An ecosystem is all the living things in an area plus the nonliving parts of their environment, such as sunlight, water, soil and climate. Ecology questions usually focus on two flows: energy, which moves one way through an ecosystem, and matter, such as carbon and nitrogen, which cycles. This guide covers both, with the food web reasoning that shows up on tests.
Roles in an ecosystem
- Producers (autotrophs): make their own food, usually by photosynthesis. Plants, algae and some bacteria. They form the base of nearly every food chain.
- Primary consumers: herbivores that eat producers, such as rabbits, grasshoppers and zooplankton.
- Secondary consumers: eat primary consumers, such as snakes that eat mice.
- Tertiary consumers: eat secondary consumers, such as hawks that eat snakes. The top predator is sometimes called an apex predator.
- Decomposers: bacteria and fungi break down dead organisms and waste, returning nutrients to the soil. Detritivores such as earthworms help by eating dead material.
Omnivores, like bears and humans, eat at more than one level.
Food chains vs. food webs
A food chain is a single path: grass → grasshopper → frog → snake → hawk. The arrows point in the direction energy flows, from the eaten to the eater. A food web links many chains together, showing that most organisms eat and are eaten by several species. Food webs are more realistic and more stable: if one food source declines, consumers can often switch to another.
Energy flow and the 10% rule
Each feeding level is a trophic level. Only about 10% of the energy at one level is passed to the next. The rest is used for the organism's own life processes (such as movement and keeping warm), lost as heat during respiration or left in parts that are not eaten or digested.
That is why food chains rarely have more than four or five levels: there is not enough energy left to support more. It is also why there are far fewer hawks than grasshoppers in an area.
Worked example: the 10% rule
A field's producers capture 50,000 kilocalories of energy. About how much reaches the tertiary consumers?
- Primary consumers: 10% of 50,000 = 5,000 kcal.
- Secondary consumers: 10% of 5,000 = 500 kcal.
- Tertiary consumers: 10% of 500 = 50 kcal.
Ecological pyramids
An energy pyramid shows energy at each trophic level and is always widest at the bottom. Pyramids of numbers and biomass usually have the same shape, though there are exceptions: one large tree can support thousands of insects, making a pyramid of numbers that is narrow at the base.
Biomagnification
Some toxins, such as mercury and the pesticide DDT, are not broken down or excreted easily. They build up in organisms and become more concentrated at each higher trophic level, because each predator eats many contaminated prey. Top predators, such as eagles and large fish like tuna, end up with the highest concentrations. DDT thinned the eggshells of birds of prey in the 20th century until it was banned in many countries.
How changes ripple through a web
Removing or adding one species can affect many others. When wolves were reintroduced to Yellowstone National Park in 1995, they reduced elk numbers and changed elk behavior, which allowed willows and aspens to recover along rivers in some areas. That in turn benefited beavers and songbirds. A species with an outsized effect like this is called a keystone species.
Matter cycles
Unlike energy, matter is recycled. In the carbon cycle, producers take in carbon dioxide during photosynthesis; consumers and decomposers release it through respiration; and burning fossil fuels releases carbon that was stored underground for millions of years. In the nitrogen cycle, bacteria convert nitrogen gas from the air into forms plants can use (nitrogen fixation), and other bacteria return it to the air.
Common mistakes
- Drawing arrows from predator to prey. Arrows show energy flow, so they point toward the eater.
- Saying energy is recycled. Energy flows one way and leaves as heat; matter cycles.
- Forgetting decomposers when describing an ecosystem.
- Assuming removing a predator always helps its prey; it can lead to overpopulation and then a crash when food runs out.
Practice questions
- In grass → mouse → owl, which organism is the secondary consumer?
- If producers store 20,000 kcal, about how much reaches secondary consumers?
- Why do top predators have the highest levels of mercury?
- What would likely happen to a rabbit population if foxes were removed?
Answers: (1) The owl. (2) About 200 kcal. (3) Biomagnification: mercury concentrates at each trophic level. (4) It would likely grow at first, overgraze its food supply and then decline.
Last reviewed 2026-10-23 by the BrainBoost team.