Ecosystems
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An ecosystem refers to a community of organisms that interact both among themselves and with elements of the non-living environment. Within an ecosystem, living entities like plants and animals constitute the biotic factors, while non-living aspects such as rainfall, temperature, and sunlight comprise the abiotic factors.
These ecosystems can vary greatly in size, ranging from expansive areas like entire rainforests to minute spaces like individual tidal pools. Organisms within an ecosystem are interconnected, engaging in both indirect and direct interactions. Indirect interactions might involve competition for the same resource, while direct interactions include instances like predation. Food chains, food webs and impact charts illustrate the relationships between the biotic and abiotic components of the ecosystem. |
A (a) tidal pool ecosystem in Matinicus Island, Maine, is a small ecosystem, while the (b) Amazon rainforest in Brazil is a large ecosystem. (credit a: modification of work by Jim Kuhn; credit b: modification of work by Ivan Mlinaric)
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Food Chains |
Two Examples of Food Chains:
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A food chain is a line-up of who eats whom in nature. It starts with plants or tiny organisms that make their own food (producers/autotrophs). Then come the plant-eaters (primary consumers/herbivores), followed by the meat-eaters (secondary consumers/carnivores). There might be even higher-level meat-eaters (tertiary consumers), and so on, until you reach the top predators (apex consumers).
Trophic levels contain animals, or groups of animals, that all get energy from the same source. Trophic levels are often divided into primary producers (plants and algae) which get energy from the sun, primary consumers (herbivores) which get energy by eating producers, secondary consumers (carnivores that eat herbivores) which get energy by eating primary consumers, and tertiary consumers (carnivores that eat other carnivores) which get energy from secondary consumers, and so on. But there's a limit to how many trophic levels a food chain can have. Energy gets "lost" as it moves up the chain and at each level of the food chain. So, there's only so much energy left for each level. Eventually, there's not enough energy to support more animals higher up. In food chains, the arrows point from the food to the thing that eats it. In the examples to the right, the arrows go from the insects (food) to the frogs because the insects are eaten by the frogs. Food Webs |
Image credit: LadyofHats, CC0, via Wikimedia Commons
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Using food chains to describe most ecosystems poses some problems. Even when organisms are categorized into trophic levels, some can feed across multiple levels, while others are both prey and predators to various species. Since food chains illustrate only one interaction per level, they oversimplify the flow of matter and energy through the ecosystem.
A more comprehensive model, known as a food web, addresses these limitations. Food webs depict all organisms that either consume or are consumed by others in the ecosystem. They highlight multiple trophic connections between species, often spanning across various trophic levels. Food webs provide a more accurate representation of the intricate interactions and the movement of matter and energy in ecosystems.
A more comprehensive model, known as a food web, addresses these limitations. Food webs depict all organisms that either consume or are consumed by others in the ecosystem. They highlight multiple trophic connections between species, often spanning across various trophic levels. Food webs provide a more accurate representation of the intricate interactions and the movement of matter and energy in ecosystems.
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Example of a food web:
Image credit: Siyavula Education, Savanna food web. Creative Commons by 2.0 DEED
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Impact Charts
Impact charts are models that show how components of the ecosystem impact other components of the ecosystem. Impacts can be direct or indirect, positive or negative.
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When drawing an impact chart, the arrows go from the thing doing the impact, to the thing that is impacted. For example, imagine a simple food chain where the grass is eaten by insects which are eaten by frogs. If a disease were to reduce the number of frogs then the disease would have a direct impact on the frog population. As a result, the population of frogs decreases there is a direct impact on the number of insects as not as many insects will be eaten. Direct impacts are represented by a solid arrow.
Using the same example we can also see an indirect impact. The disease had an indirect impact on the number of insects. The disease wasn't directly impacting and making the insects sick, but the population of insects was impacted through the frogs. Indirect impacts are like a chain reaction. They are represented by a dashed arrow. When determining if the impact is positive or negative you have to determine the direction of change. If both things are increasing or both things are decreasing then the the impact is positive. If the direction of change is in opposite directions (one increases while the other decreases) then the impact is negative. |
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Positive impacts move in the same direction. Negative impacts move in opposite directions. Looking at out disease, frog, insect example, as the amount of disease increases the frog population decreases. As these move in opposite directions (one increases while the other decreases) the impact is negative. However, as the disease increases the amount of insects also increase. These impacts move in the same direction, both increasing, so the impact is positive.
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Moving Matter and Energy through an Ecosystem
Image created with BioRender
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Ecosystems are made of many organisms that interact with each other and the environment to get energy or molecules.
Energy Flows: Energy enters the ecosystem as light energy from the sun. Plants then use the light energy to make chemical energy, in the form of carbon-carbon and carbon-hydrogen bonds in glucose. When the plants need to use the energy they perform cellular respiration. Likewise, when organisms eat plants the organisms are able to get chemical energy from the biomolecules in the plant. Organisms that eat plants can either use the chemical energy through the process of cellular respiration, or they themselves are eaten. When plants or animals die, decomposers break down the organic matter in their bodies to access the chemical energy stored in biomolecules. Eventually, all the energy is converted into heat energy, the end result of cellular respiration. Energy flows through an ecosystem. It starts as sunlight energy and ultimately ends up as heat energy. Matter Cycles: Photosynthesis is also a driving force in the movement of molecules in the ecosystem. During photosynthesis, carbon dioxide and water enter the plant. These molecules are converted into glucose, which is stored in the plant, and oxygen, which leaves the plant. Eating, deficating, and urinating, are other processes that move matter around the ecosytem. But the other main process that moves molecules through the ecosystem is cellular respiration. During cellular respiration glucose in the plant/animal is converted into water and carbon dioxide. The water and carbon dioxide then leave the organism, going back into the environment. Unlike energy, matter moves through an ecosystem in a cycle. Matter is constantly recycled. |
Pyramids of Biomass/Energy
Ecosystem structure can be shown using pyramids of biomass/energy.
Biomass pyramids measure the organic material (biomolecules) at different levels of the food chain. To find the biomass, you dry out organisms to remove the water. This leaves just the organic molecules as water is not organic.
Energy pyramids illustrate the flow of energy through trophic levels. They're always largest on the bottom and get smaller as you move up the food chain.
Energy and biomass pyramids are linked because energy is stored in biomolecules. All biomolecules have high energy C-C and C-H bonds. This means that if there are more biomolecules at a trophic level, then there will be more energy at that trophic level as well.
In general, only about 10% of energy / biomolecules are passed from one level to the next. Most of the biomolecules in each level are used for cellular respiration. This releases the energy from the biomolecules as heat that goes into the environment. Even when a large numbers of biomolecules are stored in the bodies of organisms, not all the biomolecules move to the next trophic level when that organism is eaten. When plant or animal is eaten by something higher on the food chain, not all parts of its body are consumed. When a zebra eats grass they don't eat the roots or even all the blades of grass. The biomolecules that are in the uneaten parts of the organism thus do not move up to the next trophic level.
Biomass pyramids measure the organic material (biomolecules) at different levels of the food chain. To find the biomass, you dry out organisms to remove the water. This leaves just the organic molecules as water is not organic.
Energy pyramids illustrate the flow of energy through trophic levels. They're always largest on the bottom and get smaller as you move up the food chain.
Energy and biomass pyramids are linked because energy is stored in biomolecules. All biomolecules have high energy C-C and C-H bonds. This means that if there are more biomolecules at a trophic level, then there will be more energy at that trophic level as well.
In general, only about 10% of energy / biomolecules are passed from one level to the next. Most of the biomolecules in each level are used for cellular respiration. This releases the energy from the biomolecules as heat that goes into the environment. Even when a large numbers of biomolecules are stored in the bodies of organisms, not all the biomolecules move to the next trophic level when that organism is eaten. When plant or animal is eaten by something higher on the food chain, not all parts of its body are consumed. When a zebra eats grass they don't eat the roots or even all the blades of grass. The biomolecules that are in the uneaten parts of the organism thus do not move up to the next trophic level.
In pyramids of biomass or energy you should include all the organisms at at trophic level. If there are 4 different species of producer you would include all of them in the bottom level.
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Top-down and Bottom-Up Regulation
Ecosystems can be regulated from the top-down or from the bottom-up. Top down regulation means that the things at the top of the food chain are impacting the number of organisms lower down on the food chain. On the other hand, bottom up regulation occurs when the amount of producers is controlling the number of organisms at the top of the food chain.
In general, bottom-up regulation occurs when there are limits to abiotic factors. For example, a food chain could be regulated from the bottom-up because the grass population is impacted by the amount of rainfall and how much space there is for grass to grow. In this scenario, there is a limit to how much grass there is. Organisms higher in the food chain, such as cows, rely on the grass for food. There is only enough grass to support a certain population of cows. The top of this food chain (cows) are controlled by the bottom of the food chain (grass) therefore, this is an example of bottom-up regulation.
In top-down regulation, the top of the food chain is controlling the bottom of the food chain. For the most part, top-down regulation occurs when limits are imposed by biotic factors. For example, in a grass --> cow --> human food chain the humans are controlling how many cows there are. The amount of grass is then more impacted by the number of cows than by the amount of water or space. Because the top of the food chain is controlling the organisms below it, this example shows top-down regulation.
In general, bottom-up regulation occurs when there are limits to abiotic factors. For example, a food chain could be regulated from the bottom-up because the grass population is impacted by the amount of rainfall and how much space there is for grass to grow. In this scenario, there is a limit to how much grass there is. Organisms higher in the food chain, such as cows, rely on the grass for food. There is only enough grass to support a certain population of cows. The top of this food chain (cows) are controlled by the bottom of the food chain (grass) therefore, this is an example of bottom-up regulation.
In top-down regulation, the top of the food chain is controlling the bottom of the food chain. For the most part, top-down regulation occurs when limits are imposed by biotic factors. For example, in a grass --> cow --> human food chain the humans are controlling how many cows there are. The amount of grass is then more impacted by the number of cows than by the amount of water or space. Because the top of the food chain is controlling the organisms below it, this example shows top-down regulation.