AP Environmental Science

## Learning Outcomes

- Understand why a **food web** is a more realistic representation model than a food chain
- Identify producers, consumers, detritivores, and decomposers within a food web
- Explain how energy moves through trophic levels and why less is available higher up
- Interpret arrow direction correctly in food web diagrams
- Apply the **10% rule** to estimate energy transfer between trophic levels
- Predict how changes to one species can affect other parts of a food web
- Distinguish clearly between one-way energy flow and nutrient cycling

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand the food web model, with a focus on the following syllabus points:

- Know that a **food chain** shows one linear feeding pathway, while a **food web** shows many interconnected pathways
- Identify **trophic levels**, including producers, primary consumers, secondary consumers, and tertiary consumers
- Understand that arrows in a food web show the direction of **energy transfer** from food source to consumer
- Explain why producers form the base of food webs and contain the greatest available energy
- Apply the **10% rule** to estimate energy available at higher trophic levels
- Understand that biomass and usually population size decrease at higher trophic levels
- Explain how the loss or decline of one species can alter energy flow through the rest of the web
- Distinguish between **energy flow** through food webs and **matter cycling** through habitatss

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between a food chain and a food web?
2. In a food web, which way do the arrows point?
3. Why do higher trophic levels usually contain less biomass?
4. How much energy is typically transferred from one trophic level to the next?
5. What could happen to other organisms in a food web if a major producer declines?

## Introduction

Food webs are one of the most tested ecological models in AP Environmental Science because they combine several core ideas at once: feeding relationships, trophic levels, energy transfer, and community response response to change. Students are often comfortable naming organisms in a simple chain, but AP questions usually go further. They ask you to interpret a more realistic network, track energy through multiple pathways, and predict what happens when one species rises or falls.

![Interconnected feeding relationships among producers, consumers, detritivores, and decomposers, with arrows indicating energy transfer across trophic levels.](https://static.pastpaperhero.com/images/ap-environmental-science-food-chains-and-food-webs-food-web-model_00.png)

_Food web structure links multiple trophic pathways and shows energy moving from food sources to consumers within an ecological community._

A food web matters because natural systemss are not built from single straight lines. Most organisms eat more than one thing, and many are eaten by more than one predator. That means the structure of a food web helps explain why natural systemss can sometimes absorb change and why, in other cases, one disruption can spread widely.

> **Key Term: food web**
> A network of interconnected food chains showing multiple feeding relationships and pathways of energy transfer in an habitat.

### From food chain to food web: why the model matters

A **food chain** is useful as a starting model. It shows a single pathway such as grass → rabbit → fox. This helps students identify trophic levels clearly. The problem is that real habitatss almost never work in just one line.

A rabbit may eat several plant species, not just grass. A fox may eat rabbits, mice, birds, and carrion. A hawk may eat snakes and rodents. Once you add these extra connections, the model stops being a chain and becomes a web.

> **Key Term: food chain**
> A linear sequence showing how energy passes from one organism to another through feeding.

The key reason the food web model is better is that it captures ecological choice and dependency. Organisms do not usually depend on one exact food source. This matters when conditions change. If one plant species declines, a herbivore may survive by switching to another producer. If a predator has several prey options, it may remain in the community even after one prey population falls.

That does not mean every food web is equally stable. A web with more connections often has more alternative pathways for energy flow, but AP answers should be careful. Do not claim that “more complex always means stable” without context. If a web depends heavily on one producer or one key prey species, even a complex web can be badly disrupted.

A simple grassland example shows the difference:

- Food chain: grass → grasshopper → frog → snake
- Food web: grass and wildflowers feed grasshoppers, rabbits, and mice; frogs eat grasshoppers; snakes eat frogs and mice; hawks eat snakes, mice, and rabbits

In the second model, one change can affect several species at once, and one species may occupy more than one link in the network.

> **Test Tip:**
> If an AP question shows many organisms with crossing arrows, do not reduce it mentally to one chain. Look for multiple routes by which energy can reach the same consumer.

### Trophic levels and feeding roles inside a food web

To read a food web well, you must classify organisms by feeding role. The first trophic level is made of **producers**, also called **autotrophs**. These organisms capture energy, usually from sunlight, and convert it into chemical energy by photosynthesis. In terrestrial systems, producers are plants. In aquatic systems, they are often algae or phytoplankton.

> **Key Term: trophic level**
> The feeding position an organism occupies in a food chain or food web.
> **Key Term: producer (autotroph)**
> An organism that makes its own food, usually by photosynthesis, and forms the base of a food web.

The second trophic level includes **primary consumers**, which eat producers. These are herbivores such as rabbits, deer, zooplankton, or grasshoppers.

The third trophic level includes **secondary consumers**, which eat primary consumers. Frogs eating insects or fish eating zooplankton fit here.

The fourth trophic level often includes **tertiary consumers**, which eat secondary consumers. Examples include snakes eating frogs or larger fish eating smaller predatory fish.

> **Key Term: consumer (heterotroph)**
> An organism that gets energy by eating other organisms.

Two complications often appear in AP questions.

First, some organisms are **omnivores**. They eat both plants and animals, so they may occupy more than one trophic level. A bear eating berries acts as a primary consumer. The same bear eating fish acts at a higher level. Students lose marks when they assume every organism fits into only one box all the time.

Second, decomposing pathways matter. **Detritivores** ingest dead organic matter. **Decomposers** break down dead material chemically and return nutrients to the environment. They are essential to the environment, but they are not simply the “top” of the food web. Their main role is nutrient recycling, not acting as apex consumers.

> **Key Term: detritivore**
> An organism that feeds on dead organic matter such as leaf litter or animal remains.
> **Key Term: decomposer**
> An organism, often bacteria or fungi, that breaks down dead organic matter and recycles nutrients back into the environment.

A common student mistake is to confuse energy transfer with nutrient recycling here. Decomposers do not make lost energy available again. Energy has already been dissipated, mostly as heat. What gets recycled is matter, such as carbon, nitrogen, and phosphorus.

### How to read arrows and track energy through a food web

The most common error with food webs is arrow direction. Many students think arrows point from predator to prey because they imagine the predator “going after” the prey. In ecology, arrows show the direction of **energy flow**.

That means the arrow points from the organism being eaten to the organism that eats it.

- grass → rabbit means energy moves from grass to rabbit
- rabbit → fox means energy moves from rabbit to fox

> **Key Term: energy flow**
> The one-way movement of energy through an food web, from producers to consumers and eventually lost as heat.

This rule matters because AP questions may ask you to identify what would directly affect a consumer. You should look backward along incoming arrows to find its food sources. If several arrows point toward one organism, that consumer has several energy inputs.

Suppose a web shows:

- algae → zooplankton
- algae → snail
- zooplankton → small fish
- snail → small fish
- small fish → heron

Here, the heron depends indirectly on algae through two pathways. If algae productivity drops, that reduction may spread to both zooplankton and snails, then to small fish, then to the heron.

This is why food webs are useful for cause-and-effect reasoning. You can track both direct and indirect effects. Direct effects are immediate feeding links. Indirect effects travel through intermediate species.

For example, if a predator declines:

1. Its prey may increase
2. The prey may consume more producers
3. Producer biomass may decrease
4. Other herbivores that depend on those producers may also decline

That is the logic behind a **trophic cascade**.

> **Key Term: trophic cascade**
> A chain reaction in a food web caused by changes at one trophic level affecting levels below it.

A well-known example is kelp → sea urchin → sea otter. If otters decline, urchins increase. If urchins increase, kelp declines. If kelp declines, many organisms that depend on kelp habitat also decline.

> **Test Tip:**
> In food web questions, trace effects step by step. Avoid jumping straight from “species X decreases” to a final outcome without showing the links in between.

### Energy transfer, the 10% rule, and why food webs narrow upward

A food web is not just about who eats whom. It is also about how much energy remains available at each step. The key APES principle is the **10% rule**: on average, only about 10% of the energy at one trophic level is transferred to the next.

> **Key Term: 10% rule**
> The principle that only about 10% of the energy at one trophic level is transferred to the next trophic level.

Most of the other 90% is used for life processes or lost as heat. Organisms use energy for respiration, movement, maintaining body temperature, growth, repair, and reproduction. Some biomass is also not eaten, and some eaten material is not fully digested.

That is why higher trophic levels support less biomass and fewer organisms. A producer level may store a large amount of energy, but only a small fraction reaches herbivores, and an even smaller fraction reaches predators.

Here is a worked example:

Suppose producers contain **10,000 kilocalories** of available energy.

- Primary consumers receive 10% of 10,000 = **1,000 kilocalories**
- Secondary consumers receive 10% of 1,000 = **100 kilocalories**
- Tertiary consumers receive 10% of 100 = **10 kilocalories**

So after only three transfers, available energy has fallen from 10,000 to 10.

This helps explain three patterns:

- **energy** decreases at higher trophic levels
- **biomass** usually decreases at higher trophic levels
- **population size** usually decreases at higher trophic levels

Students should also connect this to food chain length. Food chains are usually short because there is not enough usable energy to support many higher levels.

One more subtle point: biomass pyramids can sometimes look unusual, especially in aquatic systems where phytoplankton reproduce fast but do not maintain large standing biomass. But energy pyramids are always upright because usable energy decreases with each transfer.

Another related APES idea is the link to primary productivity. The amount of energy producers capture sets the upper limit for the whole web. If producer productivity falls, less energy is available to every higher level.

Finally, remember the major distinction:

- Energy flows one way
- Matter cycles

Carbon, nitrogen, water, and phosphorus move through biogeochemical cycles. Energy does not cycle back through the food web once lost as heat.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A food web is a network of interconnected food chains and is more realistic than a single linear chain
- A food chain shows one pathway, while a food web shows multiple feeding relationships
- Producers or autotrophs form the base of most food webs by capturing solar energy
- Primary consumers eat producers, while secondary and tertiary consumers eat other consumers
- Detritivores ingest dead organic matter, while decomposers break it down chemically
- Arrows in a food web point from the organism being eaten to the organism that eats it
- Arrows show the direction of energy transfer, not the direction of movement
- Energy decreases at each trophic level because much of it is lost as heat and through metabolism
- The 10% rule means only about 10% of energy is transferred to the next trophic level
- Higher trophic levels usually support less biomass and fewer organisms
- Omnivores may occupy more than one trophic level depending on what they eat
- Energy flows one way through food webss, while nutrients and matter cycle

## Key Terms and Concepts

- food web
- food chain
- trophic level
- producer (autotroph)
- consumer (heterotroph)
- detritivore
- decomposer
- energy flow
- trophic cascade
- 10% rule


Food Chains and Food Webs - Food web model