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Energy flows one way up a food chain and only about a tenth passes each step, so food chains are short, predators few, and productivity sets how much life a biome supports.
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By the end of this lesson you will be able to follow energy up a food chain with the ten percent rule and use it to explain the shape of ecosystems.
You met biomes in Grade 5 and know that climate decides which plants grow where. This lesson follows the energy the plants capture as it passes to the animals that eat them, and explains why every ecosystem has far more plants than predators.
| Term | What it means |
|---|---|
| Ecosystem | The living things of a place and the physical world they depend on. |
| Producer | A plant or alga that makes food from sunlight. |
| Consumer | An animal that gets energy by eating other living things. |
| Decomposer | A fungus or bacterium that breaks down dead matter and returns nutrients. |
| Trophic level | A step in a food chain: producers, primary consumers, and so on. |
| Productivity | The new plant growth an ecosystem makes each year, per square meter. |
Energy enters an ecosystem as sunlight, captured by producers, and passes up the food chain.
Two steps up keeps about a hundredth; three steps, a thousandth.
Another way: picture
Picture a pyramid of energy: a wide base of plants, a narrower layer of plant-eaters, a thin layer of meat-eaters, and a tiny tip of top predators. Each layer is only about a tenth of the one below, because every animal spends most of the energy it eats just staying alive.
Another way: steps
Build a catchment-to-coast explanation using this fictional record. An upland valley has scratched rock and unsorted glacial debris. A river reworks that debris and soil into sorted bars downstream. During a reference wet season the basin receives 300 millimeters of precipitation, loses 120 to evapotranspiration and exports 140 as runoff. In a later comparable wet season, after extensive clearing, the corresponding values are 300, 100 and 180. Assume no other boundary water flows. Surveys also show eroded soil below cleared slopes and a new reservoir trapping much of the river's sediment before the coast. A beach beyond the outlet narrows. These invented records are clues for an explanation, not proof of one cause.
Produce a labeled systems sketch, a water-budget table and a written explanation. Trace plate-created relief, weathering and inherited glacial deposits through soil formation, river erosion and transport, reservoir storage and coastal deposition. Identify what is directly observed and what is inferred. Calculate changes in basin water storage: the reference season gains 40 millimeters and the later season gains 20. Explain why equal rainfall does not imply equal runoff and how vegetation removal could contribute. Discuss how a sediment barrier could affect a beach, while identifying storms or changed sea level as rival influences. Distinguish this short record from evidence of a long-term climate trend.
Now compare two invented risk plans for the downstream settlement. Plan A protects 100 exposed homes with a barrier and estimates annual damaging-flood probability between 1 and 3 percent. Plan B relocates households so that 20 homes remain exposed, with probability between 2 and 4 percent. Assume one physical loss unit per exposed home per damaging flood. Calculate the expected annual loss ranges, 1 to 3 and 0.4 to 0.8, and state why the lower modeled loss does not settle questions of relocation, livelihoods, maintenance or equity. Recommend observations to monitor both water and sediment and a condition that would make you revise your recommendation. Explain which actions adapt to impacts and which additional action could mitigate climate forcing.
To prepare, examine an alternative to the clearing hypothesis: a wetter storm sequence could raise runoff even without land-cover change. Comparing equal seasonal rain totals controls one factor, but not storm intensity or antecedent soil moisture. A stronger investigation compares storm records and similar uncleared subcatchments. The same reasoning applies to a narrowing beach: compare sediment deliveries, storms and engineering, rather than assigning every change to sea level.
Human rubric: use 0 (missing or materially wrong), 1 (partly correct or unsupported), and 2 (correct, connected and qualified) for each criterion. Process chain: distinguish weathering, erosion, transport and deposition and connect glacial inheritance, soil, river and coast. Water evidence: show both storage calculations with units and relate vegetation to runoff without assuming causation. Spatial and temporal reasoning: label the sketch and distinguish observations, inferred transfers and timescale limits. Decision: calculate both risk ranges, explain residual exposure and distinguish adaptation from emissions mitigation. Evaluation: test at least one rival, address who bears relocation or protection costs, and propose monitoring with a revision trigger. Require revision wherever a criterion scores zero; record the reasoning and feedback, not just a total. The report needs a human reader. The platform cannot validate the original sketch, investigation or recommendation, and no completed human approval is implied by this rubric.
Energy transfer is one ecosystem constraint; water availability is another. For a catchment over a stated interval, change in water storage equals precipitation minus evapotranspiration minus runoff, if other boundary flows are negligible. Evapotranspiration includes evaporation and plant transpiration. Use the same area, period and depth units for every term; do not subtract a discharge in cubic meters per second from rainfall in millimeters.
In an invented monthly budget, precipitation is 90 millimeters, evapotranspiration 50 and runoff 25. Storage increases by 15 millimeters. During a dry month, precipitation of 20 with evapotranspiration of 45 and runoff of 5 implies a 30-millimeter storage drawdown, which requires stored water to be available. A negative balance does not create negative water: if stores are exhausted, actual evapotranspiration or runoff must fall.
Persistent seasonal deficits can favor drought-tolerant vegetation; reliable moisture supports different plant communities. Temperature, soils, disturbance and human choices also matter, so rainfall alone does not determine a biome. Vegetation affects the budget in return through roots, interception and transpiration. Clearing can reduce infiltration and raise rapid runoff, while regrowth can alter both water storage and evapotranspiration. Compare annual totals and seasonal timing before explaining ecological zones, and distinguish a land-cover observation from a claim about its cause.
Here is the energy in a prairie food chain, using the ten percent rule.
| Level | Example | Energy (kcal) |
|---|---|---|
| producers | grasses | 100,000 |
| primary consumers | prairie dogs | 10,000 |
| secondary consumers | coyotes | 1,000 |
| tertiary consumers | golden eagles | 100 |
By the fourth level only a thousandth of the energy captured by the grasses remains.
An animal uses most of the energy it eats to move, breathe, keep warm and repair its body. Much of that is lost as heat. Some of what it eats cannot be digested and passes out as waste. Only the energy stored in its body, as growth, is available to whatever eats it.
That is why about ninety percent is lost at every step, and why food chains rarely have more than four or five levels.
Because so little energy reaches the top, top predators are always few compared with their prey. A wolf pack needs a large herd of elk, which needs a huge area of grass.
This also explains why predators range so widely. A mountain lion may roam more than a hundred square miles to find enough deer.
When plants and animals die, decomposers break them down, returning nitrogen, phosphorus and carbon to the soil and air, where plants use them again. Nutrients go around and around.
Energy does not. Once it is lost as heat it cannot be captured again by living things, so every ecosystem depends on a steady supply of new sunlight.
Productivity measures the new plant growth an ecosystem makes each year. It is highest where it is warm, wet and sunny, as in tropical rainforests, and lowest in deserts and tundra, which lack water or warmth.
Productivity sets how much life an ecosystem can support: more plant growth at the base means more energy for every level above.
The tallgrass prairie once covered much of the central United States, from Indiana to Kansas. Its grasses grew roots many feet deep, building some of the richest soil on Earth, and fed great herds of bison.
Most of the prairie was plowed for farmland in the 1800s and early 1900s; only a few percent of the tallgrass prairie remains. Preserves such as the Tallgrass Prairie National Preserve in Kansas protect what is left.
People eat at several levels. Eating grains and vegetables takes energy from the first level; eating beef takes it from the second, after the cattle have used most of it.
That is why far more people can be fed from an acre of crops eaten directly than from the same acre used to raise animals, one of the ten percent rule's biggest effects on farming and land use.
Checking an answer. Energy must shrink at every level up. An efficiency above one hundred percent is impossible.
Dividing by ten is allowed as an approximation, because measured transfer efficiencies cluster around ten percent, often between five and twenty. Multiplying productivity by area is allowed when growth is spread evenly across the plot.
Dividing available energy by one animal's needs is allowed as an estimate of how many can be supported, though real populations also depend on water, shelter and disease.
Although they hold little of the energy, top predators can change a whole ecosystem. When predators disappear, their prey can multiply and overeat the plants; when predators return, prey change where and how they feed, and the plants recover.
Ecologists call such a chain of effects a trophic cascade. The return of wolves to Yellowstone is the most famous American example.
Ecologists measure the energy at each level by weighing living things, drying and burning samples to find the energy they hold, and counting how many of each kind live in an area. Pond studies in the twentieth century first showed that only about a tenth passes from one level to the next.
The most common slip is thinking energy is recycled like nutrients. Another is counting one step when the question asks for two.
A third is subtracting ten percent at each step instead of keeping ten percent. A fourth is dividing the lower level by the higher when finding an efficiency, giving a number above one hundred.
Ecosystems change when their physical world changes. A drought lowers productivity; a warmer climate lets some species move north and uphill; a new dam changes a river's fish.
Because every level depends on the one below, a change at the base, in the plants, spreads up through the whole food chain.
Gray wolves were hunted out of Yellowstone National Park by the 1920s. Without them, elk multiplied and browsed heavily on young willows and aspens along the rivers. In 1995 and 1996, thirty-one wolves from Canada were released into the park.
The wolves' numbers were always small compared with the elk, as the ten percent rule predicts: a large herd supports only a modest number of predators. Yet their effect spread widely. Elk numbers fell and elk changed where they grazed, and in some places willows and aspens began to recover.
Scientists still debate how much of the change came from wolves and how much from other causes, such as bears, drought and hunting outside the park. But Yellowstone became the best-known American example of a trophic cascade: a small number of top predators reshaping an ecosystem far larger than their share of its energy.
Across the United States, most cropland grows corn and soybeans, and much of that harvest feeds cattle, hogs and chickens rather than people directly. Each step up the food chain loses most of the energy, so an acre of crops eaten directly can feed far more people than the same acre fed to animals.
For beef the loss is especially large, because cattle are big, warm-blooded and live for years before they are eaten. Chickens and fish convert feed into food more efficiently, and plant foods skip the step entirely.
These numbers do not settle what anyone should eat; grazing land that cannot grow crops, taste, culture and nutrition all matter too. But they show how the ten percent rule, first worked out by ecologists studying ponds and prairies, shapes how much land it takes to feed a country.
It is natural to picture an ecosystem as a loop, with everything passed around and reused. Nutrients do cycle that way, returned to the soil by decomposers. But energy flows one way: at every step most is used and lost as heat, and only about a tenth passes on.
That one-way flow is why food chains are short, why top predators are rare, and why every ecosystem depends on a steady supply of new sunlight.
Grass captures $10000$ kilocalories. Pass the energy to rabbits.
$\dfrac{10000}{10} = 1000$
One step up.
Pass it to foxes.
$\dfrac{1000}{10} = 100$
Two steps up.
Find the share reaching the foxes.
$\dfrac{100}{10000} = 0.01$
One hundredth.
Say where the rest went.
$\text{living and heat}$
At each level.
Algae hold $50000$ kilocalories and the insects eating them $4000$. Write higher over lower.
$\dfrac{4000}{50000}$
The share passed up.
Multiply by one hundred.
$\dfrac{4000}{50000} \times 100 = 8\%$
The efficiency.
Compare with the rule.
$8\% \approx 10\%$
Close to it.
Find what fish eating the insects would get at $10\%$.
$400$
A tenth of 4000.
Check it shrinks.
$50000 > 4000 > 400$
Every level smaller.
A range's deer hold $1500$ million kilocalories available a year. Find the predators' share.
$\dfrac{1500}{10} = 150\ \text{million}$
Ten percent.
A mountain lion needs $5$ million kilocalories a year. Divide.
$\dfrac{150}{5} = 30$
Lions supported.
Say what happens without the ten percent step.
$\dfrac{1500}{5} = 300$
Ten times too many.
Say why the true number is smaller still.
$\text{other predators share the deer}$
Coyotes, bears, wolves.
Say what the lions need besides food.
$\text{space to roam}$
Large territories.
Say what losing them would do.
$\text{deer overgraze}$
A trophic cascade.
Find the primary consumers' energy.
$\dfrac{80000}{10} = 8000$
One step up.
Find the secondary consumers' energy.
$\dfrac{8000}{10} = 800$
Two steps up.
Check the share.
The grasses in a meadow capture $30000$ kilocalories of the sun's energy. About how much of it reaches the hawks that eat the mice that eat the grass?
Complete the worked solution: the plants in a forest capture $150000$ kilocalories. Using the ten percent rule, find the energy reaching the deer that eat the plants and the mountain lions that eat the deer.
Find the deer's energy.
$\dfrac{\text{plants}}{\text{ten}} =$ a
One step up.
Find the mountain lions' energy.
$\dfrac{\text{deer}}{\text{ten}} =$ b
Two steps up.
Say what the pyramid shape shows.
$\text{wide base, narrow top}$
Energy shrinks at each level.
Say why losing the lions matters.
$\text{deer overgraze}$
Top predators shape the whole system.
Match each role in an ecosystem to an example from an American prairie.
| big bluestem grass, making food from sunlight | a bison grazing the grass | a coyote hunting prairie dogs | fungi breaking down dead plants and animals | |
|---|---|---|---|---|
| producer | ||||
| primary consumer | ||||
| secondary consumer | ||||
| decomposer |
A food chain's producers capture $30000$ kilocalories. Using the ten percent rule, fill in the energy reaching the primary, secondary and tertiary consumers.
| energy | |
|---|---|
| primary consumers (kcal) | |
| secondary consumers (kcal) | |
| tertiary consumers (kcal) |
A biome's plants add about $400$ grams of new growth per square meter each year. Write the new growth on a plot, in grams a year, as a function of the plot's area $a$ in square meters.
Answer:
In a pond, the algae hold $8000$ kilocalories and the insects that eat them hold $400$. What percent of the energy passed from algae to insects?
Answer: %
Suppose the elk in part of Yellowstone National Park hold $1500$ million kilocalories of energy available each year, about a tenth of which can pass to the wolves that hunt them. A wolf needs about $5$ million kilocalories a year. About how many wolves can the elk support?
Answer: wolves
A fictional catchment receives 80 millimeters in a month, loses 50 to evapotranspiration and 40 to runoff, with no other flows. Its soil starts with 6 tens of millimeters stored. Which interpretation links its budget to vegetation appropriately?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
A biome's plants add about $400$ grams of new growth per square meter each year. Write the new growth on a plot, in grams a year, as a function of the plot's area $a$ in square meters.
Answer:
You can follow energy through an ecosystem. Explain why there are far fewer wolves than elk in Yellowstone.
26. Your turn: plants capture $80000$ kilocalories. About how much reaches the secondary consumers?, step 3
$\dfrac{800}{80000} = 0.01$
One hundredth.