Feedback Loops in AP Environmental Science

Predator and prey numbers as a negative feedback loop

The standard food web example is a predator and its prey regulating each other. Snowshoe hares and Canada lynx are the textbook pair, with populations that rise and fall in cycles of roughly ten years.

Here's the loop, step by step:

• Hares increase, so lynx have more food.
• Lynx survive and reproduce more, so the lynx population grows.
• More lynx eat more hares, so the hare population drops.
• With less food, lynx numbers fall, which lets hares recover.

Each change sets off a response that pushes the system back the other way. Neither population climbs forever or crashes to zero, which is why ecologists call negative feedback stabilizing.

Positive feedback that pushes an ecosystem further off balance

In a positive loop, the response makes the original change bigger. A good food web example is an aquatic dead zone. Excess nutrients cause an algal bloom, the algae die, and decomposers break them down while using up dissolved oxygen. Fish and other animals suffocate, their bodies add more material for decomposers, and oxygen drops even further.

The course returns to positive feedback in Topic 9.5, Global Climate Change. Polar ice and snow reflect the most solar energy back to space. When warming melts them, darker surfaces absorb more energy, which causes more warming and more melting. In the Arctic, thawing tundra also releases greenhouse gases like methane, which adds more warming on top of that. These loops explain why polar regions respond faster to climate change than the rest of the planet.

Removing or adding one species: cascade or loop

Feedback loops show up in Unit 1 (The Living World: Ecosystems) under Topic 1.11, Food Chains and Food Webs. That topic asks you to describe how energy and matter move through trophic levels and to predict what happens to the rest of a web when a single species is removed or introduced.

Wolves in Yellowstone are the go-to case. After wolves were reintroduced in 1995, they preyed on elk and changed where elk grazed, and willows and aspens recovered in some areas. That chain of effects running down the trophic levels is a trophic cascade.

A cascade and a feedback loop aren't the same thing. A cascade is a one-way sequence (wolves affect elk, elk affect plants). It becomes a loop only when the effect comes back to the starting point, such as fewer elk leading to fewer wolves, which lets elk rebound. When you call something a feedback loop, show where the arrow returns.

Labeling the loop in a food web diagram or population graph

Given a graph where the lynx line peaks just after the hare line, or a food web where hares feed lynx, you're often asked to name the loop type or predict which population rises or falls next. On the multiple-choice section, that shows up in qualitative stimulus sets built around a food web diagram or an ecosystem scenario, and in quantitative sets built around population graphs.

In the free-response section, the same thinking applies in Question 1 (Design an investigation), which starts from a visual model such as a food web, and in Question 2 (Analyze and interpret quantitative data), where you might read predator and prey lines on a graph. A strong answer does three things. It names the specific populations involved, states the direction of each change (increase or decrease), and says whether the system moves back toward balance or further away. A vague answer like "the ecosystem would be affected" earns nothing.

Why "positive" doesn't mean good

Calling the melting ice loop negative because it harms species is the classic wrong answer, and so is calling the lynx and hare cycle positive because it keeps both species alive. Positive means amplifying, and negative means counteracting. The labels describe direction, not whether the outcome is healthy.

The predator and prey cycle is negative feedback even though it keeps both species going. The melting ice loop is positive feedback even though it speeds up harm to species that depend on sea ice for food and habitat. To classify a loop, ask yourself whether the response pushes the original change bigger or smaller, and ignore whether the result sounds good or bad.