Food Chains & Food Webs in Ecosystems (College Board AP® Environmental Science): Study Guide

What are food chains & food webs?

Food chains

• A food chain shows the linear flow of energy and nutrients through an ecosystem

  • Example: grass (producer) → mouse (primary consumer) → owl (secondary consumer)

  • The source of energy in a food chain is light energy from the Sun

Food webs

• A food web is a model of interlocking food chains

  • Food webs demonstrate the complex flow of energy and nutrients between multiple organisms

• Food webs better reflect the complexity of natural ecosystems compared to simple food chains

  • Food webs are more realistic ways of showing connections between organisms within an ecosystem

  • This is because consumers rarely feed on just one type of food source

Interdependence in food webs

• Within a community, each species depends on other species for food, shelter, pollination, seed dispersal etc.

• If one species is removed it can affect the whole community

• This is called interdependence

• A stable community is one where all the species and environmental factors are in balance so that population sizes remain fairly constant

• For example, in the food web above, if the population of earthworms decreased:

  • The population of grass plants would increase as there are now fewer species feeding off them

  • The populations of frogs and mice would decrease significantly as earthworms are their only food source

  • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)

Positive & negative feedback loops in food webs

• Feedback loops describe how changes in one part of a food web can affect other parts

  • Feedback loops can either amplify or stabilize those changes

  • They are critical for understanding how ecosystems respond to disturbances

Positive feedback loops

• Positive feedback loops amplify changes in the food web

  • Example: overpopulation of herbivores can lead to overgrazing, which reduces plant availability and destabilizes the ecosystem

Negative feedback loops

• Negative feedback loops stabilize the food web by counteracting changes

  • Example: a decrease in prey population can limit predator numbers, allowing the prey population to recover over time

• Removing or adding species can significantly impact the food web’s balance

  • Example: the removal of a top predator like wolves from a forest can lead to an overpopulation of herbivores, which may cause vegetation depletion