AP Environmental Science

## Learning Outcomes

- Explain how **negative feedback loops** and **positive feedback loops** work in food webs
- Distinguish between **top-down control** and **bottom-up control**
- Describe how predator-prey interactions regulate populations over time
- Explain what a **trophic cascade** is and why it matters in natures
- Distinguish between a **keystone species** and an **apex predator**
- Apply food-web reasoning to AP-style questions about indirect effects and environment change

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand feedback loops in food webs, with a focus on the following syllabus points:

- Know that a **food chain** is a linear pathway of energy transfer, while a **food web** is a network of interconnected feeding relationships
- Identify how **negative feedback** in predator-prey systems can stabilize populations around environmental limits
- Explain that predator populations often respond to prey changes with a **lag time**
- Understand **top-down control** as regulation by predators and **bottom-up control** as regulation by resources such as light, water, and nutrients
- Explain how adding or removing a top predator can trigger a **trophic cascade**
- Know that a **keystone species** has a disproportionately large ecological effect relative to its abundance
- Distinguish clearly between a **keystone species** and an **apex predator**
- Apply food-web reasoning to examples such as sea otters, sea urchins, kelp, and wolf-driven cascades

## 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. How does a negative feedback loop operate in a predator-prey system?
3. What is the difference between top-down control and bottom-up control?
4. How can removing a predator cause a trophic cascade?
5. Why is a keystone species not always the same thing as an apex predator?

## Introduction

Food webs are not just lists of who eats whom. They are dynamic systems in which a change in one population can spread through several trophic levels. AP Environmental Science often tests this through cause-and-effect reasoning: if one species increases, decreases, is removed, or gains more resources, what happens next?

Feedback loops are one of the best ways to understand these changes. In food webs, the most important pattern is usually **negative feedback**, because natures often regulate populations rather than allowing unlimited growth. But students also need to recognize that some changes can amplify themselves, creating **positive feedback** and pushing the system farther from its starting point.

> **Key Term: feedback loop**
> A process in which a change in one part of a system causes effects that influence the original change.

## Introduction to food-web feedbacks

### Food chains, food webs, and why feedback matters

A **food chain** is a single pathway of energy transfer. A simple example is producer → herbivore → predator. This is useful for showing trophic levels, but it is too simple to describe real food webss. Most organisms eat more than one thing and are eaten by more than one thing.

A **food web** is more realistic because it shows many interconnected feeding relationships at once. That matters for feedback loops because a change in one species rarely affects only one other species. Instead, the change can spread across the network.

> **Key Term: food chain**
> A linear pathway of energy transfer from one organism to another.
>
> **Key Term: food web**
> A network of interconnected food chains that shows feeding relationships more realistically.

In APES, this matters because exam questions often describe a disturbance and ask for indirect effects. If students think only in a straight chain, they miss how one population shift can affect multiple consumers, competitors, and producers.

Energy flow also explains why food-web effects are often strongest at lower trophic levels. Producers form the base, and only about **10%** of energy is passed to the next trophic level on average. That means higher trophic levels depend on a relatively small energy supply and can be sensitive to changes below them.

A food web may be more stable than a single food chain because organisms sometimes have alternative food sources. If one prey species declines, a predator may switch to another prey. This buffering effect can reduce collapse, though it does not prevent all disruption.

> **Test Tip:** If a question asks why a food web is more realistic or more stable than a food chain, focus on **multiple feeding links**, **alternative prey**, and **buffering against disturbance**.

### Negative and positive feedback in natures

A **negative feedback loop** happens when a change triggers responses that counteract the original change. In ecology, "negative" does not mean harmful. It means stabilizing.

> **Key Term: negative feedback loop**
> A process in which a change in a system triggers responses that oppose the original change and tend to stabilize the system.

A classic example is predator-prey regulation:

- prey population increases
- more food becomes available to predators
- predator population rises after a delay
- increased predation lowers the prey population
- reduced prey availability later lowers predator numbers

This pattern does not keep both populations perfectly constant. Instead, it usually creates oscillations around an equilibrium or around the carrying capacity set by available resources.

A **positive feedback loop** happens when a change causes effects that increase the original change. In ecology, "positive" does not mean beneficial. It means self-amplifying.

> **Key Term: positive feedback loop**
> A process in which a change in a system triggers responses that amplify the original change.

In APES food-web content, the strongest standard examples are negative feedback loops. Positive feedback is often easier to show in broader Earth-system examples, such as melting ice lowering albedo and causing more warming. Within food webs, a population surge caused by abundant resources can act as a positive feedback if more individuals produce more offspring and population growth accelerates.

Students often overuse "positive feedback" when they simply mean "one thing causes another." That is not enough. To be a feedback loop, the later effect must circle back to influence the starting condition.

## Predator-prey regulation and food-web control

### Predator-prey cycles, lag time, and carrying capacity

Predator-prey systems are one of the clearest examples of food-web feedback. Suppose rabbits increase because plant growth is strong after a wet season. More rabbits means more available energy for foxes. But foxes do not increase instantly. They need time to survive longer, reproduce, and raise young. This delay is called **lag time**.

> **Key Term: carrying capacity**
> The maximum population size an environment can sustain over time.

The prey population usually peaks first. Then the predator population peaks later. This is a common exam graph pattern. Students should not assume predator and prey lines rise and fall at the same moment.

Why does this system regulate population size? Because predation is a **density-dependent** limiting factor. When prey are abundant, predators can feed more easily and their effects become stronger. When prey are scarce, predator numbers fall because food is limited.

This links food webs to carrying capacity. No population can remain above carrying capacity for long. Limiting factors such as food supply, water, disease, and predation push populations back down. Predator-prey interactions therefore act as part of a negative feedback system that keeps populations within environmental limits.

Worked sequence:

1. grass growth increases
2. rabbit population rises
3. foxes find food more easily
4. fox population rises after a lag
5. more rabbits are eaten
6. rabbit population falls
7. fox food supply declines
8. fox population falls later
9. rabbit population may recover

The key error students make is skipping steps 3 and 7. APES rewards linked reasoning, not just "more foxes, fewer rabbits."

> **Test Tip:** On a population graph, if predators peak after prey, explain the delay using **reproduction time** and **response to food availability**. The word **lag** is high value.

### Top-down control and bottom-up control

Food webs can be regulated from the top or from the bottom.

> **Key Term: top-down control**
> Regulation of lower trophic levels by consumers or predators at higher trophic levels.
>
> **Key Term: bottom-up control**
> Regulation of higher trophic levels by resource availability, such as nutrients, light, water, and producer biomass.

In **top-down control**, predators suppress herbivores, which can allow producers to increase. The direction of control is from consumers downward through the web.

Example:

- more wolves
- fewer elk
- less overgrazing
- more vegetation

In **bottom-up control**, resource supply at the producer level determines what happens higher up. More nutrients or more sunlight can increase producer growth, which can then support more herbivores and predators.

Example:

- more nitrogen and phosphorus in a lake
- more algae growth
- more zooplankton food
- potential rise in higher consumers, at least at first

These two controls are not mutually exclusive. A single community can experience both. A kelp forest may depend on nutrient availability from below and predator regulation from above. The exam may present a scenario and ask which type is more important in that case.

Clues for **top-down control**:

- predator removal or reintroduction
- herbivore release
- producer recovery after predator return

Clues for **bottom-up control**:

- fertilizer input
- water or light change
- primary productivity shift
- producer biomass drives change upward

## Trophic cascades and keystone effects

### Trophic cascades in action

A **trophic cascade** is an indirect chain of effects across trophic levels, usually caused by changes at or near the top of the food web.

> **Key Term: trophic cascade**
> An indirect chain of ecological effects across multiple trophic levels, often triggered by the addition or removal of a predator.

A standard example is the sea otter–urchin–kelp system:

- kelp are producers
- sea urchins are herbivores that graze on kelp
- sea otters prey on sea urchins

If otter numbers are high:

- otters reduce urchin numbers
- less kelp grazing occurs
- kelp forests expand

If otters decline:

- urchin numbers rise
- more kelp is eaten
- kelp declines
- species that depend on kelp habitat can also decline

This is not just a direct effect of otters on urchins. It is an **indirect effect** of otters on kelp. That indirect pathway is what makes it a trophic cascade.

Another standard case is the wolf example used in ecology discussions. Where large grazers are unchecked, vegetation can be heavily reduced. When predators return, prey numbers and behavior can shift, which can reduce browsing pressure and allow plant recovery. That plant recovery can then affect other species.

Students often oversimplify trophic cascades into "predator up, prey down, plants up." That is useful as a starting pattern, but a strong answer explains the mechanism: predation changes prey abundance and sometimes prey behavior, which then changes pressure on the next trophic level.

### Keystone species and apex predators

A **keystone species** is defined by ecological impact, not by abundance.

> **Key Term: keystone species**
> A species with a disproportionately large effect on community structure relative to its abundance.

An **apex predator** is a predator at the top of a food web with no natural predators in that habitat.

> **Key Term: apex predator**
> A top predator with no natural predators in its habitat.

These terms overlap, but they are not identical.

Comparison:

- **Apex predator** describes trophic position
- **Keystone species** describes ecological effect

So:

- some apex predators are keystone species
- not all apex predators are keystone species
- some keystone species are not apex predators

Sea otters are a classic **keystone species** because relatively few otters can strongly influence urchins, kelp, and associated biodiversity. A predator can sit at the top of a food web without having that same large system-wide-wide effect.

This distinction matters because AP questions may ask students to justify the term they use. If a species is simply the highest predator, say apex predator. If its impact is disproportionately large relative to abundance, say keystone species.

A strong justification for keystone status points to broad food-web consequences after removal:

- community structure changes
- biodiversity falls
- species interactions shift
- no other species fills the same role effectively

## Key Point Checklist

_This article has covered the following key knowledge points:_

- A food chain is a linear pathway of energy transfer, while a food web is a network of interconnected feeding relationships
- Arrows in food chains and food webs point toward the organism receiving the energy
- Negative feedback loops counteract change and tend to stabilize natural systemss
- Predator-prey interactions are a classic example of negative feedback in food webs
- Predator populations often respond to prey increases with a lag time
- Predation is a density-dependent limiting factor linked to carrying capacity
- Top-down control occurs when predators regulate lower trophic levels
- Bottom-up control occurs when resource availability regulates the food web from the producer level upward
- A trophic cascade is an indirect chain of effects across at least three trophic levels
- The sea otter–urchin–kelp relationship is a standard trophic cascade example
- A keystone species is defined by disproportionately large ecological impact, not simply by being a top predator
- An apex predator is not always a keystone species, though some species can be both

## Key Terms and Concepts

- feedback loop
- food chain
- food web
- negative feedback loop
- positive feedback loop
- carrying capacity
- top-down control
- bottom-up control
- trophic cascade
- keystone species
- apex predator


Food Chains and Food Webs - Feedback loops in food webs