Trophic Levels and Energy Pyramids
Why are there thousands of grasshoppers on a prairie but only a few hawks? A hawk needs a lot of food, and that food is already "used up" several steps down the chain. In this lesson you will follow energy from the Sun through an ecosystem and see why top predators are always rare.
Food chains and trophic levels
Every living thing needs energy. Producers (plants, algae, and some bacteria) capture energy from sunlight and store it in food through photosynthesis. Everything else must eat to get energy.
A food chain shows who eats whom. Each step is a trophic level (a feeding level).
Consumers are also grouped by diet: herbivores eat plants, carnivores eat animals, and omnivores eat both. A single animal can sit at different levels in different chains.
In a grassland ecosystem in the Great Plains, grass is eaten by grasshoppers, grasshoppers are eaten by prairie dogs, and prairie dogs are eaten by hawks. Name the…
In a grassland ecosystem in the Great Plains, grass is eaten by grasshoppers, grasshoppers are eaten by prairie dogs, and prairie dogs are eaten by hawks. Name the trophic level (producer, primary consumer, secondary consumer, or tertiary consumer) of each of the four organisms.
Emma draws an arrow from a rabbit to a coyote in a food chain diagram. What does the arrow represent? Say whether it points toward the organism that gives the energy or…
Emma draws an arrow from a rabbit to a coyote in a food chain diagram. What does the arrow represent? Say whether it points toward the organism that gives the energy or the organism that receives it.
Draw or describe a food chain with four organisms found in the Chesapeake Bay. Label each trophic level and use arrows to show the direction of energy flow. Then state…
Draw or describe a food chain with four organisms found in the Chesapeake Bay. Label each trophic level and use arrows to show the direction of energy flow. Then state where the original source of energy for the whole chain comes from.
Energy pyramids and the 10% rule
Where does the energy go? Not all of it passes up the chain. An organism uses most of the energy it takes in to move, grow, stay warm, and carry out life processes. Much of that energy leaves as thermal energy (heat) into the environment. Some food is never eaten (roots, bones, fur), and some is passed out as waste.
So only a small part is stored in the organism's body, and that stored part is all the next level can eat. A good rule of thumb: about 10% of the energy at one level reaches the next level. About 90% is lost.
An energy pyramid shows the energy available at each trophic level. Producers are always at the bottom because they bring energy into the ecosystem from the Sun. Each level above is smaller.
Because each level has only about a tenth of the energy below it, you can find the energy at any level by multiplying by 0.10 each step.
Define the term energy pyramid* in your own words. Then state which trophic level is always found at the bottom of an energy pyramid and explain in one sentence why.
Define the term energy pyramid in your own words. Then state which trophic level is always found at the bottom of an energy pyramid and explain in one sentence why.
An energy pyramid for a Florida salt marsh shows the following energy available at each level: | Trophic level | Energy available (kcal) | |---|---| | Cordgrass…
An energy pyramid for a Florida salt marsh shows the following energy available at each level:
| Trophic level | Energy available (kcal) |
|---|---|
| Cordgrass (producers) | 10,000 |
| Marsh snails (primary consumers) | 1,000 |
| Blue crabs (secondary consumers) | 100 |
| Raccoons (tertiary consumers) | 10 |
(a) What percentage of the energy at one level is available to the next level? (b) Name two ways energy from the cordgrass is "lost" before reaching the snails.
A producer level in a forest contains 50,000 kcal of energy. Assume that only 10% of the energy is passed to each next level. Calculate the energy available to the…
A producer level in a forest contains 50,000 kcal of energy. Assume that only 10% of the energy is passed to each next level. Calculate the energy available to the primary consumers and to the secondary consumers. Show each step.
Why top predators are rare
Now we can answer the opening question. Each step up loses about 90% of the energy, so the top level has very little left. Little energy supports few organisms, so there are fewer and fewer individuals as you go up. Most ecosystems have only 3 to 5 trophic levels, because after that there is too little energy left to feed another one.
Food chains also depend on each other. If grasshoppers drop sharply, mice have less food, then snakes, then hawks. Fewer hawks result from a lower energy supply below them.
Mateo notices that a lake ecosystem supports thousands of algae and insects, a few hundred fish, but only a handful of large predatory birds. Use the idea of energy…
Mateo notices that a lake ecosystem supports thousands of algae and insects, a few hundred fish, but only a handful of large predatory birds. Use the idea of energy transfer between trophic levels to explain this pattern. In your answer, include the approximate percentage of energy passed on, at least two ways energy is lost, and what this means for the maximum number of trophic levels an ecosystem can support.
A wildlife manager in Yellowstone studies two food chains: Chain A: grass → elk → wolf Chain B: grass → grasshopper → mouse → snake → hawk Assume 10% of energy passes to…
A wildlife manager in Yellowstone studies two food chains:
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Chain A: grass → elk → wolf
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Chain B: grass → grasshopper → mouse → snake → hawk
Assume 10% of energy passes to each level and the grass in both chains starts with 100,000 kcal. (a) Calculate the energy available to the top consumer in each chain. (b) Explain why a region can usually support more individuals of the top consumer in Chain A than in Chain B, and predict what would happen to the hawk population if the grasshopper population dropped sharply.
Recycling matter
Energy flows in one direction: from the Sun, through the food chain, and out as thermal energy. Matter is different, because it is recycled. Atoms of carbon, nitrogen, and other elements are used again and again.
Decomposers (fungi and bacteria) break down dead plants and animals and wastes. They return nutrients to the soil and water, where producers take them up again. Without decomposers, nutrients would stay locked in dead bodies and producers would run out. The whole biosphere (all the places on Earth where life exists) depends on this cycle.