Biology for AP Courses · Ecosystems
Energy Flow through Ecosystems
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In 30 seconds
Every organism needs energy, but almost no organism makes its own. The energy that powers nearly all ecosystems arrives as sunlight and is captured only by autotrophs (producers) such as plants, algae, and cyanobacteria through photosynthesis. From there, energy moves through the ecosystem as organisms eat one another, and it is progressively lost at every step as heat — the inevitable cost of metabolism, movement, growth, and maintaining body temperature. This one-way march from sunlight to heat is what ecologists mean by energy flow, and it sets hard limits on how many organisms an ecosystem can support and how long food chains can be.
This topic follows the energy half of the rule from the previous topic — energy flows; matter cycles. Here we trace who captures energy, how it is transferred, how much survives each transfer, and why the shape of ecosystems follows directly from that math. The chapter's third topic follows the matter.
Two terms carry most of the analytical weight. Gross primary productivity (GPP Gross primary productivity: total energy captured by producers Full entry →) is the total energy captured by producers in an ecosystem. Net primary productivity (NPP Net primary productivity: GPP minus producer respiration Full entry →) is what remains after producers use some of that energy for their own respiration: NPP = GPP − R. NPP is the energy actually available to every consumer and decomposer in the ecosystem — the "budget" the rest of the community lives on.
Why this matters
Energy flow explains patterns you can observe in the real world. It is why there are always more plants than herbivores, why food chains rarely exceed four or five links, and why top predators are rare. It also explains why eating lower on the food chain feeds more people: a field of grain feeds far more people directly than the same field does after being converted into beef, because each transfer wastes most of the energy. Energy flow also carries a safety-relevant side effect: persistent toxins such as DDT and mercury biomagnify — they become more concentrated at each trophic level, so top predators (and the people who eat them) receive the heaviest doses. On the AP exam, expect questions that give you productivity numbers and ask you to calculate the energy available at a trophic level or to explain why a pyramid cannot be inverted.
The college version
Core Concepts
Autotrophs, heterotrophs, and the three ways of eating
Autotrophs build their own organic molecules from inorganic sources — plants, algae, and cyanobacteria via photosynthesis (sunlight), and a few bacteria via chemosynthesis (chemical energy). Heterotrophs must obtain organic carbon by consuming other organisms. Consumers are classified by what they eat: primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Two often-forgotten groups recycle energy: detritivores (earthworms, vultures) eat dead organic matter, and decomposers (bacteria, fungi) break it down into inorganic nutrients. All heterotrophs — including humans — ultimately depend on energy captured by autotrophs.
Food chains and food webs
A food chain is a single linear path of energy transfer (grass → grasshopper → mouse → hawk). A Food web All interconnected feeding relationships in an ecosystem Full entry → is the realistic, interconnected map of all the feeding relationships in an ecosystem, because most organisms eat and are eaten by several species. Food webs are more stable than chains: if one prey species collapses, a predator with multiple prey options can switch. A Keystone species Species whose removal causes disproportionately large changes Full entry → is one whose removal causes disproportionate change — sea otters controlling sea urchins, or wolves controlling elk — and its effects can be traced through the web.
Productivity: GPP, NPP, and the energy budget
Productivity is the rate at which energy is captured or stored. GPP = total energy fixed by photosynthesis. NPP = GPP − respiration of the producers themselves. NPP is the food for everything else; it is the number to track for crops, forests, and fisheries. The difference between GPP and NPP is large — plants use a substantial share of what they fix just to stay alive — so confusing the two is a classic exam error.
The 10% rule: why pyramids are pyramids
At each trophic transfer, roughly 90% of the energy is lost — used for metabolism, growth, and heat — leaving only about 10% to be incorporated into new biomass. This "10% rule Commonly taught approximation that ~10% of energy transfers per level Full entry →" is a textbook reference approximation; actual transfer efficiency varies with ecosystem and species. The consequence is geometric: if producers capture 10,000 units of energy, primary consumers receive about 1,000, secondary consumers about 100, and tertiary consumers about 10. That is why pyramids of energy are always upright and why food chains are short — by the fifth or sixth level, too little energy remains to support another predator.
Pyramids of numbers, biomass, and energy
Ecologists draw three kinds of pyramids. Pyramids of energy show the energy available at each level and are always upright. Pyramids of biomass show the dry mass at each level and are usually upright, but can be inverted in some aquatic systems (in the open ocean, the standing crop of phytoplankton is small while the fish above it are larger). Pyramids of numbers count individuals and can be inverted when one tree supports many insects. When a question shows an inverted pyramid, ask which kind it is — that determines whether it is normal or impossible.
Biomagnification of persistent toxins
Energy is not the only thing that moves up food chains. Fat-soluble toxins that organisms cannot break down or excrete accumulate in body tissues; each consumer inherits the combined toxin loads of its prey, so concentration rises at every level. That is why top predators like eagles and tuna carry the highest levels of compounds such as DDT and mercury — and why fish-consumption advisories focus on large, long-lived predatory species.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| GPP | NPP | GPP is total energy fixed by producers; NPP subtracts producer respiration and is what consumers actually get |
| Food chain | Food web | A chain is one linear path; a web is the full network of feeding relationships |
| Pyramid of numbers | Pyramid of energy | Numbers count individuals (can be inverted); energy is always upright because ~90% is lost per level |
| Decomposer | Detritivore | Detritivores eat dead matter (worms, vultures); decomposers (bacteria, fungi) chemically break it down into nutrients — both recycle, at different steps |
| "Energy is recycled" | "Energy flows one way" | Energy is not recycled; it is progressively lost as heat. Only matter cycles |
| Top predator abundance | Top predator importance | Top predators are few (low energy) but ecologically powerful (keystone effects) |
| Bioaccumulation | Biomagnification | Bioaccumulation is buildup in one organism; biomagnification is concentration increasing up the food chain |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Energy flow is like a game of "pass the snack" where the snack shrinks every time it is passed. The sun gives plants a giant basket of snacks; a mouse that eats plants keeps only a small handful; a fox that eats mice gets an even smaller handful, because every animal uses up most of the snack's energy just to move, grow, and stay warm. By the top of the chain, almost nothing is left — that's why there are always far more plants than wolves.
Worked example
A grassland researcher measures the producers in a one-hectare field and finds that the plants fix a total of 10,000 kilocalories per year — that is the field's GPP. But the plants themselves respire away much of it; after subtracting producer respiration, the researcher reports an NPP of 1,000 kcal/yr. That 1,000 kcal is the entire energy budget for everything else in the field.
Using the textbook 10% approximation: grasshoppers (primary consumers) convert about 10% of the NPP they eat into their own biomass — 100 kcal. Mice (secondary consumers) get about 10 kcal. A hawk (tertiary consumer) gets about 1 kcal. Two things stand out. First, each level is a tenth of the one below — the pyramid of energy is upright by construction. Second, the hawk's level is tiny: one hawk needs a huge territory of mice and grasshoppers to meet its energy needs, which is exactly why top predators are rare, wide-ranging, and the first to vanish when a habitat shrinks. Now add a toxin: if the field is sprayed with a persistent pesticide, the grasshoppers carry trace amounts, the mice carry more, and the hawk — having eaten hundreds of mice — carries the highest concentration of all. Same food web, two consequences of trophic structure: energy scarcity and poison concentration at the top.
Key takeaways
- NPP = GPP − R (producer respiration). NPP is the energy available to consumers and decomposers.
- Only autotrophs add new energy to an ecosystem; all heterotrophs depend on their production.
- ~10% of energy transfers between trophic levels (approximation; actual efficiency varies).
- Pyramids of energy are always upright; pyramids of biomass and numbers can be inverted in special cases.
- Food webs, not chains, describe real ecosystems; keystone species have outsized effects.
- Persistent toxins biomagnify up food chains — top predators carry the highest concentrations.
- Eating lower on the food chain is more energy-efficient for feeding human populations.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the difference between gross primary productivity and net primary productivity, and which one feeds the rest of the ecosystem?
Show answer
GPP is the total energy captured by producers through photosynthesis. NPP is GPP minus the energy producers use in their own respiration (NPP = GPP − R). NPP is the energy available to consumers and decomposers — the part that feeds everything else.
Why are pyramids of energy always upright, while pyramids of numbers sometimes are not?
Show answer
Pyramids of energy are always upright because ~90% of energy is lost as heat at each trophic transfer, so each level must contain less energy than the one below. Pyramids of numbers can be inverted because a single large producer (one tree) can support thousands of small consumers (insects).
Why do food chains rarely have more than four or five trophic levels?
Show answer
Cumulative energy loss: with only ~10% transferred per level, after four or five transfers too little energy remains to support another trophic level's metabolism and reproduction.
Explain how a persistent pesticide can end up most concentrated in a top predator that never touched the pesticide itself.
Show answer
The pesticide is persistent (not broken down) and fat-soluble, so it accumulates in each organism's tissues (bioaccumulation). Each predator eats many prey and inherits their combined loads, so concentration increases at every trophic level (biomagnification) — the top predator ends up with the highest dose.
A field's producers fix 20,000 kcal of energy per year, and producer respiration uses 15,000 kcal. What is the NPP, and roughly how much reaches the primary consumers (using the 10% rule)?
Show answer
NPP = 20,000 − 15,000 = 5,000 kcal/yr. Primary consumers receive roughly 10% of the NPP they consume, so about 500 kcal/yr reaches the primary consumer level.
Why is a food web a more accurate description of an ecosystem than a food chain?
Show answer
Real organisms eat and are eaten by multiple species; a web captures those branching relationships and the stability they provide (alternative food sources), while a chain shows only one simplified path.
Study toolsKey vocabulary
Key vocabulary
- Autotroph (producer)
- Organism that builds its own food from inorganic sources (photosynthesis or chemosynthesis)
- Heterotroph (consumer)
- Organism that obtains energy by eating other organisms
- GPP
- Gross primary productivity: total energy captured by producers
- NPP
- Net primary productivity: GPP minus producer respiration
- 10% rule
- Commonly taught approximation that ~10% of energy transfers per level
- Food web
- All interconnected feeding relationships in an ecosystem
- Keystone species
- Species whose removal causes disproportionately large changes
- Biomagnification
- Increasing concentration of persistent toxins at higher trophic levels
Sources & references
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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