How do food chains combine into food webs, and what happens when one species is removed?
Topic 1.11 Food Chains and Food Webs: describe how food chains and food webs represent the flow of energy and matter, and predict the effects of changes to a food web.
A focused answer to APES Topic 1.11, covering food chains and food webs, how energy and matter flow through them, keystone species, trophic cascades, and predicting the effects of removing a species, with a worked food-web disruption question.
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What this topic is asking
The College Board (Topic 1.11) wants you to describe how food chains and food webs represent the flow of energy and matter, and to predict the consequences of changes to a web, such as removing a species. The big ideas are that webs are interconnected, that some species (keystone species) matter far more than their numbers suggest, and that disturbances can ripple through the web as trophic cascades.
Food chains and food webs
A food web is more realistic than a single chain because most organisms eat, and are eaten by, several others. This interconnection gives ecosystems resilience but also means disturbances can spread.
Energy and matter in the web
Keystone species
Classic examples include the sea otter (which controls sea urchins and so protects kelp forests) and top predators such as wolves. Keystone species often hold a web together by controlling the populations beneath them.
Predicting the effects of change
Because a food web is interconnected, removing or adding a species sets off knock-on effects. When a top predator is removed, its prey can increase, and that increase then reduces the prey's own food source, an indirect effect known as a trophic cascade. The classic kelp-forest example runs: remove sea otters, sea urchins boom, urchins overgraze the kelp, and the kelp forest collapses, taking with it the many species that depended on it. Cascades can also work the other way, with the return of a predator restoring a community. This predictive reasoning is exactly what the exam asks for: trace the arrows, decide which populations rise and which fall, and follow the effect through the web. It is also the bridge from Unit 1 to Unit 2, because the loss of a keystone species is one of the most powerful ways biodiversity, and the ecosystem services that depend on it, can be lost.
Try this
Q1. Describe one advantage of a food web over a food chain for representing an ecosystem. [1 point]
- Cue. A food web shows the many interconnected feeding relationships, which is more realistic than a single linear chain because most organisms have several food sources and predators.
Q2. Explain why losing a keystone species can transform an ecosystem. [2 points]
- Cue. A keystone species has effects far larger than its abundance; removing it (for example a top predator) lets its prey boom and triggers a cascade that changes populations throughout the web.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). A food web includes sea otters, sea urchins and kelp. Sea otters eat sea urchins, and sea urchins eat kelp. (a) Describe the difference between a food chain and a food web. (b) Predict what happens to the sea urchin and kelp populations if sea otters are removed. (c) Explain why the sea otter can be considered a keystone species. (d) Define a trophic cascade using this example.Show worked answer →
A 4-point FRQ on food-web dynamics.
(a) Describe (1 point): a food chain is a single linear pathway of energy from one organism to the next; a food web is the network of many interconnected food chains showing all the feeding relationships in a community.
(b) Predict (1 point): without otters, sea urchins increase (no longer eaten), and the larger urchin population overgrazes the kelp, so kelp declines or disappears.
(c) Explain (1 point): the sea otter is a keystone species because its presence has a large effect on the whole community relative to its abundance, controlling urchins and so allowing the kelp forest and the species that depend on it to persist.
(d) Define (1 point): a trophic cascade is a chain of indirect effects passing down the food web when a top predator is changed, here otters affecting urchins affecting kelp.
Markers reward the chain-versus-web distinction, the urchin-up and kelp-down prediction, the keystone definition applied to otters, and a correct trophic-cascade definition.
AP 2020 (style)1 marksSection I (multiple choice). The removal of a top predator causes its prey to increase and the prey's food source to decrease. This series of indirect effects is best described as (A) resource partitioning (B) a trophic cascade (C) primary succession (D) eutrophication. Justify your choice.Show worked answer →
A 1-point MCQ on food-web effects. The answer is (B).
A trophic cascade is a chain of indirect effects that ripples through trophic levels when a predator (or other key species) is changed. Resource partitioning (A) is how competitors divide resources; primary succession (C) is colonization of bare ground; eutrophication (D) is nutrient enrichment of water. The trap is confusing a trophic cascade with a direct predator-prey effect; the cascade includes the indirect knock-on effect further down the web.
Related dot points
- Topic 1.9 Trophic Levels: describe the trophic levels of an ecosystem and explain the roles of producers, consumers and decomposers in transferring energy and matter.
A focused answer to APES Topic 1.9, covering producers, primary, secondary and tertiary consumers, decomposers and detritivores, autotrophs and heterotrophs, and how energy and matter move through trophic levels, with a worked classification question.
- Topic 1.10 Energy Flow and the 10% Rule: explain how energy is lost between trophic levels, apply the 10% rule, and calculate energy transfer and ecological efficiency.
A focused answer to APES Topic 1.10, covering the one-way flow of energy, the 10% rule, why energy is lost as heat and through respiration, ecological efficiency, and energy pyramids, with full worked multi-level energy calculations.
- Topic 1.8 Primary Productivity: define gross and net primary productivity, explain the factors that control them, and calculate net primary productivity from data.
A focused answer to APES Topic 1.8, covering gross and net primary productivity, respiration, the GPP-NPP relationship, limiting factors, productivity across biomes, and ecological efficiency, with a full worked NPP calculation.
- Topic 1.1 Introduction to Ecosystems: explain how species interactions, including predation, symbiosis and competition, shape ecosystems and influence the survival of organisms.
A focused answer to APES Topic 1.1, covering ecosystems, predator-prey relationships, the three symbioses (mutualism, commensalism, parasitism), competition and resource partitioning, with a worked FRQ on interpreting interaction data.
- Topic 2.7 Ecological Succession: distinguish primary and secondary succession, describe how communities change over time, and explain the roles of pioneer, keystone and indicator species.
A focused answer to APES Topic 2.7, covering primary and secondary succession, pioneer species, the path to a climax community, keystone and indicator species, and the effects of succession on biomass and biodiversity, with a worked succession-sequencing question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)