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Lab 6
ENERGY FLOW IN ECOSYSTEMS: by Drs. Edith C. Marsh-Matthews and Michael T. Dixon |
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The past two weeks we have looked at factors that affect the size of populations including birth and death rates, carrying capacity and the interactions between predators and prey. This week we will look at the interaction between predators and prey from a different angle.
What Do You Eat?
All living organisms require energy to sustain metabolism, to grow and to reproduce. Some organisms (including most plants and some protists and bacteria) have the ability to capture solar energy and convert it into chemical bonds. This is photosynthesis. These chemical bonds make sugars out of carbon dioxide and water. Photosynthetic organisms obtain energy directly from the sun. We call these photosynthesizers "producers" because they make the carbohydrates (chains or polymers of sugars) on which all of the rest of life relies. They can also be called "autotrophs" which means self-feeders because they make their own carbohydrates and don't have to eat them. In a very few places on Earth we find bacteria that can create sugars from chemical sources instead of the sun. We will ignore these in this lab.
All other organisms, the non-producers, may be termed "consumers" because they must consume the carbohydrates that they can't make. They are also called "heterotrophs" which means they feed on others.
Consumers that feed on plants (or other producers) may be thought of as "primary consumers" because they feed directly on the producers. Most of these are also "herbivores," or plant-eaters. An animal that feeds on other animals is a "secondary consumer" (a consumer feeding on a consumer) or "carnivore," meat-eater. The terms "tertiary consumer" and "quaternary consumer" are also used sometimes for carnivores that feed on other carnivores.
It is often helpful to think of these as layers sitting on top of each other. In the pyramid at the top of this page, plants (producers) support the populations of rabbits (primary consumers) which are food for the snakes (secondary consumers) which get eaten by hawks (tertiary consumers). We call these layers "trophic levels" (troph refers to food or feeding as in heterotroph). Most ecosystems have at least three trophic levels:
- Producers that get their energy from the sun,
- Primary consumers (herbivores) that eat the primary producers, and
- Secondary consumers (carnivores) that eat the primary consumers.
Finally, the nutrients and waste products from all the trophic levels will be consumed by detritivores (detritus is non-living organic material including dead organisms and droppings). This helps to recycle the organic material.
Points for the lab will be earned from a pre-lab quiz and lab activity.
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