Predation, Herbivory and Trophic Cascades
Predators and prey drive each other's numbers in cycles, and removing a top predator can change vegetation several links away. Whether a community is controlled from the top or the bottom is a question that field manipulation can answer.
What a learner can do afterwards
- Explains the lag between predator and prey peaks in a cycle.
- Traces a cascade through at least three links and names the direction of each effect.
- Designs an observation or removal that separates top-down from bottom-up control.
1 · Read
Predators and prey chase each other in cycles. In the classic lynx and snowshoe hare record, numbers swing on a roughly 10 year loop with the predator lagging 1 to 2 years behind. Hares multiply, well-fed lynx raise more young, heavy hunting then crashes the hares, and starving lynx fall after them. With few hunters left, hares recover and the loop restarts, so the prey peak always leads.
A trophic cascade carries one change down the chain link by link, flipping direction each step. Remove the top predator and herbivores surge upward, then overgrazed plants collapse downward. Bring the predator back and each arrow reverses: herbivores shrink, plants recover. Tracing a cascade means naming every link and stating whether each step rises or falls.
Picture two valleys side by side. In one, you remove the top predator and watch herbivores climb while plants thin. In the other, you change nothing and use it as a control. If prey rise only where predators went missing, the community is ruled from the top down. If instead adding fertilizer lifts every level at once, the rule runs from the bottom up.
To read any cascade, write the chain and alternate the arrows: predator down means herbivore up means plant down. Never skip a link, and check each direction separately.
Prey peaks lead, effects alternate down the chain, and removal plots tell top-down from bottom-up.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.