Life Science · Unit 4: Matter and energy in ecosystems
Food Webs and Energy
Why can the Chesapeake Bay hold large schools of small fish but only a limited number of the ospreys that eat them? You sort the living and nonliving parts of an ecosystem, build an energy pyramid in a simulation, and read real energy numbers from a Florida spring. Afterwards you can draw a food web with the arrows the right way and explain why each level has less energy.
Where this lesson sits
- Virginia SOL LS.5 b
- Virginia SOL LS.5
- Practices LS.1 c, LS.1 d, LS.1 e
Essential question: Why does each step up a food web, from plankton to fish to ospreys, have less energy to live on?
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Life Science (grade 7) LS.5 b, LS.5 You are here
This lesson
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Biology
Preview it at high school level
In Biology you will give the levels new names: primary, secondary and tertiary consumers. Each feeding step is called a trophic level. You will also build biomass pyramids, which stack the mass of living things at each level instead of the energy. You will learn that a biomass pyramid can sometimes be upside down while the energy pyramid stays right side up. Then you will work through a real lake budget from 1942, Raymond Lindeman’s Cedar Bog Lake in Minnesota. Its numbers show that the one-tenth figure is a rough rule of thumb, not a fixed law.
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Biology II Biology II: Ecology, EC.3; Advanced Topics, BIOII.6 a-c
Preview where it goes next
In Virginia’s Biology II courses (Ecology, EC.3, and Advanced Survey, BIOII.6 a-c), energy flow is tied to the laws of thermodynamics. You measure gross primary productivity, how fast producers capture energy, and net primary productivity, what is left after their own respiration. You calculate ecological efficiency from matching measurements at two levels. You meet organisms that get energy from chemicals, or that both make and eat food, and you follow carbon, nitrogen and phosphorus through an ecosystem.
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College
Watch first Less Energy at Every Step · 2:39
Every spring and summer, ospreys nest on channel markers and dead trees along the Chesapeake Bay. They plunge feet-first into the water to catch fish, and in parts of the Bay one fish they often bring home is the Atlantic menhaden.
Menhaden are small, oily fish. They swim in large schools and filter plankton, tiny living things that drift in the water. So what limits how many ospreys the Bay’s fish can feed? First, decide what you think now.
What do you think? Decide before you read.
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Scientists disagree: Matter such as carbon and water is used again and again. Energy is not. It comes in as sunlight, passes from producers to consumers, and leaves as heat at every step. Most ecosystems need a steady supply of new sunlight.
What scientists think
Scientists disagree: Matter such as carbon and water is used again and again. Energy is not. It comes in as sunlight, passes from producers to consumers, and leaves as heat at every step. Most ecosystems need a steady supply of new sunlight.
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Scientists disagree: A food chain is a simple model of one path. In a real ecosystem, many chains join into a food web. Menhaden are eaten by ospreys and by larger fish, and they eat plankton, so a change in menhaden can reach many other living things.
What scientists think
Scientists disagree: A food chain is a simple model of one path. In a real ecosystem, many chains join into a food web. Menhaden are eaten by ospreys and by larger fish, and they eat plankton, so a change in menhaden can reach many other living things.
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Scientists disagree: Predators matter, but they depend on others. Producers such as plants and plankton capture the energy that every other level uses. Decomposers such as bacteria and fungi break down dead matter and return nutrients. Each role is needed.
What scientists think
Scientists disagree: Predators matter, but they depend on others. Producers such as plants and plankton capture the energy that every other level uses. Decomposers such as bacteria and fungi break down dead matter and return nutrients. Each role is needed.
Explore: build an energy pyramid
In this simulation you build an energy pyramid for a Bay food chain. You set the sunlight the producers capture. You also set the percent of energy passed up at each step (5 to 20) and the number of levels (3 or 4). Bars show the energy at each level. Red arrows show heat leaving. A counter shows how many ospreys the top bar could feed, using a made-up amount of energy for each osprey.
- Ospreys the top bar could feed
- 10
- Share of the producers’ energy at the top
- 0.1 percent
- The arithmetic (units per square meter per year)
- 10,000 × 0.10 × 0.10 × 0.10 = 10
Each step passes 10 percent up.
In this sketch, every step passes the same percent, each level is one bar and decomposers are not drawn. Amounts are made-up units, and 1 unit feeds 1 osprey (also made up; the count is rounded down to whole ospreys). Wide bars are drawn to scale; any bar under 3 pixels is drawn 3 pixels wide so you can see it, so very thin bars look equal but are not. Red arrows show that much of the energy leaves as heat (all the same size, not to scale); in real food chains some energy not passed up also goes to decomposers.
Rule to keep: each step passes only about one-tenth up, and much of the rest is given off as heat, so the top level has the least energy. Energy limits top predators but gives no exact count.
Try this.
- Set the percent to 10 and use 4 levels. Predict what share of the producers’ energy reaches the top bar. Then run it.
- Predict how the osprey counter will change if you lower the percent to 5. Then test it.
- Double the sunlight. Predict whether the osprey counter doubles too. Then check.
Explain: living and nonliving parts of an ecosystem
- ecosystem
- biotic factor
- abiotic factor
- producer
- first-order consumer
- second-order consumer
- third-order consumer
- decomposer
- food web
- energy pyramid
An ecosystem is all the living things in an area plus their nonliving surroundings. The Chesapeake Bay, a pond and a forest are all ecosystems.
A biotic factor is any living or once-living part of an ecosystem. That includes organisms, their parts and their wastes:
- A menhaden or an osprey.
- A fallen leaf or a fish scale.
- Osprey droppings.
- Parasites and diseases.
An abiotic factor is a nonliving part of an ecosystem. Abiotic factors decide which kinds of organisms can live in a place, and how many. Examples:
- Temperature.
- How much rain falls in a year.
- The pH of a lake or pond (how acidic or basic the water is).
- Minerals in the soil.
- How much light reaches different depths of the ocean.
Light fades as you go deeper in water, so deep down plankton capture little energy. A change in temperature or rainfall can also change what lives in a place.
Explain: food chains and food webs
Nearly every food web you will meet starts with a producer, an organism that captures energy from sunlight and stores it in food. In the open water of the Bay, the main producers are phytoplankton, tiny drifting algae. You saw how they capture energy in Photosynthesis Feeds the World.
Consumers get energy by eating other organisms:
- A first-order consumer eats producers. Many zooplankton, tiny drifting animals, eat phytoplankton.
- A second-order consumer eats first-order consumers.
- A third-order consumer eats second-order consumers.
A decomposer, such as many bacteria and fungi, breaks down dead matter and wastes from every level. You will study them in Producers, Consumers and Decomposers.
A food chain is a straight-line model of one path that energy takes. The arrows point from the food to the eater. They show which way the energy moves:
sunlight → phytoplankton → zooplankton → menhaden → osprey
In this chain, the osprey is a third-order consumer. But menhaden also eat phytoplankton directly. In the chain phytoplankton → menhaden → osprey, the osprey is a second-order consumer.
A food web shows many food chains linked together. Larger fish eat menhaden too. If a disease killed many menhaden, the ospreys, the larger fish and the plankton could all change. A web shows those links.
- phytoplankton
- zooplankton
- menhaden
- osprey
- larger fish
- decomposers
- phytoplankton
- blank
- blank
- osprey
- blank
- blank
Word bank: decomposers, larger fish, menhaden, zooplankton
- blank
- blank
- blank
- blank
- blank
- blank
Check your labels
- phytoplankton
- zooplankton
- menhaden
- osprey
- larger fish
- decomposers
Explain: why each level has less energy
Stack the energy of each level as a block, with producers on the bottom. Each block is smaller than the one below. This stack is an energy pyramid, a model of how much energy is available at each level.
Why does each level have less? Most of the energy in the plankton a menhaden eats does not become new menhaden.
- Much of it is given off as thermal energy (heat) during the fish’s life processes. These include swimming and breaking down sugar in respiration. You met respiration in Releasing Energy from Food.
- Some of the food passes through the fish without being digested. Decomposers can use that energy.
- Only the rest is built into the fish’s body, where an osprey can get it.
No energy change is perfect. Energy is not used up. It changes form, and much becomes heat that spreads into the water and air. No living thing can use that heat to make food.
As a rough rule, about one-tenth of the energy in one level is available to the next. Real measurements vary from place to place. Here is a model with made-up numbers and exactly one-tenth per step:
| Level | Energy (units per square meter per year) |
|---|---|
| Producers | 10,000 |
| First-order consumers | 1,000 |
| Second-order consumers | 100 |
| Third-order consumers | 10 |
Only 10 of the 10,000 units reach the top. That helps explain why a place can feed fewer top predators than small fish. But an energy pyramid is not a head count. One large tree can feed many insects. Energy limits top predators but does not give an exact number.
In most ecosystems, energy flows one way: in as sunlight, out as heat. Matter is different. Decomposers return carbon and nutrients that producers use again, as you will see in The Carbon and Water Cycles.
Elaborate: real energy numbers from a Florida spring
In the 1950s, the ecologist Howard T. Odum studied the clear water at Silver Springs, Florida. He estimated the energy flowing into each level for each square meter of the spring in a year. His unit was the kilocalorie. One kilocalorie is one Calorie on a food label, about 4,184 joules.
| Level at Silver Springs | Energy (kilocalories per square meter per year) | Percent of the level below |
|---|---|---|
| Producers (plants and algae) | 20,810 | starting level |
| First-order consumers (plant eaters) | 3,368 | 16.2 |
| Second-order consumers (meat eaters) | 383 | 11.4 |
| Third-order consumers (top meat eaters) | 21 | 5.5 |
| Decomposers (bacteria and others) | 5,060 | a side branch fed by all levels |
Source: Howard T. Odum, Ecological Monographs, 1957, Figure 7, yearly estimates for the upper Silver Springs run in Florida. Producers: energy captured from sunlight. Consumers: energy taken in. Odum called these estimates tentative. We calculated the percents.
- Describe the trend in energy from producers to third-order consumers.
- Which step was closest to 10 percent? Which was farthest from 10 percent, counting in percentage points?
- Decomposers took in more energy than the plant eaters. Where could that energy come from?
Claim, evidence, reasoning
Make a claim that answers the essential question: why can a place feed many small animals but few top predators? Use the Silver Springs data.
Start with: Fewer top predators can live in a place because blank.
Start with: At Silver Springs, producers captured blank kilocalories per square meter per year, but third-order consumers took in only blank.
Start with: At each step, much of the energy is blank, so blank is left for the next level.
Compare with a model answer
A place can support fewer top predators than the organisms below them because less energy reaches each higher level. At Silver Springs, producers captured 20,810 kilocalories per square meter per year, but third-order consumers took in only 21, about one-thousandth as much. At each step much of the energy was given off as heat, and some went to decomposers. One limit: this is one Florida spring in the 1950s, not the Bay, and Odum called his numbers tentative. Energy is also not a count of animals.
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Design problem
Mealworms are beetle larvae that eat oat bran. Plan an investigation to compare how much food mealworms eat with how much mass they gain.
One way to do it
- Question: How does the mass of bran eaten compare with the mass the mealworms gain?
- Independent variable: whether a container has mealworms in it or not.
- Dependent variable: mass of the mealworms at the start and end, and mass of the bran left after sifting out the mealworms and their waste, in grams, over 7 days.
- Constants: 20 mealworms and 20 grams of bran per container, same temperature, same container size.
- Control: a container of bran with no mealworms. It shows how much mass the bran loses by drying out.
- Trials: three containers with mealworms and three without. Report the mean.
- Safety: wash your hands after handling, keep containers closed, and never release mealworms outdoors.
If the mealworms gain much less mass than the bran they ate, most of the food did not become new mealworm. Respiration and waste account for much of the rest.
Evaluate: check your thinking
The energy in an osprey began as sunlight captured by plankton. At each step, from plankton to menhaden to osprey, much of the energy was given off as heat, so only roughly one-tenth passed on. Less energy at the top means a place can feed fewer ospreys than the small fish they eat.
Science practices you used: testing predictions with a model, finding percents in a real data table, making a claim from evidence, and planning a fair test.
How we measure it
Measuring oxygen in flowing water
At Silver Springs, Howard Odum compared the oxygen in the water upstream and downstream, and corrected for oxygen moving between the water and the air. Day and night readings together showed how fast the plants and algae were capturing energy by photosynthesis.
Cameras at osprey nests
In a 2021 Virginia study, small trail cameras at nests took a picture every five minutes in daylight. Researchers used the pictures to identify the fish the adults brought to their chicks.
Finish line
Test yourself
8 questions
Practice and MCAT-style, with explained answers.
Next · Lesson 13
Producers, Consumers and Decomposers
Leaves fall in the forest every autumn, so why don’t the old ones just keep piling up?
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