Energy in Ecosystems

Along the Chesapeake Bay, ospreys raise chicks in nests and carry fish home to their chicks. One important fish they bring is the Atlantic menhaden, a small oily fish that filters plankton out of the water.

The plankton caught the energy first, from sunlight. Then a menhaden ate the plankton, and an osprey ate the menhaden. How much of the plankton’s energy ever reaches the osprey chick, and where does the rest go?

Explore: build an energy pyramid

The simulation starts with sunlight falling on a patch of bay water. Producers capture some of it, and you choose how many feeding levels sit above them and what share of energy passes from one level to the next, from 5 to 20 percent. The pyramid shows the energy at each level and the heat given off, and a switch shows carbon moving back through decomposers.

Share passed up each step: 10 percent Feeding levels, producers included: 3

Carbon switch: off Reset

Captured by producers (level 1)

Top level reached

Heat or uneaten, all steps added

Heat or uneaten + top level

In this sketch the producers start with 100,000 kJ per square meter per year (a made-up number), every step passes the same share, and bar widths are not to scale. Real measurements vary: the lesson reports about 13 and 21 percent for Cedar Bog Lake and about 6 percent as the average of a 2026 synthesis.

Rule to keep: about 10 percent is a rough rule of thumb, not a fixed law. Energy flows one way, from sunlight to heat, and much is lost at each step, so each level has less. Matter such as carbon cycles and is used again.

Try this.

1. Set the transfer to 10 percent and add a fourth level. Predict how much of the producers’ energy reaches the top, then test it.
2. Change the transfer to 5 percent, then 20 percent. Predict which setting can support more levels.
3. Add up the heat lost at every level. Predict whether the heat plus the energy at the top level equals what the producers captured.
4. Turn on the carbon switch. Predict whether carbon and energy follow the same path, then watch.

Explain: who eats whom in the Bay

An ecosystem is all the living things in an area together with the nonliving environment they interact with, such as water, light and temperature. The Chesapeake Bay is one ecosystem, and a single marsh inside it can be studied as another.

Every living thing in it gets energy in one of three ways.

• A producer captures energy from sunlight and stores it in food. In the open water of the Bay, the main producers are phytoplankton, tiny algae and bacteria that drift in the water. Underwater grasses are producers too. A few producers in other places use energy from chemicals instead.
• A consumer gets energy by eating other living things. A primary consumer eats producers. A secondary consumer eats primary consumers, and a tertiary consumer eats secondary consumers.
• A decomposer, such as many bacteria and fungi, gets energy from dead matter and wastes.

A trophic level is a feeding step: producers are the first level, primary consumers the second, and so on.

A food chain follows one feeding path. A food web links many food chains together. Arrows point from the food to the eater, the way the energy moves:

sunlight → phytoplankton → zooplankton → Atlantic menhaden → osprey

Menhaden filter both phytoplankton and zooplankton, tiny animals that drift in the water. Striped bass eat menhaden too. Because menhaden eat at two levels, the osprey is a secondary consumer on one path and a tertiary consumer on another. A web shows this; a single chain hides it. You will add bacteria and their recycling in Matter Cycles.

Explain: why each level gets less

Phytoplankton store light energy in sugars by photosynthesis, which you met in Photosynthesis. When a zooplankton eats a phytoplankton cell, most of that energy does not become new zooplankton.

• Some is released as heat during cellular respiration, the process that releases energy from food, explained in Respiration and ATP.
• Some is in parts that are not eaten or not digested. Much of that material ends up with decomposers.
• Only the rest becomes new body tissue that the next level can eat.

As a rule of thumb, about 10 percent of the energy at one level ends up in the next. Real measurements are often higher or lower. A 2026 synthesis of 2,052 estimates from 122 studies found an average energy transfer of about 6 percent, lower on land than in the sea. The simulation lets you try 5 to 20 percent.

Here is a worked example with made-up starting numbers. Suppose phytoplankton in a model patch of bay add 100,000 kJ per square meter per year, and 10 percent passes at each step.

Level	Calculation	Energy (kJ per square meter per year)
Phytoplankton	given	100,000
Zooplankton	100,000 × 0.10	10,000
Menhaden	10,000 × 0.10	1,000
Osprey	1,000 × 0.10	100

Only one thousandth of the producers’ energy reaches the osprey level. Loss at each step is a major reason food chains rarely have more than about five levels.

An energy pyramid stacks the energy at each level, producers at the bottom. It always narrows going up. A biomass pyramid stacks biomass, the mass of living things at each level, such as grams per square meter. Biomass pyramids usually narrow too, but not always. In the English Channel, phytoplankton divide so fast and are eaten so quickly that, at any moment, the zooplankton can outweigh them. That pyramid is upside down, while the energy pyramid stays upright.

Explain: energy flows, matter cycles

Energy is never made or destroyed; it changes form. Light becomes chemical energy in food, and food energy becomes motion and heat. Heat spreads into the water and air, and no living thing can capture it to build food again. So energy flows one way: in as sunlight, out as heat. Nearly every ecosystem needs a steady supply of new energy, which in the Bay is sunlight; a few, such as deep-sea vent communities, run on chemical energy instead.

Matter behaves differently. Decomposers release much of the carbon in a dead menhaden as carbon dioxide, and phytoplankton can take that carbon up again.

	Energy	Matter (such as carbon)
Where it enters	sunlight captured by producers	already present in air, water and soil
What happens at each level	much is released as heat	atoms are passed on or released, not lost
Path	one way	a cycle, used again

Elaborate: real energy budgets and osprey diets

Lindeman measured how much each feeding level in a small Minnesota lake added in a year.

Cedar Bog Lake level	Energy added (gram-calories per square centimeter per year)
Producers	111.3
Primary consumers	14.8
Secondary consumers	3.1

Source: Lindeman, Ecology, 1942, corrected values in Tables II and IV, from about four years of study at Cedar Bog Lake. A gram-calorie is a unit of energy, about 4.2 joules. Lindeman printed 22.3 percent for the second step, which differs from what these values give (3.1 divided by 14.8 is about 20.9 percent); rounding does not explain the gap, and the table does not say why.

Ospreys show that the menhaden link varies from place to place and year to year.

Osprey sample (lower Chesapeake Bay)	Years	Menhaden share (%)
Lower-estuary nests, share of fish counted	2006–2007	24.2
Upper-estuary nests, share of fish counted	2006–2007	1.5
Lower-estuary nests, share of prey energy	2006–2007	44.7
Upper-estuary nests, share of prey energy	2006–2007	4.5
Mobjack Bay, mean of 7 nests	1985	67.3
Mobjack Bay, mean of 8 nests	2006–2007	24.7
Mobjack Bay, mean of 4 nests	2021	30.2

Sources: the first four rows are from nest videos of 25 osprey pairs (Glass and Watts, Journal of Raptor Research, 2009). The last three are mean percentages of the diet, from fish recorded at nests, by Watts and colleagues, Frontiers in Marine Science, 2024.

1. Calculate the percent of energy passed from producers to primary consumers, and from primary to secondary consumers, at Cedar Bog Lake.
2. Compare the menhaden share by fish count and by energy at the lower-estuary nests. Why might they differ?
3. How did the Mobjack Bay mean change from 1985 to 2021?

Design problem

Plan a study to test whether osprey chicks fed extra fish are more likely to survive to flying age. Name your variables, control, constants, sample size and how you protect the birds.

One way to do it

• Question: does extra food raise the share of nests that raise young?
• Independent variable: extra menhaden placed at the nest, or none.
• Dependent variable: whether each nest raises at least one chick to flying age, and how many.
• Control: nests that get no extra fish but are visited the same way.
• Constants: the same area, the same season, and nests that started with eggs at about the same time.
• Allocation: assign nests to groups at random. If some nests can only be control, as when boats cannot reach them, say so, because it could bias the result.
• Sample size: at least 15 nests per group, and repeat in more than one year.
• Ethics and safety: work under wildlife permits, keep visits short, and stay out of the nest during heat or storms.
• What results mean: higher success in the fed group supports food limitation at that site. It does not say why fish are scarce.

Evaluate: check your thinking

The osprey chick’s energy began as sunlight caught by phytoplankton. At every step, from plankton to menhaden to osprey, much of the energy was released as heat or left in uneaten and undigested matter, so only a small share, often around 10 percent, passed upward. Much of the carbon in those fish can cycle back to the plankton, but the energy will not.

Science practices you used: making predictions and testing them with a model, calculating percentages from a real data table, comparing samples from different places and years, planning a fair test with a control and random groups, and naming the limits of one study’s data.