---
title: "Food Web — AP Biology Definition & Exam Guide"
description: "A food web maps all the interconnected feeding relationships in an ecosystem, showing how energy flows from producers through consumers and decomposers in AP Bio Unit 8."
canonical: "https://fiveable.me/ap-bio/key-terms/food-web"
type: "key-term"
subject: "AP Biology"
unit: "Unit 8"
---

# Food Web — AP Biology Definition & Exam Guide

## Definition

In AP Biology, a food web is a network of interconnected food chains that shows the many feeding relationships and energy pathways in an ecosystem, linking producers, consumers, and decomposers across multiple trophic levels.

## What It Is

A food web is what you get when you stop pretending [energy](/ap-bio/unit-3/environmental-impacts-on-enzyme-function/study-guide/Q8PevM3BI76060aoWtit "fv-autolink") moves in a single straight line. A [food chain](/ap-bio/unit-8/energy-flow-through-ecosystems/study-guide/A1PeQD1Zy3BIMu1zSMzd "fv-autolink") is one path (grass to rabbit to fox). A food web is all those chains tangled together, because most organisms eat more than one thing and get eaten by more than one thing.

Under CED topic 8.2, a food web shows how energy flows and matter cycles through [trophic levels](/ap-bio/key-terms/trophic-levels "fv-autolink"): producers (autotrophs that capture energy from sunlight or chemicals), then primary, secondary, tertiary, and quaternary consumers, plus decomposers and scavengers that break everything back down. The arrows in a food web always point in the direction energy travels, from the thing being eaten to the eater. Remember the rule that comes with it: only about 10% of the energy at one trophic level makes it to the next, so the higher up the web you go, the less energy is available.

## Why It Matters

Food webs live in [Unit 8](/ap-bio/unit-8 "fv-autolink"): Ecology, specifically topic 8.2 Energy Flow Through Ecosystems. They're the visual backbone for [[AP Bio](/ap-bio "fv-autolink") 8.2.B], which asks you to explain how energy flows and matter cycles through trophic levels, and for [AP Bio 8.2.C], which is about how changes in energy availability ripple through populations, communities, and ecosystems. The CED literally lists food chains/webs and trophic pyramids as illustrative examples. If you understand a food web, you can predict what happens when one piece changes: knock out the producers and every level above them shrinks. That cause-and-effect reasoning is exactly what 8.2.C wants you to do.

## Connections

### Decomposers and Decomposition (Unit 8)

Food web diagrams often forget the cleanup crew, but [decomposers](/ap-bio/key-terms/decomposer "fv-autolink") are what close the loop. They break down dead organisms at every trophic level and return matter to the environment, which is why energy flows one way but matter cycles.

### [Biogeochemical Cycles (Unit 8)](/ap-bio/key-terms/biogeochemical-cycles)

A food web shows energy moving through organisms, but the atoms in that food, like carbon and nitrogen, are being recycled through the [carbon cycle](/ap-bio/key-terms/carbon-cycle "fv-autolink") and other biogeochemical cycles at the same time. Energy is a one-way street; matter goes in circles.

### Biomass and Trophic Pyramids (Unit 8)

The 10% rule from food webs is why a [trophic pyramid](/ap-bio/key-terms/trophic-pyramid "fv-autolink") is shaped like a pyramid. Each level holds less biomass than the one below it because so much energy is lost as heat with every transfer.

### Autotrophs and Chemosynthesis (Unit 8)

Every food web starts with an autotroph capturing energy. Usually that's a photosynthetic producer using sunlight, but in a deep-sea hydrothermal vent web, chemosynthetic bacteria do the job using inorganic molecules with no sunlight at all ([AP Bio 8.2.D]).

## On the AP Exam

On MCQs, food webs show up as diagrams or tables where you trace energy and predict consequences. A 2017 Short FRQ (Q4) gave a table showing how much each organism relied on different food sources and asked you to work out the feeding relationships from data. Practice questions push the same skills: figuring out where a bioaccumulated toxin would be most concentrated (answer: the top consumers, since toxins build up as you move up the web), and predicting the immediate consequence when an algal bloom blocks light and starves the producers at the base. What you actually have to DO: read the web, follow the arrows, and reason about what changes when one level grows or collapses.

## food web vs food chain

A food chain is a single linear path of who-eats-whom (one producer, one path up to a top consumer). A food web is many of those chains overlapping, because real organisms eat from and feed into multiple chains. If a diagram has branching arrows, it's a web; if it's one straight line, it's a chain.

## Key Takeaways

- A food web is a network of overlapping food chains that shows all the feeding relationships and energy pathways in an ecosystem.
- Arrows in a food web point from the organism being eaten to the organism eating it, tracing the direction energy flows.
- Only about 10% of energy passes from one trophic level to the next, so higher trophic levels hold less energy and biomass.
- Fat-soluble toxins bioaccumulate and reach their highest concentrations in top consumers at the upper end of the web.
- Energy flows one way through a food web while matter cycles back through decomposers and biogeochemical cycles.
- Reducing producers at the base of a web shrinks the number and size of every trophic level above them ([AP Bio 8.2.C]).

## FAQs

### What is a food web in AP Biology?

A food web is a network of interconnected food chains that shows the multiple feeding relationships and energy pathways in an ecosystem, from autotrophic producers up through consumers and decomposers. It's a topic 8.2 concept in Unit 8: Ecology.

### Is a food web the same as a food chain?

No. A food chain is a single straight path (grass to rabbit to fox), while a food web is many chains overlapping because most organisms eat and are eaten by more than one species. A web has branching arrows; a chain is one line.

### Where does a toxin end up highest in a food web?

In the top consumers. Many toxins bioaccumulate and biomagnify, meaning their concentration increases at each higher trophic level, so the organisms at the top of the web carry the most.

### Why does a food web shrink if you remove the producers?

Producers capture the energy that feeds every level above them, so removing them cuts off the energy supply. Per [AP Bio 8.2.C], a drop in producers reduces the number and size of every higher trophic level.

### Do food webs always start with sunlight?

Not always. Most start with photosynthetic producers using sunlight, but in deep-sea hydrothermal vent ecosystems, chemosynthetic bacteria start the web by capturing energy from inorganic molecules with no sunlight at all.

## Related Study Guides

- [8.2 Energy Flow Through Ecosystems](/ap-bio/unit-8/energy-flow-through-ecosystems/study-guide/A1PeQD1Zy3BIMu1zSMzd)

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