Deer and Wolves on the Ridge
A wildlife class studies a ridge where deer browse young trees. Wolves are the main predator of deer. In year 1, wolves are common and young trees are abundant. In year 2, wolves are removed. By year 3, deer numbers rise sharply and young trees are heavily browsed. By year 5, deer numbers crash, and tree seedlings are scarce. A student says, 'The wolves disappearing caused the deer crash.' Another says, 'The trees caused everything.' The teacher asks the class to analyze the food web and the timing of events.
1. Explain how removing wolves could lead, over several years, to both a deer increase and a later deer decline, using food-web reasoning.
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Removing wolves reduces predation on deer, so deer survival rises and the population grows (year 3). More deer browse young trees heavily, reducing tree regeneration. With fewer young trees, future food and cover for deer decline. Eventually food limitation, and possibly disease or starvation, causes the deer population to crash (year 5). This is a trophic cascade plus resource limitation: the predator's removal indirectly affects plants, which then feed back on the herbivore. The deer crash is not caused directly by wolves but by the loss of trees the deer depend on.
2. A student claims the trees 'caused everything.' Evaluate this claim.
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Partly supported but incomplete. Trees are the resource that limits deer once wolves are gone, so they drive the later decline. But the initial trigger was wolf removal, which changed deer numbers. The trees did not cause the wolf loss or the initial deer increase. A complete explanation includes both the predator's role and the plant-herbivore feedback. The claim captures the feedback but ignores the trigger.
3. Predict one additional observable change by year 5 and justify it with the food web.
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Acceptable: increase in animals that eat tree seedlings or insects dependent on young trees (decline), or changes in birds that nest in young trees. Justification must trace a path through the food web - e.g., fewer young trees → less habitat/food for insects and nesting birds → their numbers decline. The prediction must follow from the tree decline, not from the wolves directly.