AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/1/1-11)
Unit 1 · Topic 1.11
1.11 Food Chains and Food Webs
A food chain shows one path of energy and matter through an ecosystem, while a food web shows how many chains connect. Because species are linked, removing or adding one can ripple through the whole web, and feedback loops can either steady or amplify those changes.
Key terms
- food chain
- food web
- autotroph
- heterotroph
- detritivore
- feedback loop
Food chains and food webs
A food chain is a single path of feeding: grass → grasshopper → frog → snake → hawk. The arrows point in the direction energy and matter move, from the organism eaten to the organism that eats it. A common mistake is drawing arrows backward.
Real ecosystems are more tangled. A food web combines many food chains to show all the feeding links in a community. Most consumers eat more than one thing and are eaten by more than one predator. Food webs show how a change in one population can affect others it never directly touches.
Food webs with many links tend to be more stable. If a predator has several kinds of prey, it can switch to another when one becomes scarce, so the loss of one species is less likely to cause a collapse. Simple food webs, like those in the Arctic, are more easily disrupted.
Autotrophs, heterotrophs and detritivores
- Autotrophs (producers) make their own food, usually by photosynthesis.
- Heterotrophs (consumers) eat other organisms: herbivores, carnivores and omnivores.
- Detritivores eat detritus, which is dead plant and animal material, like earthworms and crabs.
- Decomposers, mainly bacteria and fungi, chemically break down dead matter and release nutrients.
Ripple effects in a food web
When a species is removed, its prey may boom and its predators may decline or switch to other food. When a new species is added, it may outcompete natives for food or eat species that had few defenses.
A classic example comes from the Pacific coast. Sea otters eat sea urchins, and sea urchins eat kelp. When fur hunters wiped out otters in many areas, urchin numbers exploded and they grazed kelp forests down to bare rock, called urchin barrens. Where otters returned, urchins declined and kelp forests regrew, along with the fish that shelter in them. This chain of effects running down through several levels is called a trophic cascade.
Feedback loops
A feedback loop happens when a change causes effects that loop back to influence the original change.
- Negative feedback loops push a system back toward balance. Example: more prey leads to more predators, which eat more prey, which brings prey numbers back down. Negative feedback stabilizes ecosystems.
- Positive feedback loops amplify a change, pushing the system further from where it started. Example: warming melts Arctic sea ice; open ocean is darker than ice and absorbs more sunlight; that causes more warming, which melts more ice.
| Feedback type | Effect | Ecosystem example |
|---|---|---|
| Negative | Stabilizes; reverses the change | Predator-prey population cycles |
| Positive | Amplifies; pushes the change further | Melting ice → darker surface → more warming |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Predicting a ripple effect
In a food web, owls and foxes both eat mice, mice eat seeds, and foxes also eat rabbits, which eat grass. A disease kills most of the owls. Predict the effects on mice, seeds and rabbits.
Show the solutionHide the solution
- Step 1: Start with the organism directly affected: owls eat mice, so fewer owls means less predation on mice, and mice numbers rise.
- Step 2: Follow the mice: more mice eat more seeds, so the seed supply drops.
- Step 3: Look for indirect links: foxes now have more mice to eat, so they may eat fewer rabbits, letting rabbits increase. (Foxes could also increase with more food.)
- Step 4: On the exam, a prediction plus a clear reason for each link earns credit.
Answer: Mice increase, seeds decrease, and rabbits may increase because foxes shift to eating the more plentiful mice.
- Example 2
Positive or negative?
Identify each feedback loop as positive or negative. (a) Warming thaws permafrost, which releases methane, which causes more warming. (b) A deer population grows, overgrazes its food, and then declines from lack of food, letting plants recover.
Show the solutionHide the solution
- Step 1: Ask whether the result pushes the original change further or reverses it.
- Step 2: (a) Warming leads to more warming: the change is amplified, so it's positive feedback.
- Step 3: (b) Growth leads to food shortage, which leads to decline: the change is reversed, so it's negative feedback.
Answer: (a) Positive feedback; (b) negative feedback.
Common mistakes
- Drawing food chain arrows from predator to prey. Arrows point toward the eater, showing the direction energy flows.
- Thinking ‘positive’ feedback means good. Positive feedback amplifies change, which is often harmful; negative feedback stabilizes.
- Predicting only direct effects. Food web questions usually want you to follow the change at least two links.
On the exam
- Food web questions often ask what happens if one species is removed. Trace at least two links and explain each with ‘because’.
- Be ready to identify or give an example of a positive or negative feedback loop, especially ones related to climate change.
Connected topics
Videos
Check yourself
4 questions on 1.11 Food Chains and Food Webs. Pick an answer to see if you got it, and why.
In a lake, green algae are eaten by zooplankton, zooplankton are eaten by minnows, and minnows are eaten by largemouth bass. Mayfly larvae feed on dead leaves and other detritus on the lake bottom, and minnows also eat mayfly larvae.
Over several years, anglers remove most of the bass from the lake.
Described scenario
Which sequence of changes is most likely after most of the bass are removed?
What role do the mayfly larvae play in this ecosystem?
Ecosystem A has a simple food web in which each predator eats only one kind of prey. Ecosystem B has a complex food web in which most consumers eat several kinds of food. If one prey species declines sharply in each ecosystem, which outcome is most likely?
Which of the following is an example of a positive feedback loop?
0 of 4 answered