Concepts of Biology · Ecosystems and the Biosphere
Energy Flow through Ecosystems
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In 30 seconds
Every living thing needs energy, yet almost nothing on Earth produces its own. The energy that runs nearly all ecosystems begins as sunlight, and only autotrophs — plants, algae, and cyanobacteria — capture it through photosynthesis. From there, energy passes from organism to organism as they eat one another, and at every step most is lost as heat, the unavoidable cost of staying alive. This one-way journey from sunlight to heat is energy flow — and it explains why food chains are short and top predators are rare.
The organizing rule of ecosystem ecology is energy flows; matter cycles. Two terms carry the analysis: gross primary productivity (GPP Gross primary productivity: total energy captured by producers) is the total energy producers capture; net primary productivity (NPP Net primary productivity: GPP minus producer respiration) is what remains after producers spend energy on their own respiration (R): NPP = GPP − R. NPP is the energy budget every consumer lives on.
Why this matters
Energy flow connects to daily life and to the biggest questions in food and environmental science. It is why beef costs the environment far more than grain — animals waste most of the plant energy they eat, so eating lower on the food chain feeds more people with less land and water. The same logic drives fisheries, agriculture, and conservation. Energy flow also has a safety-relevant side: persistent, fat-soluble pollutants such as DDT and mercury biomagnify, concentrating at each step of the food chain, so top predators and the people who eat them get the heaviest doses.
The college version
Core Concepts
Producers, consumers, and decomposers
Autotrophs (producers) build their own food from inorganic materials using sunlight (photosynthesis) or, in a few deep-sea bacteria, chemical energy (chemosynthesis). Heterotrophs must eat other organisms: primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Also essential: detritivores (earthworms, vultures) eat dead organic matter, and decomposers (bacteria, fungi) break it into inorganic nutrients.
Food chains and food webs
A food chain is a single linear path: grass → grasshopper → mouse → hawk. A Food web All interconnected feeding relationships in an ecosystem is the full map of feeding relationships — more realistic, because most organisms eat and are eaten by several species, and more stable: if one prey crashes, a predator can switch. A Keystone species Species whose removal causes disproportionately large changes is one whose removal causes outsized changes — sea otters controlling sea urchins, wolves shaping elk behavior.
Productivity: GPP and NPP
Productivity is the rate at which energy is captured or stored. GPP is the total energy fixed by photosynthesis; NPP subtracts the energy producers use to keep themselves alive (respiration): NPP = GPP − R. NPP is the food available to everything else — and confusing the two is a classic exam mistake.
The 10% rule and three kinds of pyramids
When energy moves from one Trophic level A feeding position in a food chain (producer, primary consumer, etc.) Full entry → (feeding level) to the next, roughly 90% is lost — used for metabolism, movement, growth, and heat — leaving about 10% to be built into new biomass. This "10% rule Commonly taught approximation that ~10% of energy transfers between levels Full entry →" is a commonly taught approximation; actual efficiency varies. The arithmetic is severe: if producers capture 10,000 units of energy, primary consumers get about 1,000, secondary about 100, tertiary about 10. That is why pyramids of energy are always upright and why food chains rarely exceed four or five levels.
Ecologists also draw pyramids of biomass and numbers. Pyramids of biomass show dry mass at each level and are usually upright, but can invert in some open-ocean situations where the standing crop of tiny phytoplankton is smaller than the fish above them. Pyramids of numbers count individuals and can invert when one tree supports thousands of insects. On an exam, always ask which kind of pyramid is inverted.
Biomagnification
Energy is not the only thing that moves up food chains. Toxins that organisms cannot break down or excrete accumulate in body fat; each consumer eats many prey and inherits their combined toxin loads, so concentration rises at every level. This is why eagles and tuna carry the highest levels of 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 captured; 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 relationships |
| Pyramid of numbers | Pyramid of energy | Numbers count individuals (can invert); energy is always upright because ~90% is lost per level |
| Decomposer | Detritivore | Detritivores eat dead matter; decomposers (bacteria, fungi) chemically break it into nutrients |
| "Energy is recycled" | "Energy flows one way" | Energy is not recycled; only matter cycles |
| Bioaccumulation | Biomagnification | Buildup within one organism vs. increase up the food chain |
| Top predators are rare | Top predators are unimportant | Rare due to low energy, but ecologically powerful |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Energy flow is like "pass the snack," where the snack shrinks every time it is passed. The sun gives plants a giant basket; a mouse that eats plants keeps a small handful; a fox that eats mice gets even less, because every animal uses up most of the energy to move, grow, and stay warm. By the top of the chain, almost nothing is left — that's why there are far more plants than wolves.
Worked example
Imagine a small farm pond. Its algae and plants fix 50,000 kilocalories per year — the pond's GPP — but the producers respire away most of it, leaving an NPP of 5,000 kcal/yr for everything else in the pond.
Using the textbook 10% approximation: zooplankton (primary consumers) build about 500 kcal into their bodies; small fish (secondary consumers) get about 50 kcal; a large bass (tertiary consumer) gets about 5 kcal. Each level is a tenth of the one below, so the pyramid of energy is upright by construction, and the top is tiny — one bass needs many small fish, which is why big predators are rare and need large habitats. Now suppose the pond sits near a farm spraying a persistent pesticide: zooplankton carry trace amounts, small fish more, and the bass — having eaten hundreds of small fish — the most of all. Same food web, two consequences: scarce energy at the top, concentrated poison at the top.
Key takeaways
- Energy flows one way; matter cycles. Energy enters as sunlight and is lost as heat.
- NPP = GPP − R. Net primary productivity feeds consumers and decomposers.
- Only autotrophs add new energy to an ecosystem; all heterotrophs depend on them.
- ~10% of energy transfers between trophic levels (commonly taught approximation; actual efficiency varies).
- Pyramids of energy are always upright; biomass and numbers pyramids can invert in special cases.
- Food webs are more realistic and stable than food chains, which are short because cumulative loss leaves too little for top levels.
- Persistent toxins biomagnify — top predators (and their human consumers) get the highest concentrations.
- Eating lower on the food chain is more energy-efficient for feeding people.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What is the difference between GPP and NPP, and which one feeds the rest of the ecosystem?
Show answer
GPP is total energy captured by producers; NPP subtracts producer respiration (NPP = GPP − R) and is what feeds consumers and decomposers.
Why are energy pyramids always upright, but numbers pyramids sometimes not?
Show answer
Energy pyramids are always upright because ~90% of energy is lost as heat at each transfer. Numbers pyramids can invert because one tree can support thousands of insects.
Why do food chains rarely exceed four or five levels?
Show answer
Cumulative energy loss: with only ~10% transferred per level, after four or five transfers too little remains to support another trophic level.
How can a persistent pesticide sprayed on crops end up most concentrated in a top predator that never touched it?
Show answer
The pesticide is persistent and fat-soluble, so it accumulates in tissues (bioaccumulation). Each predator eats many prey and inherits their combined loads, so concentration rises at every level (biomagnification) — the top predator gets the highest dose.
A forest's producers fix 40,000 kcal of energy per year, and producer respiration uses 30,000 kcal. What is the NPP, and roughly how much reaches the primary consumers (using the 10% rule)?
Show answer
NPP = 40,000 − 30,000 = 10,000 kcal/yr. Primary consumers receive roughly 10% of that, about 1,000 kcal/yr.
Why is a food web more accurate than a food chain?
Show answer
Real organisms eat and are eaten by multiple species; a web captures those relationships and their stability, while a chain shows one path.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Autotroph (producer)
- Organism that builds its own food from inorganic sources
- Heterotroph (consumer)
- Organism that gets energy by eating other organisms
- Trophic level
- A feeding position in a food chain (producer, primary consumer, etc.)
- GPP
- Gross primary productivity: total energy captured by producers
- NPP
- Net primary productivity: GPP minus producer respiration
- Food web
- All interconnected feeding relationships in an ecosystem
- Keystone species
- Species whose removal causes disproportionately large changes
- 10% rule
- Commonly taught approximation that ~10% of energy transfers between levels
- Biomagnification
- Increasing concentration of persistent toxins up the food chain
- Detritivore
- Organism that eats dead organic matter (worms, vultures)
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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