical rainforests are highly diverse- roughly 2/3 of Earth's terrestrial species can be found in the tropical rainforest biome. |
Tropical Seasonal Forest
Tropical seasonal forests not only experience high temperatures, but also distinct seasons. Unlike seasons in temperate seasonal forests that are centered around temperate changes, tropical seasonal forest seasons have distinct wet and dry seasons. Therefore, precipitation is the major limiter of plant growth which explains the diversity of tropical seasonal forest landscapes. Some forests with short dry seasons are dominated by dense stands of shrubs and trees, while other drier forests are mostly grassland with a few scattered trees. |
Subtropical Desert
As their name suggests, subtropical deserts are prone to hot temperatures with little annual rainfall. This climate leads to sparse, low growing, but hearty vegetation such as cacti or model CAM plant designed to maximize carbon dioxide uptake, while also limiting water loss through transpiration. Subtropical deserts tend to form around 30 degrees North and South latitude due to air circulation patterns, not unlike boreal forests. While rain does not occur often, when it does plants make the most of it, exploding in growth before water disappears again. |
The Structure of Terrestrial Biomes
Climate and Latitude
Terrestrial biomes are land based and defined primarily by their floral and faunal communities as well as climate. Of these variables, climate is the most important, the average temperature and rainfall of an area controls the plant structure of the plant community, which ultimately determines the types of animals that can be found in the environment. One of the interesting trends that we can observe with biomes is that the climate of a biome is very strongly correlated with the latitude at which we find that particular biome. The reason for this is actually pretty simple and involves the angle of incidence, the angle at which incoming sunlight strikes a land surface. A steeper angle of incidence (closer to 90 degrees) means that sunlight is more concentrated. This ultimately raises the temperature of the area as the light energy is |
absorbed and later remitted as thermal energy. A more shallow angle of incidence (closer to 0 degrees) spreads sunlight out over a greater surface area. This disperse the energy over more area, resulting in comparatively cooler temperatures. Ultimately what this means is that biomes near the equator receive more average annual solar radiation (sunlight) than biomes near the poles. The reason we say " average annual solar radiation" is because the amount of solar radiation a land area receives changes over the course of the year. As the Earth rotates around the sun, the angle of incidence of various latitudes changes. This is because the Earth is titled on its axis by 23.5 degrees which means land surfaces can be oriented toward or away from the sun at different times of the year. For example, during the June solstice, the Northern Hemisphere is tilted toward the sun, while the Southern Hemisphere is
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tilted away from the sun. This results in a steep angle of incidence and warm temperatures in the Northern Hemisphere and a much shallower angle of incidence and colder temperatures in the Southern Hemisphere. In other words, we experience summer as the Southern Hemisphere experiences winter. The reverse of this occurs during the December solstice; it is winter in the Northern Hemisphere and summer in the Southern Hemisphere. During the March and September equinoxes, the angle of incidence is highest at the equator. This is the reason that the tropics are so much warmer than the other latitudes; even through the latitude with the highest angle incidence shifts over the course of the year it is highest the steepest at the equator twice a year resulting in warmer annual temperatures in the tropics.
Albedo: Reinforcing Global Climate Patterns
Albedo refers to the amount of light reflected by an object or surface. Materials with higher albedos reflect more light, while surfaces with lower albedos will reflect less light and absorb more of it. In general, dark colored objects have lower albedos, while more lightly colored objects have higher albedos. Albedo plays an important role in global climate patterns through plant foliage and ice and snow. Plants by their nature have low albedos, which makes sense since plants are photosynthetic and absorb light for photosynthesis. On the other hand, ice and snow have much higher albedos and reflect up to 90% of incoming light. This results in a positive feedback loop that reinforces the global climate patterns established by latitude and the angle of incidence. The tropical biomes are dominated by abundant plant life that absorbs more of the incoming light energy, while the polar biomes |
are dominated by ice and snow which reflect more of the incoming light. So not only do the tropics receive more light energy as a result of the higher angles of incidence found there, but they are also able to stored more of that energy thanks to the lower albedo resulting in even warmer temperatures. In contrast, the polar regions receive less light and reflect away more of than energy resulting in cooler temperature.
Feedback Loops
A feedback loop occurs whenever the outputs of a system are rerouted to be inputs of the same system. In a positive feedback loop, any change in the system becomes exaggerated, any change in the system generates further change. A great example of a positive feedback loop would be the impact of albedo on climate as described above. Here a warmer climate permits more plant growth which lowers albedo and allows the absorption of solar energy and an increase in temperature which allows more plant growth. A negative feedback loop is the reverse, any change in the system is mitigated, any change in the system causes the system to respond by returning to its original state. A great example of a negative feedback loop would be homeostasis such as when we sweat. When our body temperature rises, we sweat which cools us and returns our internal conditions to their original state. The terms positive and negative do not mean good/bad in this context.
A feedback loop occurs whenever the outputs of a system are rerouted to be inputs of the same system. In a positive feedback loop, any change in the system becomes exaggerated, any change in the system generates further change. A great example of a positive feedback loop would be the impact of albedo on climate as described above. Here a warmer climate permits more plant growth which lowers albedo and allows the absorption of solar energy and an increase in temperature which allows more plant growth. A negative feedback loop is the reverse, any change in the system is mitigated, any change in the system causes the system to respond by returning to its original state. A great example of a negative feedback loop would be homeostasis such as when we sweat. When our body temperature rises, we sweat which cools us and returns our internal conditions to their original state. The terms positive and negative do not mean good/bad in this context.
Floral Communities and Climate
The climate of any ecosystem or biome is what ultimately determines the composition of the flora or plant community. This is because temperature and water availability are two of the major limiting factors for photosynthesis, the foundation of any plant's metabolism. If temperatures are too cold, photosynthesis is inhibited, but if temperatures are too hot, the enzymes that allow photosynthesis to operate effectively like ATP synthase, rubisco and NADP+ reductase can become denatured and stop working, which also halts photosynthesis. In the case of water, not only can water serve as a limiting factor for photosynthesis, but water availability also controls plant height. Taller plants like trees require more water to grow, which leaves low growing shrubs and grasses as the dominate plant types in more arid terrestrial biomes like shrublands an grasslands.
The climate of any ecosystem or biome is what ultimately determines the composition of the flora or plant community. This is because temperature and water availability are two of the major limiting factors for photosynthesis, the foundation of any plant's metabolism. If temperatures are too cold, photosynthesis is inhibited, but if temperatures are too hot, the enzymes that allow photosynthesis to operate effectively like ATP synthase, rubisco and NADP+ reductase can become denatured and stop working, which also halts photosynthesis. In the case of water, not only can water serve as a limiting factor for photosynthesis, but water availability also controls plant height. Taller plants like trees require more water to grow, which leaves low growing shrubs and grasses as the dominate plant types in more arid terrestrial biomes like shrublands an grasslands.
Coniferous vs. Deciduous Trees
Trees are perennial plants with an elongated stem or trunk and one of the most recognizable plants on the planet. All trees are vascular plants, meaning they contain conductive tissues like xylem and phloem to distribute nutrients. In general, there are two major types of trees: coniferous and deciduous trees. Coniferous trees generally do not shed their leaves, making them evergreen and include pine, spruce, cedar and redwood trees. Conifers are cold-tolerant and often referred to as "soft-wood" because wood produced from conifers is softer than wood produced from deciduous trees. On the other hand, deciduous |
trees like maple and oak trees are known as "hardwoods" and shed their leaves for at least part of the year. Unlike coniferous leaves that are needle-like in shape, deciduous tree are broad-leafed and designed to maximize sunlight capture making them more efficient at photosynthesis.
While both coniferous and deciduous trees produce seeds, the way in which seeds are produced and stored is very different. Coniferous trees are an example of gymnosperms which means "naked seed"; conifers store their seeds on scales in cones with no other protective tissues or vessels. In contrast, deciduous trees are angiosperms which means "covered seed"; seeds in deciduous trees encase their seeds in a vessel or ovary like a fruit. Angiosperm seeds also have cotyledons, seed
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leaves, to help give the new plant a start once it germinates, a feature absent from gymnosperm seeds. Angiosperms seeds can either have one or 2 seed leaves. Angiosperms that have a single cotyledon are known as monocots, while those with two are called dicots. The image to the right illustrates the difference between monocots and dicots. In addition to lacking cotyledons, gymnosperm seeds also tend to be much smaller than angiosperms seeds. This allows gymnosperms like conifers to use wind to disperse their seeds. Angiosperms rely on animals that eat their fruit to distribute their seeds.
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Examples in Conifers: The Redwoods (Sequoia)
Redwoods are a group of enormous conifers that grow in the Pacific Northwest in location including Northern California, Oregon, Washington and Canada. Redwoods (scientific name Sequoia) are actually the tallest and largest organisms on the planet, dwarfing blue whales and even the Statue of Liberty. What is truly amazing is that these trees are largest than should be possible; they are so tall it should not be possible to transport water up the plant. To compensate for their |
enormous size, the Sequoia are actually able to draw moisture out of the air, specifically from coastal fog that dominates the Pacific Northwest. This makes it easier to transport water to the leaves for photosynthesis.
Examples in Deciduous Trees: The Southern Beech (Nothofagus)
At first glance, you might confuse the beech trees you find in the woods around here (genus Fagus) with the so called Southern Beech trees (genus Nothofagus) that dominate many of the continents in the Southern Hemisphere including South America, Australia and New Zealand. Nothofagus is a very ancient group of trees that can trace their origins all the way back to Gondwana, one of the super continents that formed following the break up of Pangaea. Climate change was expected to wipe out these trees, as they are adapted to cooler |
climates. However, Nothofagus has proved resilient, especially because of it ability to grow clones. An individual tree can reproduce via cloning, forming stands of trees with identical genomes. This has earned Nothofagus the unofficial nickname, "the immortal tree". An individual tree may die after 1,000 or 2,000 years, but its genome can live on through the clones.
Trends in Tree Types and Climate Controls
Something you may have noticed when examining the different terrestrial biomes earlier is that coniferous trees tend to dominate forests with lower temperatures, while deciduous trees control forests with warmer climates. This is not an accident, but rather a product of the design of these plants. As you know deciduous trees have broad leaves that maximize available surface area for light absorption. This makes deciduous trees more efficient at photosynthesis. To compensate for the increased amount of light available for photosynthesis, deciduous trees need to supply more water to their leaves. Their xylem tissue (the vascular tissue responsible for conducting water) is much wider than the xylem seen in conifers, which allows for increased water movement. The type of xylem seen in deciduous trees are called vessels, compared to the thinner trachieds of conifers.
Something you may have noticed when examining the different terrestrial biomes earlier is that coniferous trees tend to dominate forests with lower temperatures, while deciduous trees control forests with warmer climates. This is not an accident, but rather a product of the design of these plants. As you know deciduous trees have broad leaves that maximize available surface area for light absorption. This makes deciduous trees more efficient at photosynthesis. To compensate for the increased amount of light available for photosynthesis, deciduous trees need to supply more water to their leaves. Their xylem tissue (the vascular tissue responsible for conducting water) is much wider than the xylem seen in conifers, which allows for increased water movement. The type of xylem seen in deciduous trees are called vessels, compared to the thinner trachieds of conifers.
The drawback to vessels is that why they can move more water up the plant, they are more susceptible to an embolism- a blockage in vascular tissue. In trees, embolisms are generally caused by air bubbles; the air bubble breaks the water column in the xylem effectively interrupting cohesion and capillary action. The larger diameter of the vessels in deciduous trees actually promotes the formation of air bubbles that can form when the water in the xylem freezes. In contrast, the thinner tracheids actually reduce the size of the air bubbles, allowing the coniferous trees to be more cold tolerant. In short, this is one of the major reasons conifers dominate colder biomes like the boreal forests and deciduous trees are more prevalent in warmer forest biomes.
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Aquatic Biomes
In addition to the 9 terrestrial biomes there are 8 aquatic biomes. Aquatic biomes are characterized by salinity (salt concentration), depth and water flow. There are two major groups of aquatic biomes- freshwater and marine- making salinity one of the most important traits of an aquatic environment. The flowchart below outlines the 8 aquatic biomes.
Streams and Rivers
The trademark of any stream or river is the presence of flowing fresh water. The major difference between a stream (sometimes referred to as a creek) and a river is the width of the moving water; rivers are much larger and flow more slowly than streams narrow, fast moving streams. Cold, turbulent (rough) water increases the amount of dissolved oxygen available in the water which supports animal life like salmon and trout, but reduces the productivity of producers like algae or plants. Therefore the foundation of stream and river ecosystems usually comes from organic matter that enters the flowing water from terrestrial sources. Slower flowing water will have less available oxygen and will support other types of organisms like catfish. |
Wetlands
Wetlands are freshwater, aquatic biomes that are submerged or saturated with water for at least part of the year. As a result, vegetation must be adapted to living in saturated soils with low oxygen availability. Wetlands include swamps, bogs and freshwater marshes like the Meadows Lands in East Rutherford and Okefenokee Swamp in Florida. Wetlands are one of the most productive ecosystems on the planet and provide a number of important ecosystem services, natural actions of the ecosystem that benefit people both economically and culturally. For instance, wetlands are able to absorb large amounts of rainfall, preventing flooding that could damage properties and homes. |
Lakes and Ponds
Lakes and ponds are bodies of fresh, still water. While vegetation is common in both ponds and lakes, there will be areas of the pond/lake that are simply too deep to allow for plant life. This allows us to divide ponds/lakes into distinctive zones. For example, the more shallow water is part of the photic zone which supports plant growth through photosynthesis and the deeper water is part of the aphotic zone. Within the photic zone are the littoral and limonitic zones. The littoral zone refers to the shallow waters along the shores of a pond/lake while the limentic zone indicates open water. The profundal zone exists below the littoral zone within the photic zone and finally the muddy bottom of the lake is referred to as the benthic zone. |
Mangrove Swamps
Mangrove swamps occur along tropical coast lines and are dominated primarily by mangrove trees that grow in soils saturated with salt water. Unlike most other trees, mangroves are salt tolerant (remember water availability is a major limiting factor for tree growth). The mangroves are able to exclude salt from the roots, thereby preventing or at least limiting the uptake of the unwanted, excess salt. Additionally, the mangroves are even able to excrete excess salt through there leaves. Much like wetlands, mangrove swamps provide ecosystem services and help to prevent coastal flooding by regulating surface waters and reduce erosion, securing soil with their roots and preventing the soil from moving away due to tides and other natural forces. |
Salt Marshes
Much like mangrove swamps, salt marshes form along coasts, but in more temperate climates. And like wetlands, salt marshes are one of the most productive ecosystems in the world. Salt marshes are often found in estuaries, areas where rivers empty into the ocean (an estuary is a physical location as opposed to a biome like a salt marsh). The combination of fresh and salt water results in brackish water which is saltier than freshwater, much does not have the salinity of ocean water. Rivers tend to carry lots of nutrients with them and so when the river empties its water into an estuary, the result is a highly fertile ecosystem that allows for abundant plant life and as a result, animal life. In fact, salt marshes provide habitat for the larval stage of 2/3 of all marine fish and shellfish. As with other coastal biomes, salt marshes provide protection from flooding. |
Intertidal Zones
Intertidal zones are transient ecosystems that exist between the high and low tide marks of coastlines. As you would expect, conditions during high tide are favorable for marine life, but at low tide, the ocean water recedes and conditions in the intertidal zone becomes harsh and unforgiving. Organisms living here must be adapted to exposure to direct sunlight, high temperatures and desiccation (extreme dehydration), not to mention the physical abuse they will take from crashing waves. As a result, intertidal zones tend to be populated by armored organisms like shellfish, sponges and arthropods. |
Coral Reefs
Coral reefs are the most biologically diverse marine biome and form in warm, tropical waters. Corals are invertebrate organisms of the phylum Cnidaria, making them cousins of jellyfish and hydra. These organisms secrete calcium carbonate to form a tough, rigid and durable exoskeleton that serves as the foundation of coral reef ecosystems even after the coral die. As adults, coral are sessile (immobile) and form a mutualistic relationship with algae. The algae is provided algae and resources in exchange for glucose the algae produces via |
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photosynthesis. Unfortunately, corals are dying at an alarming rate because of a phenomenon called coral bleaching, largely due to global climate change. The algae living with the corals cannot tolerate environmental change such as changes in temperature or pH caused by excess carbon dioxide emissions. The excess carbon dioxide raises temperatures and causes the oceans to become more acidic. When the algae become stressed, they eject from the coral, turning the coral white and ultimately killing the invertebrate. Current estimates suggest that 27% of the world's coral reefs have already been lost to climate change, with another 60% expected to be lost in 30 years. Even the Great Barrier Reef off the eastern coast of Australia has been heavily impacted with as much as 93% of the ecosystem damaged by bleaching.
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Open Ocean
The open ocean exists far from shore and contains deep water devoid of sunlight. Just like ponds and lakes the open ocean can be divided into multiple regions or zones such as a photic and aphotic zone. There is no littoral, limnetic or profundal zones in the ocean, however the benthnic zone still refers to the bottom. Many portions of the open ocean are devoid of sunlight and so photosynthesis cannot serve as the foundation of ecosystems. Instead chemosynthesis replaces photosynthesis as the metabolic pathway of autotrophs. This can be seen in the bacteria that form a mutualism with Riftia worms near hydrothermal vents on the ocean floor. The bacteria are provided a place to live inside the worms, in exchange for glucose. The bacteria use hydrogen sulfide from the vents as an energy source to convert water and carbon dioxide (dissolved in the ocean water) to produce organic |
sugars. In addition to chemosynthesis, whale falls also organisms living at the bottom of the ocean with food. A whale falls refers to when a large, marine organism (usually a whale) dies and sinks to the ocean floor. Organisms living on the ocean floor will quickly scavenge the carcass and make use of the organic tissues for energy and carbon. In this way, a whale fall is similar to rain in a desert; there will be rapid growth and activity following the whale fall, but then the organisms will disperse once more in search of food once again. It might sound eerie but the ocean floor is lined with the skeletal remains of many, many whales much like a graveyard.
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Population Dynamics
Population Growth
The size of a population at any given time relies on a few key factors, most notably its growth rate. The rate of growth of a population determines if the population size is expanding, contracting or remaining stable. Obviously a population expands with each birth in the population and decreases with every death. However, migration between populations also has an important role to play in determining population size. Immigration occurs whenever individuals from outside populations enter the population of study and emigration occurs when individuals exit the population of study to join outside populations. This means that the birth rate and immigration rate positively impact population size, while the death rate and emigration rate negatively influence the population's size. Mathematically speaking, population size can be defined by the following formula: A population's size in a given year (Nt) = the previous year's population size (Nt-1) + B (birth rate) + I (immigration rate) - D (death rate) - E (emigration rate). |
Fertility Rate
The fertility rate of a population describes the average number of offspring each female will have in her lifetime. High fertility rates will cause populations to expand, while low fertility rates will cause populations to decrease in size. At replacement level fertility, the number of deaths that occur within a population are offset by the fertility rate, there is not net change in the population's size. The replacement level fertility rate can vary between populations since it is dependent on the death rate, but it is generally 2.1 in human populations. This means that in a stable population, on average each woman will have 2.1 offspring. This number accounts for the death of the woman and her mate and the remaining 0.1 accounts for infant mortality. Keep in mind that the infant mortality can vary by species and even within population of the same species. For example, the replacement level fertility rate is much lower in the U.S. or other developed nations than it is in developing nations like Kenya.
The fertility rate of a population describes the average number of offspring each female will have in her lifetime. High fertility rates will cause populations to expand, while low fertility rates will cause populations to decrease in size. At replacement level fertility, the number of deaths that occur within a population are offset by the fertility rate, there is not net change in the population's size. The replacement level fertility rate can vary between populations since it is dependent on the death rate, but it is generally 2.1 in human populations. This means that in a stable population, on average each woman will have 2.1 offspring. This number accounts for the death of the woman and her mate and the remaining 0.1 accounts for infant mortality. Keep in mind that the infant mortality can vary by species and even within population of the same species. For example, the replacement level fertility rate is much lower in the U.S. or other developed nations than it is in developing nations like Kenya.
Population Growth
Generally speaking, population tend to grow in predictable ways or patterns. Two notable examples are exponential and logistic growth. Exponential growth occurs in populations that have an abundance of resources and no predators, allowing all individuals to reproduce with in the population. A great example is bacteria; when grown in the lab with the right conditions every bacteria cell in a population is able to reproduce in about 20 minutes. That means that if there were 100 bacteria at the start, after 20 minutes there will be 200 bacteria and after 40 minutes there will be 400. With each generation the population doubles, growing wildly out of control, earning the name J shaped curve. This is a problem because eventually the population will exhaust it resources no matter how abundant those resources were at the start. Eventually the population will have to crash and return to sustainable size.
In logistic growth populations can expand rapidly when resources are abundant, but the growth rate decreases the larger the population becomes and eventually stabilizes. This forms an S-curve where the population begins to level off as it approaches its carrying capacity. This is very different from what occurred in exponential growth because the carrying capacity of the population was ignored leading to a population crash.
Generally speaking, population tend to grow in predictable ways or patterns. Two notable examples are exponential and logistic growth. Exponential growth occurs in populations that have an abundance of resources and no predators, allowing all individuals to reproduce with in the population. A great example is bacteria; when grown in the lab with the right conditions every bacteria cell in a population is able to reproduce in about 20 minutes. That means that if there were 100 bacteria at the start, after 20 minutes there will be 200 bacteria and after 40 minutes there will be 400. With each generation the population doubles, growing wildly out of control, earning the name J shaped curve. This is a problem because eventually the population will exhaust it resources no matter how abundant those resources were at the start. Eventually the population will have to crash and return to sustainable size.
In logistic growth populations can expand rapidly when resources are abundant, but the growth rate decreases the larger the population becomes and eventually stabilizes. This forms an S-curve where the population begins to level off as it approaches its carrying capacity. This is very different from what occurred in exponential growth because the carrying capacity of the population was ignored leading to a population crash.
Carrying Capacity
Every ecosystem has a specific amount of resources with in it and therefore can only support so many organisms. Resources in an environment include food, water, physical living space and energy. The carrying capacity (denoted by the letter K) is the maximum number of organisms an environment can support indefinitely given the resources (food, water, physical space, etc.) available. The more resources available, the more organisms the environment can support. The carrying capacity of an ecosystem is generally correlated with the latitude of the ecosystem. Environments at lower latitudes receive more annual sunlight and therefore more overall energy. This is why ecosystems like tropical rainforests can support more life than tundra or a desert- there is simply more energy and resources available. |
Although the carrying capacity of an ecosystem is a fixed number it is not a concrete line; populations can expand beyond the carry capacity, but only temporarily. For example, a particular prairie may support 80 bison (this would be the carrying capacity), but one year there could be 84 bison living on the prairie. This is called an overshoot, a temporary expansion of a population past the carrying capacity of its environment. Inevitably, the population will have to decrease because there simply are not enough resources available to support 84 bison. The extra bison will starve or the high population density will make it easier for predators to kill off some of the bison- carrying capacity can be enforced in a variety of ways. Regardless of how it actually happens, this rapid decline of a population is known as a die-off. An example can be seen above, notice how the population may fluctuate above and below the carrying capacity.
Population Growth Patterns
The interactions of population and the carrying capacity of their environment's through overshoots and die-offs, produces interesting and repeated population growth patterns. While every population can expand and contract in its own manner, there are a few typical patterns we can observe in natural. One of the most common patterns is population oscillations also known as the "boom and bust" cycle). |
Oscillating populations experience regular intervals of overshoots (booms) and die-offs (busts). A classic example of this can be seen in predator-prey interactions (a biotic factor), like those between the hare and lynx as seen in the graph above. The general pattern in this oscillation or boom and bust cycle is that a high prey population (the hares) allows the predator population (the lynx) to expand. As the lynx population expands, more and more hare are consumed. This causes a die-off of the are population, which generates a die-off in the lynx population as well. With a reduced number of predators, the hare population expands again, which of course allows the lynx population to grow and the whole process begins again.
Population oscillations can follow a variety of patterns including chaos, stable limit cycle and damped oscillation. In chaos, the population sizes alter without any real pattern; the populations rises and falls randomly. Populations experiencing stable limit cycles are more predictable: they exhibit regular repeated overshoot and die-off events similar to the
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hare and lynx example above. Unlike like under the chaos model, stable limit cycles are more predictable, growth follows decline and vice versa. In damped oscillation, a population experience overshoots and die-offs that become progressively less intense over time. Under this model, the population will eventually reach equilibrium with the environment and maintain a population at or near carrying capacity.
A boom and bust cycle can also be generated by abiotic factors. Resource availability can change dramatically within a population. Consider for example a desert, most of the time the desert is dry and arid which suppresses growth (this is your bust). But when it does rain, plants take advantage and there is a sudden bloom. (this is the boom). This uneven availability of resources will generate population oscillations. In this context, the resource could be anything, even sunlight. In short, when resources are limited (like in a drought), a bust with ensure; when resources are plentiful, there will be a boom.
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Survivorship Curves
Just as populations tend to grow in predictable ways, the individuals that make up those populations tend to operate in predictable ways and form specific survivorship curves. Survivorship refers to the probability that an organism will survive (not die) as it ages. Type I organisms like humans and most carnivores have low birth rates and high survivorship early in life, but experience a dramatic decrease in survivorship late in life. A female black bear may only give birth to a cub or two per year, but those cubs are likely to survive well into adulthood. This high |
survivorship is largely because of a long gestation period to ensure well developed offspring and parental care. Type II organisms like prairie dogs and most herbivores tend to have moderate birth rates and a constant rate of survivorship. A rabbit may have upwards of 12 offspring in a single year, but they will die off at a fairly constant rate due to predation. Humans actually used to follow a Type II curve, but the invention and use of medicine and better hygiene in the 20th century increased our survival rate. Type III organisms like sea turtles, shellfish and insects tend to have very high birth rates and low survivorship early in life, but those organisms that make it past the first few years will see a dramatic increase in survivorship later in life. A female sea turtle can lay hundreds of eggs in a single year, but only a small percentage will make it past their first few years of life (for example, only 80% of the eggs laid will hatch). However, the few sea turtles that make it out of this dangerous time of their lives and long enough to reach reproductive maturity will live long full lives. The survivorship of a species has important ecological consequences and is inherently tied to population size and growth.
r and K Strategists
To maximize their survival, some species have adopted specific life strategies. K-selected species have slow intrinsic growth rates and steady population sizes at or near the carrying capacity of the environment, hence the name K-selected. K-stratgists favor survivorship over birth rate. In contrast, r-selected species have high intrinsic growth rates (in ecology r is the variable used for intrinsic growth rate, the maximum growth rate of a species under ideal conditions), but experience dramatic overshoots and die-off events. r-strategists favor birth rate over survivorship. Both strategies work in their own way when they are implemented correctly. K-strategists follow the Type I survivorship curve- they have low birth rates and high survivorship. K-strategists tend to be large organisms (especially mammals), live in stable environments, become sexually mature late in life, have low numbers of offspring that require a large amount of |
energy and resources to produce and have long life expectancies. On the other hand, r-strategists like most insects, fish and invertebrates are the opposite: they follow Type III survivorship, tend to be small organisms, live in variable environments, achieve sexual maturity early in life, have large numbers of offspring that require little energy and resources to produce and have short life expectancies. The graphic to the right summarizes this trend; the more offspring per year the more r-selected the species is considered, the fewer offspring the more K- selected.
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In addition to following the Type I survivorship curve, K-selected species tend to express logistic growth, never expanding far beyond their carrying capacity (K). Of course, r-selected species follow a different pattern, expressing exponential growth and exhibit population oscillations, the "boom and bust pattern". Because of their expansive birth rate, r-selected species will rapidly increase in size (the boom), pushing past their carrying capacity before returning to sustainable levels due to low survivorship (the bust)- a great example of carrying capacity being enforced.
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Niches
Niche
A niche describes an organism's role or job in its environment. All organism's fall into one of two very basic categories: autotrophs and heterotrophs. Autotrophs produce their organic food from inorganic carbon and energy sources, while heterotrophs must consume organic tissues for energy and carbon. Most autotrophs undergo photosynthesis, converting inorganic carbon dioxide and water into glucose and oxygen using sunlight as an energy source.
A niche describes an organism's role or job in its environment. All organism's fall into one of two very basic categories: autotrophs and heterotrophs. Autotrophs produce their organic food from inorganic carbon and energy sources, while heterotrophs must consume organic tissues for energy and carbon. Most autotrophs undergo photosynthesis, converting inorganic carbon dioxide and water into glucose and oxygen using sunlight as an energy source.
Autotrophs that conduct photosynthesis are know as photoautotrophs (photo = light). The other major type of autotroph is the chemoautroph. Unlike photoautotrophs (such as photosynthetic plants, algae and cyanobacteria), chemoautrophs produce glucose using chemosynthesis. Chemosynthesis is very similar to photosynthesis except instead of using sunlight, chemoautotrophs using chemical energy. For example, Riftia worms (giant tube worms) live along the mid-ocean ridge of the Atlantic Ocean where the continental plates separate. This action generates hydrothermal vents, allowing gases to
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escape from deep within the Earth, including hydrogen sulfide. Special, chemosynthetic bacteria, living with the worms use the chemical energy of the hydrogen sulfide to convert carbon dioxide (dissolved in the ocean water) and oxygen (also dissolved in the ocean water) into carbohydrates. Chemosynthesis provides an alternative to photosynthesis as a foundation to ecosystems, allowing life to exist even in the absence of solar energy. Of course, photosynthesis still serves as the primary means of converting inorganic compounds into organic molecules for metabolism.
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Heterotrophs can also be divided into two major groups: photoheterotrophs and chemoheterotrophs. Both types of heterotrophs acquire carbon from organic tissues (like glucose), but photoheterotrophs get their energy from sunlight, while chemoheterotrophs use the chemical energy of the organic tissues they consume. Photoheterotrophs are exceptionally rare and we really will not be focusing on them, but they are certainly interesting and worth mentioning. Chemoheterotophs represent the vast majority of heterotrophic life including humans. Heterotrophs are highly dependent on autotrophs for survival, which sets up for unique species interactions and community dynamics that we shall explore.
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An organism's realized niche describes the abiotic and biotic conditions under which a species actually lives. A realized niche is more specific than a generic niche and can realistically only be filled by a single species. When species have overlapping realized niches competition ensues and the species will either need to evolve to fill new, distinct niches or the weaker species (less adapted) will go extinct. The example to left shows a group of three different species of warblers. Each species has developed to fill its own unique role (realized niche) in the environment. They may all live and feed within the spruce tree, but each occupies a different location within the tree, thus maintaining a unique realized niche.
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Niche Types- Producers
As we have seen, a niche is a highly detailed description of a species' role in its environment that includes both abiotic and biotic factors. We cannot evaluate each individual, unique species niche, but we can study some of the major niche types that are repeated in ecosystems. By far the most common metabolisms are photoautotrophic (like most plants) and chemoheterotrophic (like most animals and fungi). But in terms of ecological niches, the most basal and arguably the most important to the stability of the ecosystem would be the autotrophs which in ecology are often referred to as producers, organisms that produce their own food from nonliving sources. As you know, producers can be photosynthetic or chemosynthetic and form the foundation of virtually all ecosystems. Heterotrophic organisms are completely dependent on producers not just for oxygen, but for sources of organic energy and carbon.
As we have seen, a niche is a highly detailed description of a species' role in its environment that includes both abiotic and biotic factors. We cannot evaluate each individual, unique species niche, but we can study some of the major niche types that are repeated in ecosystems. By far the most common metabolisms are photoautotrophic (like most plants) and chemoheterotrophic (like most animals and fungi). But in terms of ecological niches, the most basal and arguably the most important to the stability of the ecosystem would be the autotrophs which in ecology are often referred to as producers, organisms that produce their own food from nonliving sources. As you know, producers can be photosynthetic or chemosynthetic and form the foundation of virtually all ecosystems. Heterotrophic organisms are completely dependent on producers not just for oxygen, but for sources of organic energy and carbon.
Niche Types- Consumers
In the context of ecology, consumers are heteortrophic organisms, organisms that consume organic tissues and compounds in order to acquire both energy and carbon. There are a variety of different types of consumers that we shall examine including: herbivores, carnivores, omnivores, apex predators, scavengers, decomposers and detritivores. Each of these types of organisms consumes organic tissues, whether they are living or nonliving at the time of consumption. |
Primary Consumers- Herbivores
Organisms that feed exclusively on autotrophs are known as herbivores (from the roots "herb" meaning plant and "more" meaning mouth). Herbivores are considered primary consumers because they are the first level of consumers in an ecosystem. Most herbivores have eyes oriented on the sides of the skull such as in the Fanghorn deer skull to the left. This provides the organism with a wide field of view to make detecting predators easier. Of course, a wider field of view provides |
herbivores with little if any depth perception. Luckily, plants are immobile so the lack of depth perception has little negative impact on these organisms. The teeth of herbivores are flat and square like molars to make grinding tough plant matter easier. Reinforced enamel helps to prevent teeth from wearing down over time.
consumer in the ecosystem. Carnivores sport pointed, conical (cone shaped) teeth like canines to help stab and hold prey. Carnivores typically have eyes oriented at the front of the skull that face forward, like in the fox skull depicted above. This causes the field of view of each eye to overlap, enhancing the depth perception of the organism. This is known as binocular vision and it helps carnivores track their prey. Typically, carnivorous predators will only get one shot at taking down their prey and a failed attempt means waste resources (energy). Binocular vision provides depth perception to allow the carnivore to better track its prey and determine how far away the prey is. Patient predators are often the most successful, constantly assessing if their target is within striking distance and limiting energy waste.
Transient Consumers- Omnivores
Some organisms eat both plants and animals and are known as omnivores (from the root "omni" meaning all and "more" meaning mouth). Because of their diet, omnivores do not fit neatly into a any single consumer level since they can act as primary or secondary consumers. For this reason, omnivores are referred to as transient consumers, since they can occupy multiple consumer levels in an ecosystem. |
As one might expect, omnivores are a blended intermediate of herbivores and carnivores not only in terms of their diet, but also their physical structure. Like herbivores, omnivores possess squared molars grinding plant based food. And like carnivores, omnivores use sharp, conical canines to consume animal prey. Most omnivores have eyes positioned at the front of the skull to enable binocular vision. This can be seen in the raccoon skull in the example above.
Tertiary or Higher Consumers- Apex Predators
Apex predators are the predators of carnivores, they are the top of the food chain and have no predators themselves. Apex predators have no natural predators that eat them, their dominance is absolute and uncontested in the ecosystem. Examples of apex predators include orca whales, the grey wolf and many raptors. These top predators are referred to as tertiary consumers since they are the third level of consumer organisms in an ecosystem. Of course, apex predators could reside at even higher levels such as quaternary (fourth level), but as we shall see in our discussion of trophic levels, consumer levels are finite and few ecosystems have more than 4 levels of consumer. |
Scavengers, Decomposers and Detritivores
The consumers we have seen so far all hunt and eat their own food. But in every ecosystem, there is going to be waste and some organisms have evolved to take advantage of that resource as their niche. These are the scavengers, decomposers and detritivores, the recyclers of the environment that break down living tissues, opening up space and providing fertilizer for new growth. Scavengers are animals that consume dead organisms that they themselves did not kill. Vultures, hyenas and in many cases coyotes are all examples of scavengers. Decomposers such as fungi and bacteria break down dead organisms, discarded plant matter like fallen leaves and animal waste through |
decomposition. This provides the decomposer with energy and carbon but also produces detritus, organic waste that has been processed by decomposers into small, simpler parts. This detritus is what detritivores like worms will feed on, completing the recoiling process of the ecosystem. These organism play an enormously important role in all ecosystems. Without the decomposition (the decay of organic tissues) performed by scavengers, decomposers and detritivores important resources would become permanently locked up in dead organisms and waste products, depriving the organisms of the ecosystem vital nutrients for growth. Decomposition returns important elements like nitrogen and phosphorus to the soil to support new growth of plants and by extension animals. In addition, decomposition opens up physical space- without scavengers, decomposers and detritivores, ecosystems would be inundated with waste and dead tissues with no room for living organisms.
A classic example of the impacts of a keystone species would by the case of the sea otter and sea urchin. Sea otters are marine mammals that feed on sea urchins and other echinoderms. The sea urchins feed on kelp, a type of brown algae or seaweed. The sea otters function as a keystone species by keeping the other species in check: the sea otters feed on the sea urchins, helping to regulate the sea urchin population which in turn feeds on the kelp, regulating that population. But when sea otters are removed from the environment the whole system collapses. With no predators to keep their population from expanding, the sea urchins grow out of control. The kelp is overgrazed and the population collapses. With no more food, the now expanded sea urchin population suffers are die off due to starvation. Sea otters prevent all of this by enforcing population balance and stability.
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Many keystone species are apex predators that keep prey populations in balance. For example, when the last grey wolf was removed from Yellow Stone Nation Park in 1926, the remaining species in the park fell out of balance. With their natural predator now absent the elk population exploded in size generating degradation of the land through overgrazing and erosion. The elk decimated populations of willow and aspen, two of the most important trees for beavers to build dams.
Beavers are known as ecosystem engineers, organisms that create and maintain habitat for other species and can also function as keystone species. With the beaver populations suffering, beaver dams collapsed, altering the landscape even further. The chain reaction continued, harming populations of aquatic organisms like fish and amphibians. With no competition for food from the wolves, the coyote population rose and drastically reduced the pronghorn antelope population of Yellowstone. It was only after reintroduction of the grey wolf in 1995 that the Yellowstone community began to fully recover. The graphic to the left summarize the interconnectedness of the Yellowstone community, with the grey wolf at its core. |
Trophic Pyramids: Energy Limitations of Ecosystems
Trophic Levels
All ecosystems or rather the organisms contained within them are centered around energy to support themselves. But energy is not equally distributed within an ecosystem; often the amount of energy available to an organism is dependent on the position of the organism in its environment. Because of this, we can form trophic levels to describe the energy available to an organism based on its position in the ecosystem. Trophic level 1 is the most basal trophic level, forming the base of an energy pyramid. Trophic level 1 consists of autotrophs (producers) that produce their own food. Organisms in trophic level 1 have the largest amount of available energy. The next trophic level |
would be trophic level 2 which exists above trophic level one and contains the next largest amount of available energy. Trophic level 2 consists of primary consumers (herbivores) that feed on autotrophs. Trophic level 3 exists above trophic level 2 and contains the secondary consumers (carnivores). Trophic level 4 is often the highest level of the energy pyramid, but contains the least available energy. Tertiary consumers such as apex predators including keystone species like orca whales or the grey wolf occupy trophic level 4.
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You may have noticed that some organisms such as omnivores, scavengers, decomposers and detrivores do not appear in the trophic levels depicted above (the trophic levels together are called trophic pyramids). These organisms are more challenging to place in energy pyramids because they have multiple sources of energy as they eat a variety of kinds of food. By definition, omnivores occupy multiple trophic levels and so are often placed in intermediate levels. For example, humans occupy trophic level 2.5 in-between the primary and secondary consumer levels. Scavengers, decomposers and detritivores on the other hand exist outside the energy pyramid as they can consume from all trophic levels at any time. You should also note that energy pyramids do not have to stop after the fourth trophic level.
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Theoretically, there could be an infinite number of trophic levels, but that would require infinite energy since energy is lost at each transition. Most ecosystems only have enough initial solar energy to support four or possibly five trophic levels.
Trophic Pyramids
The key concept to the energy pyramid and trophic levels is that as you advance up from one trophic level to the next (1 to 2; 2 to 3 and so on), the amount of available energy decreases significantly (about a 90% decrease during each transition). This is because of the second law of thermodynamics; most energy is lost as heat before being consumed by a predator. This is how energy pyramids earned their name, as you move up the pyramid, less and less energy is available to organisms and the trophic levels become more and more narrow. In fact, each trophic level can only support about 10% of the biomass the one below it could |
support. This explains why predators are always outnumbered by their prey. Organisms at lower trophic levels simply have more access to energy and can support large population sizes. But at the top of the pyramid, there is less energy available and so populations must be smaller. For example, there might be billions of individual grass plants living in Yellow Stone National Park, but only a few hundred wolves at the top of the food web.
A common misconception is that because only 10% of the energy is passed along from trophic level to trophic level, predators (herbivores, carnivores, etc.) only consume 10% of the prey population. This is not true. Predators can eat all of the available prey populations, not just 10%. Again, the only reason the trophic levels become smaller near the top is because energy is "lost" as heat during cellular respiration, limiting the amount of energy that can advance.
A common misconception is that because only 10% of the energy is passed along from trophic level to trophic level, predators (herbivores, carnivores, etc.) only consume 10% of the prey population. This is not true. Predators can eat all of the available prey populations, not just 10%. Again, the only reason the trophic levels become smaller near the top is because energy is "lost" as heat during cellular respiration, limiting the amount of energy that can advance.
Food Webs
Purpose
Food webs help to characterize the relationship between different species living within an ecosystem. In the example to the right, plants form the basis of the food we, obtaining their energy from the sun (not shown). The primary consumers (gazelle, giraffe, zebra, hare wildebeest and hyena) feed on the grass and trees. These primary consumers are in turn eaten by secondary consumers (carnivores) like the lion, cheetah and hyena. Scavengers, decomposers and detritivores including the lion, vulture, hyena, bacteria, fungi and beetle recycle dead organic tissues. Notice that some organisms can occupy multiple positions within the food web based on their diets. |
Structure
Food webs are structured so that autotrophs (producers) are always at the bottom. This is because they are the foundation of ecosystems: as you move up the food web, you advance in trophic level. As such, top predators (apex predators) are always found at the top of a food web. The arrows of a food web show who eats who or stated another, more scientific way, the movement of energy through an ecosystem. Because energy flows from the prey to the predator, the arrows should point toward the consumer in a food web.
Food webs are structured so that autotrophs (producers) are always at the bottom. This is because they are the foundation of ecosystems: as you move up the food web, you advance in trophic level. As such, top predators (apex predators) are always found at the top of a food web. The arrows of a food web show who eats who or stated another, more scientific way, the movement of energy through an ecosystem. Because energy flows from the prey to the predator, the arrows should point toward the consumer in a food web.
Relationship with Trophic Levels and Energy Pyramids
While a food web may not physically resemble an energy pyramid, the two models draw heavily from one another. In effect they represent the same information just displayed in a different way. Kind of like how we can illustrate a population trend as a bar graph or a line graph. It is up to you to decide when each would be most appropriate. Food webs are best for illustrating the relationship between species and flow of energy, while energy pyramids are typically better for understanding broader ecological concepts like limitation of resources and carrying capacity.
While a food web may not physically resemble an energy pyramid, the two models draw heavily from one another. In effect they represent the same information just displayed in a different way. Kind of like how we can illustrate a population trend as a bar graph or a line graph. It is up to you to decide when each would be most appropriate. Food webs are best for illustrating the relationship between species and flow of energy, while energy pyramids are typically better for understanding broader ecological concepts like limitation of resources and carrying capacity.
Primary Productivity
In an ecosystem, productivity refers to the generation of biomass while primary productivity, as the name suggests, refers specifically to autotrophs. In other words, ecosystems with high primary productivity have larger populations of plants and higher rates of photosynthesis which can in turn support larger communities of organisms, making primary productivity the most important type of productivity in any ecosystem. It should be noted that primary productivity is not exclusive to plants, but refer to any autotroph. This means that chemosynthesis would also be considered part of primary productivity. However, we will be focusing primarily on plants and photosynthesis.
Productivity can be further differentiated into gross primary productivity (GPP) and net primary productivity. Gross primary productivity (GPP) refers to the total amount of solar energy converted into plant biomass (glucose) through photosynthesis and net primary productivity (NPP) refers to the amount of solar energy converted to biomass after taking into account energy "loss" through cellular respiration (in economics gross refers to the total amount of income before subtracting expenses, while net refers to income after subtracting expenses). As you can see in the graphic to the right, photosynthesis is not a very efficient process, only about 1% of the total available amount of solar energy is captured by plants (GPP) and of that energy 60% is "lost" through cellular respiration in the plants. This means that only 40% of the GPP will be used to build new tissues (biomass) in the producers and will be available to be transferred to other trophic levels.
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It should again be noted that energy is not created or destroyed. As we saw with the trophic pyramids, energy is not truly "lost" from ecosystems but rather is converted into an unusable form. So when we say that "60% of GPP is lost to respiration", the energy did not disappear but is just no longer useable by the organism (most often it has been converted and dispersed as heat).
The graphic to the left summarizes the net primary productivity of various terrestrial and aquatic ecosystems and biomes. Notice that swamps, marshes, coral reefs and tropical forests dominate the list and offer highly productivity ecosystems. This high amount of productivity can support an expansive community of organisms and promote high biological diversity. On the other hand, tundra, deserts and the open ocean offer the lowest rates of net primary production. These ecosystems generally lack the resources needed for growth; desert plant growth is limited by temperature and precipitation. |
Species
What is a Species?
A species is one of the those concepts that seems so familiar to us, but in reality is difficult to put an exact definition too. You would probably define a species as all of the organisms of the same type. That's the easy part; the challenge is drawing the line between one species and another. This is something ecologists (scientists who study ecology) have wrestled and struggled with for a long time. Below are four different definitions of a species, known as species concepts. Evaluate them all and consider which is the most clear, objective and scientific at differentiating between species.
A species is one of the those concepts that seems so familiar to us, but in reality is difficult to put an exact definition too. You would probably define a species as all of the organisms of the same type. That's the easy part; the challenge is drawing the line between one species and another. This is something ecologists (scientists who study ecology) have wrestled and struggled with for a long time. Below are four different definitions of a species, known as species concepts. Evaluate them all and consider which is the most clear, objective and scientific at differentiating between species.
Biological Species Concept
The biological species concept focuses on the ability of organisms to interbreed. If two organisms are unable to successfully interbreed, then they are not members of the same species. There are a variety of barriers that could prevent organisms from interbreeding and the barriers are cumulative meaning each builds off the the previous one. It is important to note that these barriers are not absolute- sometimes organisms will overcome the barrier, but still be regarded as different species. That can be confusing but that is how the natural world works, nothing is ever clear cut. The best way to think it is this: members of the same species can get past all of the barriers, all of the time.
Barriers can be divided into two major types: pre-zygotic barriers and post-zygotic barriers. Pre-zygotic barriers prevent a zygote from every being established between individuals meaning the barrier is in place before any fertilization attempt occurs. Post-zygotic barriers prevent the zygote/offspring from surviving and/or reproducing. The graphic below summarizes the major types of barriers, although sometimes uses different names than discussed on this site.
The biological species concept focuses on the ability of organisms to interbreed. If two organisms are unable to successfully interbreed, then they are not members of the same species. There are a variety of barriers that could prevent organisms from interbreeding and the barriers are cumulative meaning each builds off the the previous one. It is important to note that these barriers are not absolute- sometimes organisms will overcome the barrier, but still be regarded as different species. That can be confusing but that is how the natural world works, nothing is ever clear cut. The best way to think it is this: members of the same species can get past all of the barriers, all of the time.
Barriers can be divided into two major types: pre-zygotic barriers and post-zygotic barriers. Pre-zygotic barriers prevent a zygote from every being established between individuals meaning the barrier is in place before any fertilization attempt occurs. Post-zygotic barriers prevent the zygote/offspring from surviving and/or reproducing. The graphic below summarizes the major types of barriers, although sometimes uses different names than discussed on this site.
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- Ecological Barriers: This type of barrier exists when species live in the same ecosystem, but occupy different regions or niches within it. An ecological barrier is very similar to a geographic barrier in that sense that both barriers prevent organisms from coming into contact with one another. One example would be bull frogs that live in permanent ponds and the red-legged frog which lives in fast moving streams.
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- Gamete Barriers: Gamete barriers exist when organisms from different species are able to successfully mate, but their gametes are unable to fuse and therefore no fertilized egg or embryo can form.
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Evolutionary Species Concept
The evolutionary species concept focuses on the evolutionary history of an organism. Organisms are grouped into species according to their genetic relatedness, evolutionary history and fossil record evidence of a recent common ancestor. Like the ecological species concept, the evolutionary species concept is very inclusive and the barriers between species are blurry. For instance, marsupials like the koala, kangaroo and wombat of Australia would all be considered the same evolutionary species because of their recent common ancestral species. In a similar way, humans and chimpanzee would be grouped in the same evolutionary species because of their nearly identical DNA (99% similar). Phylogenetic trees are extremely useful tools designed to help organize different species according to their genetic relatedness and their evolutionary history, making them very useful for evaluating species under the evolutionary species concept. This video does a great job explaining how phylogenetic trees are made and why they are so useful. |
Ecological Species Concept
Under the ecological species concept, organisms are considered a part of the same species if they have the same ecological niche. As you know an ecological niche is the role and position a species has in its environment; how it meets its needs for food and shelter, how it survives, and how it reproduces. In theory, every species has its own unique niche, but nothing in nature is ever black and white and species can have overlapping niches. Like the evolutionary species concept, the ecological species concept is far more inclusive than the biological species concept and allows song birds and fruit bats to be considered the same species since they both fly and eat fruit. The boundary between species becomes very blurry and difficult to objectively define.
Under the ecological species concept, organisms are considered a part of the same species if they have the same ecological niche. As you know an ecological niche is the role and position a species has in its environment; how it meets its needs for food and shelter, how it survives, and how it reproduces. In theory, every species has its own unique niche, but nothing in nature is ever black and white and species can have overlapping niches. Like the evolutionary species concept, the ecological species concept is far more inclusive than the biological species concept and allows song birds and fruit bats to be considered the same species since they both fly and eat fruit. The boundary between species becomes very blurry and difficult to objectively define.
Recognition Species Concept
The recognition species concept focuses on organisms' ability to recognize one another as potential mate. This works very well for the animal kingdom (in particular reptiles, birds and mammals) because the prevalence of elaborate courtship displays like the Bison rut and Bird of Paradise dance. Each species has its own unique courtship display that acts like a fingerprint to identify it. An example would be how different species of crickets have unique songs. The females of one species will not respond to the calls of a male from a different species. Interestingly, the biological species concept actually draws some of its characteristics from the recognition species concept as seen with behavioral barriers. The major down side of the recognition species concept is that not all organisms rely on courtship rituals during mating. Many plants for instance release pollen which disperse through the wind and collected to fertilize eggs without the plants ever identifying each other as potential mates. This makes applying the recognition species concept very difficult.
The recognition species concept focuses on organisms' ability to recognize one another as potential mate. This works very well for the animal kingdom (in particular reptiles, birds and mammals) because the prevalence of elaborate courtship displays like the Bison rut and Bird of Paradise dance. Each species has its own unique courtship display that acts like a fingerprint to identify it. An example would be how different species of crickets have unique songs. The females of one species will not respond to the calls of a male from a different species. Interestingly, the biological species concept actually draws some of its characteristics from the recognition species concept as seen with behavioral barriers. The major down side of the recognition species concept is that not all organisms rely on courtship rituals during mating. Many plants for instance release pollen which disperse through the wind and collected to fertilize eggs without the plants ever identifying each other as potential mates. This makes applying the recognition species concept very difficult.
Species Interactions
All species interact with each other in some way; this is one of the unwritten laws of nature and we can see that in the food webs we discussed above. These intimate, long term interactions between species are known as symbioses which can be expressed in a variety of ways. Ecologists have classified these symbioses into 6 major categories.
Neutralism (0/0)
A neutralism is a hypothetical relationship between species in which they have no impact on one another. The truth, is this never happens in nature- all species have an impact on each other whether that impact be very small or very obvious. Remember, all life is connected within through biosphere and nutrient cycling (carbon and water cycles) so in some small way all species interact. The figure to the right illustrates this: all organisms will eventually die and their bodies will be decomposed, breaking down tissues into their base components, elements and molecules. These will become a part of the soil and integrated into new plant life which often serves as the foundation of the ecosystem. Plants help to drive the water cycle forward through transpiration and release oxygen as they perform photosynthesis. Aerobic organisms will consume that oxygen for their own metabolism. |
Ammensalism (-/0)
In an ammensalism, one species is negatively impacted while the other remains unaffected. For example, consider ungulates (hoofed mammals) in Australia. Historically, Australia is home to marsupials and did not have hoofed animals (horses, cows, goats, etc.) until the Europeans imported them as they colonized the continent. The Australian landscape was unprepared for their arrival and has suffered greatly since the introduction of the ungulates. As they walk, ungulates tear up the soil, killing delicate native plant life and increasing soil erosion. As the soil erodes away, it becomes more and more difficult for plant life to grow. Yet the ungulates remain unaffected from the interaction, they are simply walking. |
Another example of an amensalism is nature is allelopathy. In allelopathy, plants release chemical into the environment that inhibit the growth of other plants. Plants that exhibit allelopathy are attempting to avoid competition for resources like water or light with other plants and the chemicals they release do not harm themselves, only other plants. This can be seen in black walnut, a tree that releases the chemical juglone into the soil.
Commensalism (+/0)
In a commensalism, one species benefits from the interaction while the other remains largely unaffected. For instance, consider barnacles growing on a Humpback whale. The barnacles are provided a physical place to grow and transportation around the oceans which helps them breed. Clearly this is beneficial for the barnacles and the whales are harmed or benefitted from the experience. |
Mutualism (+/+)
This is probably an interaction you are probably already familiar with. In a mutualism, both species benefit from the interaction. For example, predatory fish such as grouper and even sharks will have their teeth cleaned by a tiny little fish known as a cleaner wrasse. The wrasse enters the other fishes mouth and eats the small bits of food caught in the teeth. In this way the cleaner wrasse gets a free meal and the predatory fish have their teeth cleaned to avoid cavities or infections. Another great example of a mutualistic partnership is the relationship between clownfish and sea anemones. The clownfish is immune to the |
anemone's sting which provides the clownfish with protection from predation. In exchange, clownfish will lure fish to the seas anemone to make for a quick meal.
Antagonism (+/-)
In this type of species interaction, one species benefits at the expense of the other. As the name implies, an antagonistic relation requires an antagonist or a villain to prey upon a victim; a winner and a loser. This type of interaction is typified by predation and parasitism. In predation, a predator gains a meal at the cost of the life or limbs of the prey. Predation applies to both herbivores and carnivores; both plants and animals can be considered prey. In a parasitism, a parasite gains sustenance by literally draining the life force of its host. A major difference between predation and parasitism is that generally a parasite will not kill its host in order to maximize the amount of resources it can gather. In either case, the antagonist is gaining from the experience and the victim is being harmed. |
Competition (-/-)
Wherever more organisms exist than can possibly survive, competition exists and organisms will compete to acquire as many resources as possible and ultimately survive. Because resources are limited in an ecosystem, the more of the resource acquired by one organism, the less there is available for another, it is a zero sum game. in this way, competition can be considered a "lose-lose" scenario as both sides suffer from the competition and will be unable to acquire the full amount of resources they need. Resources for organisms include physical space, food and even mates for reproduction. There simply is not enough of a resource to go around and both species suffer as a result.
Some ecologists will argue that competition is actually a form of amensalism because species do not always compete on an evenly playing field, one is usually better adapted the the other(s). In this case, the stronger species will be largely unaffected by the presence of the weaker one, acquiring resources with relative ease. However, the weaker one will be hurt by the presence of the stronger.
Wherever more organisms exist than can possibly survive, competition exists and organisms will compete to acquire as many resources as possible and ultimately survive. Because resources are limited in an ecosystem, the more of the resource acquired by one organism, the less there is available for another, it is a zero sum game. in this way, competition can be considered a "lose-lose" scenario as both sides suffer from the competition and will be unable to acquire the full amount of resources they need. Resources for organisms include physical space, food and even mates for reproduction. There simply is not enough of a resource to go around and both species suffer as a result.
Some ecologists will argue that competition is actually a form of amensalism because species do not always compete on an evenly playing field, one is usually better adapted the the other(s). In this case, the stronger species will be largely unaffected by the presence of the weaker one, acquiring resources with relative ease. However, the weaker one will be hurt by the presence of the stronger.
Competition can occur in two major forms depending on what kinds of organisms are involved. Interspecific competition occurs when two or more different species have overlapping niches and fight for the same resource as is the case with lions and hyeans. Both species are top carnivores that will literally fight for access to food. Species that compete against one another are subject to the competitive exclusion principle which says that two or more competing species cannot coexist.
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According to this principle, species that compete are left with two possible outcomes: the resource can be partitioned or the better adapted species will outcompete the other(s) and drive them to extinction. In resource partioning, the species with overlapping niches will divide up a resource based on differences in their morphology (physical design) or their behavior. This can be seen in the figure to the right. Species 1 and 2 are both competing for the same resource an undisclosed type of seed. In this scenario, Species 1 has been identified as the stronger competitor and Species 2 is the weaker. Species 1 consumes small to medium sized seeds, while Species 2 consumes medium to large sized seeds. The two species only compete for the medium sized seeds, which Species 1 is better adapted as collecting. Rather than continue competing for the medium sized seeds fruitlessly (no pun intended), Species 2 will shift its behavior and consume only larger sized seeds. The interspecific competition ends will both species intact and the resource effectively partitioned.
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The second way interspecific competition can end is through the collapse of the weaker species. This is illustrated in the example to the left where two species of Paramecium are grown in the same environment. In isolation, Paramecium aurelia and Paramecium caudatum thrive and exhibit S shaped curves. However, when grown together, Paramecium caudatum proves to be the weaker of the two species and is unable to effectively compete against Paramecium aurelia. The Paramecium caudatum population quickly plummets while Paramecium aurelia quickly establishes dominance. However, note the difference in growth rate between the Paramecium aurelia grown in isolation vs. in the presence of Paramecium caudatum. Paramecium aurelia grows much more slowly during competition and its maximum population density is slightly lower than when it was grown in isolation. Clearly in this scenario both species have been negatively impacted by competition.
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The second type of competition is intraspecific comepetition. Intraspecific competition exists within a species and is centered around reproductive rights during breeding, something not seen in interspecific competition. Intraspecific competition is typified by males competing for a female mate. This type of competition does not produce natural selection like we saw in interspecific competition, but rather sexual selection and usually results in sexual dimorphism. Sexual dimorphism occurs when males and females of the same species become morphologically (physical) distinct from one another. This can be seen in song birds, like the American cardinal to the right, where males have bright, colorful plumage compared to the drab plumage of females.
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Differences in size between the genders is another example of sexual dimorphism with males usually (but not always) larger than females. We will explore both natural selection and sexual selection further in our unit of evolution.
Interspecific vs. Intraspecific Competition
As you can imagine, competition is going to be hugely important in driving the change and development of species. But not all forms of competition are equal; generally speaking intraspecific competition will be much more influential than interspecific competition. This is because greater overlap in the niches of competing organisms results in more intensive competition. For example, interspecific competition is centered around competition between individuals of different species. By the definition of a species, these organisms cannot be identical, there are critical differences in their morphology and behavior that differentiates them. This results in more unique niches which provides more flexibility when species compete, increasing the likelihood of resource partitioning compared to extinction of the weaker competitor. In contrast, members of the same species will inherently be very similar and occupy nearly identical if not identical niches. This stimulate more fervent competition which leads to greater changes in a species.
As you can imagine, competition is going to be hugely important in driving the change and development of species. But not all forms of competition are equal; generally speaking intraspecific competition will be much more influential than interspecific competition. This is because greater overlap in the niches of competing organisms results in more intensive competition. For example, interspecific competition is centered around competition between individuals of different species. By the definition of a species, these organisms cannot be identical, there are critical differences in their morphology and behavior that differentiates them. This results in more unique niches which provides more flexibility when species compete, increasing the likelihood of resource partitioning compared to extinction of the weaker competitor. In contrast, members of the same species will inherently be very similar and occupy nearly identical if not identical niches. This stimulate more fervent competition which leads to greater changes in a species.
Ecological Disturbance
Ecological disturbance
No ecosystem is ever static; all ecosystems experience change. These changes are known as ecological disturbances, events that are physical, chemical or biological in origin and result in a change of size for a population or the composition of an ecosystem. Disturbances are evaluated based on their frequency and intensity or severity which tend to be negatively correlated: more intense disturbances happen less often while less intense ones happen more frequently. An ecological disturbance can be natural like a forest fire or anthropogenic like with pollution or climate change. But no matter how it happens, a disturbance alters the biodiversity of an ecosystem. |
Biodiversity
Biodiversity describes the total diversity of an ecosystem, the sum of all species in the community. Higher biodiversity within an ecosystem translates to greater adaptability and survivorship and is a measure of global stability. High biodiversity require high species richness and evenness. Species richness refers to the total number of unique species living in an environment, while species evenness refers to how equally represented each species is in the environment. The graphic to the left demonstrate this: both communities have equal richness, but Community 2 is more even, giving it a higher biodiversity. |
Evaluating Disturbances
Just as disturbances are evaluated based on their severity and frequency, ecosystems can be evaluated based on their ability to respond to these disturbances through resistance and resilience. Ecosystems that are capable of enduring a disturbance with little damage or change in composition are said to be highly resistant. On the other hand, ecosystems that experience a disturbance and are able to rapidly recover as said to be highly resilient. This is summarized in the graphic to the left. The blue line represent a resistant ecosystem that experiences a disturbance with little if any alteration of the ecosystem. The dotted red line represents a resilient ecosystem, that changes dramatically in the face of a disturbance but rapidly recovers and returns to its original state afterwards. It is important to note that the resistance or resilience of an ecosystem are not absolute, a strong enough disturbance can wipe out an ecosystem completely or drastically alter it. |
Fire Ecology: An Example of Resistance and Resilience
Fire is one of the most common forms of ecological disturbance and is experienced by almost every terrestrial ecosystem especially grasslands and shrublands. While animals can flea from approaching wildfires, plants have no choice but to stand their ground. Luckily, plants have developed a number of strategies and defenses to deal with fires. A common defense against fire is the lignotuber, woody swellings that form at the base of plants including trees. The extra mass insulates vital tissues like meristems, effectively plant stem cells, protecting them from the heat of the fire. |
Serotiny is another adaptive trait of plants and occurs when plants disperse seeds only after a fire had moved through an area. All conifers including pine trees house their seeds on the scales of their cones, fire adapted, serotinous pine trees will only open their cones when they experience heat and smoke, signaling a recent fire. A few hours after the fire, the cones open and disperse the seeds to take advantage of the new resources and space provided by the fire. Lignotubers are examples of resistance since they minimize the impact of the wildfire, while serotiny is an example of resilience because it helps the ecosystem recover.
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Something we should note though is that fire is not really a "bad thing for an ecosystem and in fact provides a number of important services for the ecosystem. Not unlike decomposers, fire helps to recycle nutrients and open up new space for growth. As trees and plant grow, they often lose branches, twigs and leaves that can choke the ecosystem floor. Fire helps to clear away this debris and free up resources like nitrogen and phosphorous that can enrich soils and foster new plant growth. Moderate occurrences of fire also help to prevent less frequent but much more severe fires that can truly devastate ecosystems even ones that are resistance/resilient. These low intensity fires clear away debris that can build up in the absence of fire to unsafe levels and fuel severe fires later on.
Intermediate Disturbance Hypothesis
At this point you might be thinking to yourself that all disturbances are bad, but this is not the case. Obviously too much disturbance and destroy communities, but too little disturbance encourages only a few species to dominate. This results in low biodiversity and low overall health. The truth is that moderate amounts of disturbance are actually beneficial to an ecosystem an idea known as the intermediate disturbance hypothesis. The graphic to the left demonstrates this what this looks like, species richness is highest under moderate disturbance and lowest under very low and high rates of disturbance. |
Ecological Succession
Ecological succession is an observable, predictable process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire), or even millions of years after a mass extinction. Succession typically focuses on the plant community's structure as this is what controls the composition of the animal community. Succession begins with some catastrophic event (the disturbance) that wipes out life in an ecosystem. The event could be a wildfire, volcanic eruption or some other disturbance. In the absence of a disturbance, the plant community will become dominated by progressively taller plants and smaller plant species will die out or become reduced in prevalence.
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There are two main types of succession: primary and secondary succession. These two processes differ only in how they begin, the plant community will still develop in the same way.
Primary Succession
Primary succession begins with a catastrophic ecologic disturbance (like a volcanic eruption) that wipes out all plant and animal life AND even removes the soil from an ecosystem. Primary succession therefore begins with only exposed, bare rock. Pioneer species are the first to recolonize an ecosystem following a disturbance. A classic example of pioneer species in primary succession would be lichen, algae living within fungi. Lichen are highly adaptable and unlike most most plants, |
they do not require soil to grow in. Instead lichen can grow on other plants such as trees and even bare rock. In primary succession pioneer lichen will grow directly on the exposed rock and begin to lay the foundation for soil development which will enable other plants like grasses to invade. Lichen come in a variety of shapes, size, colors and textures and seen in the image to the left. The diagram above illustrates primary succession.
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Secondary Succession
As with primary succession, secondary succession begins with an ecological disturbance that wipes out the plant and animal life BUT leaves the soil intact and often enriched. Often secondary succession begins with a forest fire that "resets" the ecosystem and allows the process of succession to begin again. Because the soil remains intact, secondary succession generally progresses faster than primary succession but still takes years to occur. |
Pioneer species will quickly move in to establish themselves following a disturbance in secondary succession just as they did in primary succession. However, the pioneer species are usually fast growing trees like aspen or cherry as opposed to lichen. These trees will grow rapidly and do better in full sun than other plant species, before being replaced by other species. In the absence of further disturbance, the ecosystem will eventually succeed into a climax community, dominated by tall, but slow growing deciduous trees. This can take hundreds of years to occur.
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Prescribed Fires
As you know, many ecosystems are adapted to fire especially grasslands and shrublands. These habitats require frequent disturbances to be maintained, otherwise they will succeed into other ecosystem types. In this way, fire arrests the development of ecosystems through the process of succession, effectively preventing grasslands and shrublands from succeeding into forest communities. But for a long time, fire of any type was viewed as a inherently destructive and was indiscriminately suppressed. Ironically, this strategy only made fires worse. Less intense fires were put out, allowing fuel loads (sticks, branches and leaves) to build to catastrophically high levels so that when a fire did eventually start, it was highly intense and out of control. These intense wildfires devastated ecosystems that were used to more moderate wildfires.
As you know, many ecosystems are adapted to fire especially grasslands and shrublands. These habitats require frequent disturbances to be maintained, otherwise they will succeed into other ecosystem types. In this way, fire arrests the development of ecosystems through the process of succession, effectively preventing grasslands and shrublands from succeeding into forest communities. But for a long time, fire of any type was viewed as a inherently destructive and was indiscriminately suppressed. Ironically, this strategy only made fires worse. Less intense fires were put out, allowing fuel loads (sticks, branches and leaves) to build to catastrophically high levels so that when a fire did eventually start, it was highly intense and out of control. These intense wildfires devastated ecosystems that were used to more moderate wildfires.
Today, wildlife officials realize that fire is an important part of ecology. But fire is still dangerous and needs to be treated with care from a public safety perspective. To compromise, prescribed fires are often implemented to control the fire, preventing them from harming people, while also allowing fire to play its role for ecosystems. In a prescribed burn, officials carefully design a fire plan and intentionally light a fire in the ecosystem are controlled conditions. Different variables like wind, relative humidity and rainfall are considered to better control the spread of the flames. This allows the fire to remove fuel loads, clear debris and recycle nutrients while also keeping the public safe from harm.
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