1.11 Food Chains and Food Webs

A food chain shows a single, linear path of energy and nutrients from producers up through consumers, while a food web links many overlapping food chains to model how energy and nutrients actually move through an ecosystem. Because everything is connected, removing or adding one species can ripple through the whole web through positive and negative feedback loops.

Why This Matters for the AP Environmental Science Exam

This topic builds your ability to read and describe models, which is a core science skill in AP Environmental Science. Food webs and food chains are exactly the kind of visual representations you may be asked to interpret, and you should be able to identify members by trophic level and explain how energy and nutrients flow through them. You should also be ready to predict what happens to the rest of a food web when a species is removed or added, using feedback loops to support your reasoning.

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Key Takeaways

• A food chain is a linear model of who eats whom; a food web is a model of interlocking food chains that shows energy and nutrient flow through two or more chains.
• Organisms are grouped by trophic level: producers, primary consumers, secondary consumers, tertiary consumers, plus decomposers and scavengers.
• Food webs are more accurate than food chains because most organisms eat more than one thing and are eaten by more than one predator.
• Removing or adding a species can affect the entire food web, sometimes triggering a trophic cascade.
• Negative feedback loops push a system back toward balance; positive feedback loops amplify change and can destabilize an ecosystem.
• "Positive" and "negative" describe whether feedback increases or decreases change, not whether it is good or bad.

Energy Transfers in Ecosystems

A food chain shows how organisms in an ecosystem depend on each other for food and how energy is transferred in a straight line. In a grassland, grass is a producer that uses sunlight to make energy through photosynthesis. A rabbit eats the grass, making it a primary consumer. A hawk eats the rabbit, making the hawk a secondary consumer.

A food chain is an oversimplified model, though. In real ecosystems, an organism often eats several kinds of prey or gets eaten by several different predators, sometimes at different trophic levels. If both owls and hawks eat the rabbit in our grassland, a single chain cannot show two predators feeding on the same organism.

To represent energy transfers more accurately, we use food webs. A food web is a model of interlocking food chains that shows all the overlapping trophic interactions between species in an ecosystem. Because populations at any level can affect other species, food webs let you see how a change in one place can spread.

Trophic Cascades

A trophic cascade is a chain of effects that happens when a change at one trophic level, often near the top, impacts populations at lower levels. For example, in a marine ecosystem, if the sea otter population (a predator) increases, the sea urchin population (their prey) decreases. Fewer urchins means more kelp, since urchins graze on kelp. More kelp provides more habitat for other species, creating a ripple effect through the ecosystem.

Trophic cascades can have positive or negative effects on an ecosystem, and they can be triggered by natural events or human activity. Understanding how species interact is an important part of conserving and managing ecosystems.

The diagram below shows an example of a food web from Lake Ontario. Primary producers are green, primary consumers are orange, secondary consumers are blue, and tertiary consumers are purple.

Response to Disturbance

Feedback Loops

Natural systems try to stay in a steady state by responding to changes in their inputs and outputs. These responses are called feedback. There are two types: negative feedback loops and positive feedback loops.

Negative Feedback Loops

Negative feedback loops are more common. They happen when a system responds to a change by trying to return to its original state, or by slowing the rate of change. For example, if drought causes a lake to shrink, the smaller surface area leads to less evaporation. When there is more precipitation, the lake grows, which increases evaporation. The lake tends to stay about the same size.

Negative feedback applies to food webs too, where it helps maintain balance and stability.

Take a population of herbivores, like deer. If the deer population grows, they eat more plants, which decreases the plant population. With less food available, the deer population then decreases. This back-and-forth keeps any single species from becoming too abundant and disrupting the ecosystem.

Positive Feedback Loops

Positive feedback loops are the ones scientists tend to worry about, because the change keeps increasing. For example, rising temperatures cause ice caps to melt. As darker ground and water are exposed, more sunlight is absorbed instead of reflected, which causes even more melting. Positive feedback loops are very hard to slow once they start.

In a food web, a positive feedback loop happens when a change in one part of the ecosystem triggers a response that amplifies the original change. This can make a system unstable and more likely to undergo major shifts.

Invasive species can set off this kind of loop. As an application of this idea, brown tree snakes arrived on the island of Guam, likely as stowaways in ship cargo. With no natural predators there, the snake population grew exponentially and wiped out many of Guam's native bird species. That initial change set off a series of further changes that reinforced and amplified the original impact across the ecosystem.

The words positive and negative can be confusing, but they do not mean one type of feedback is bad and the other is good. In fact, negative feedback loops usually point to healthy, stable environments. Remember it this way: positive feedback loops cause more change, while negative feedback loops cause less change.

When one species is removed from or added to a specific food web, the rest of the food web can be affected.

How to Use This on the AP Environmental Science Exam

MCQ

Expect questions that hand you a food web diagram and ask you to identify trophic levels or trace energy flow. Match each organism to its role: producer, primary consumer, secondary consumer, tertiary consumer, decomposer, or scavenger. Remember that arrows point in the direction energy flows, from the organism being eaten toward the one eating it.

Free Response

If asked what happens when a species is removed or added, name the affected species step by step and explain the direction of each effect. State whether the situation describes a trophic cascade and whether the feedback involved is positive or negative. Tie your explanation back to energy and nutrient flow rather than just listing organisms.

Common Trap

When describing positive feedback, do not say it is "good." Positive means the change keeps growing, which is often harmful. Make sure your answer shows the change amplifying itself, not returning to balance.

Common Misconceptions

• "Positive feedback is good and negative feedback is bad." The labels only describe whether feedback increases or decreases change. Negative feedback usually keeps ecosystems stable.
• "A food web and a food chain are the same thing." A food chain is one linear path; a food web links many chains together to show how energy and nutrients really move.
• "Energy in a food web moves in both directions." Energy flows one way, from producers up to higher trophic levels. Following the arrows shows that direction.
• "Removing one species only affects what it ate or what ate it." A single change can ripple across the whole web through connected chains and feedback loops.
• "Trophic cascades only come from natural causes." Both natural events and human activities can set off a cascade.

Related AP Environmental Science Guides

• 1.1 Introduction to Ecosystems
• Unit 1 Overview: The Living World: Ecosystems
• 1.2 Terrestrial Biomes
• 1.4 The Carbon Cycle
• 1.5 The Nitrogen Cycle
• 1.3 Aquatic Biomes