Predation, Herbivory and Trophic Cascades
Predation and herbivory can change population sizes and create indirect effects through food webs. Some predator-prey populations cycle. Controlled, replicated field manipulations can test contributions from predators and resources, which may act together.
What a learner can do afterwards
- Explains the lag between predator and prey peaks in a cycle.
- Traces an indirect effect through three trophic levels and names the direction of each population change.
- Designs controlled, replicated tests of top-down and bottom-up contributions.
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Predator and prey populations can cycle, but many do not. In the classic lynx and snowshoe hare trapping record, cycles last about 10 years and lynx peaks typically follow hare peaks by 1 to 2 years. More hares can support more lynx; fewer hares can limit lynx later. Predation alone does not explain the hare cycle. Density-dependent changes in hare reproduction can also contribute.
Predation is a predator killing and eating prey. Herbivory is an animal eating plants or plant parts. Spines and toxins can deter feeding. Camouflage and some forms of mimicry help prey avoid detection or attack; warning colors can signal defenses.
A trophic cascade is an indirect effect across feeding links. Consider a simple model where wolves limit deer numbers and deer browsing limits young trees. Keep other conditions fixed. Removing wolves predicts more deer, more browsing and fewer young trees. These directions depend on the stated links, not on a rule that every food web must alternate.
In the same simple model, suppose returning wolves reduce deer numbers while other conditions stay fixed. Fewer deer means less browsing, so the model predicts more young trees. In a real valley, measure deer numbers, browsing and tree growth rather than assuming recovery. Other influences or lasting changes can delay or prevent a return to the earlier vegetation.
Bottom-up effects begin with resources supporting producers, such as nutrients supporting plants. Suppose fertilizer increases plants, herbivores and predators across replicated plots compared with untreated controls. This supports a bottom-up contribution from nutrients. It does not show that all levels rise at once or that top-down effects are absent. Predator and resource effects can coexist.
For predator exclusions, compare several matched treatment and control plots before and after treatment. Assign treatments at random when feasible. Include controls with similar fences that still allow predator access to test fence effects. Shared weather, seasons and initial differences can also explain changes. Consistent treatment differences support a top-down contribution; they do not prove that predators alone control the community.
Some populations cycle. Simple food-chain models predict indirect effects. Replicated tests can reveal both top-down and bottom-up contributions.
Lesson, questions and flashcards written for this dataset.
Original readings written for this dataset. Video references: Khan Academy and TED-Ed.
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