Deer and Wolves on an Isolated Island
On an isolated island, a deer population feeds on shrubs. Wolves were introduced to control the deer. Over five years, deer numbers fell from 2,000 to 800, shrub cover increased, and wolf numbers rose then leveled off. A ranger proposes removing the wolves to protect the deer, arguing that fewer deer means a struggling population. A biologist notes the island has no other large predator, that shrubs had been heavily browsed before wolf introduction, and that the deer herd now shows more disease and lower body mass than before. The island's carrying capacity for deer is estimated at about 1,500.
1. Explain how the wolf introduction changed the deer population and the shrub community, using predator-prey and carrying-capacity concepts.
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Before wolves, the deer population at 2,000 exceeded the estimated carrying capacity of about 1,500, so resources were overused and shrub cover declined from heavy browsing. Introducing wolves added a predation pressure that reduced deer numbers toward 800, below carrying capacity. With fewer deer, browsing pressure fell and shrub cover recovered. The wolf population rose because prey was initially abundant, then leveled off as deer numbers declined - a typical predator-prey lag in which predator abundance tracks prey with a delay. The system moved from resource-limited (bottom-up) control to a mix of predation and resource limitation. The lower deer density also means each deer has more food per individual, which can improve body condition over time, though the current low body mass and disease suggest the transition period was stressful.
2. Evaluate the ranger's proposal to remove the wolves, predicting likely consequences for deer, shrubs, and wolves.
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Removing wolves would release the deer from predation. With no other large predator and a carrying capacity near 1,500, the deer population would likely grow rapidly and overshoot that level, then decline through starvation and disease - a boom-and-bust pattern. Shrubs would again be heavily browsed, reversing the recovery and possibly reducing future food supply, which worsens later deer declines. The wolf population would lose its food base and decline or disappear. The ranger's concern that 800 deer is "struggling" misreads the data: a population below carrying capacity with recovering habitat is not necessarily in trouble, and the disease and low body mass may reflect past overabundance or density-related stress rather than wolf predation. A better approach is monitoring rather than removal, since the predator is performing a stabilizing ecological role.