Deer, Shrubs, and a Severe Winter on an Island
Wildlife biologists monitor a deer herd on a 50 km² island with no predators. From 2000 to 2010, they estimate population size each spring and record winter severity. In 2000, there are 120 deer. The population grows steadily to 480 by 2005. In 2006, a mild winter and abundant browse allow growth to 620. In 2007, a severe winter with deep snow is followed by a count of 390. In 2008, the population is 410; 2009, 430; 2010, 425. During the same period, the biomass of preferred shrubs declines from 8,000 kg to 2,500 kg. Biologists also find that fawn survival is lower when deer density exceeds 400. A manager proposes introducing wolves to control the herd, while a resident argues that the 2007 drop proves the population is already regulated by weather and needs no management.
1. Estimate the carrying capacity of the island for deer. Justify your estimate with at least two pieces of data from the scenario.
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Carrying capacity is the population size an environment can support over time, not a fixed number. The best estimate here is about 400–430 deer. After the 2007 crash from 620 to 390, the population stays near 400–430 for three years instead of continuing to grow, which suggests it has reached a ceiling. The shrub biomass fell from 8,000 kg to 2,500 kg, indicating that food has been reduced and may limit further growth. Fawn survival is lower when density exceeds 400, which is direct evidence that the habitat cannot support many more than about 400 deer. The 2006 peak of 620 was an overshoot above K, followed by a crash. A precise K would require more years and data on food regrowth, but 400–430 is the most reasonable estimate.
2. Was the 2007 winter a density-independent or density-dependent factor? Explain how it interacted with deer density to produce the crash.
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The severe winter itself is density-independent: snowfall and temperature affect deer regardless of how many there are. However, its effect was amplified by density-dependent food competition. By 2006 the herd had reached 620, well above the 400–430 level that the shrub data suggest can be sustained. High density had already reduced browse, so deer entered winter in poorer condition and had less food available under deep snow. The winter killed many, especially fawns, and the population fell to 390. Thus the weather was the trigger, but density-dependent resource depletion made the population vulnerable. This interaction is common: a density-independent event can cause a crash whose severity depends on density.
3. Evaluate the manager's proposal to introduce wolves and the resident's argument that weather already regulates the herd. What evidence would help decide?
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The resident is partly right that weather can reduce the herd, but the data show the population overshot its food supply and then stabilized near 400, which is consistent with food limitation, not just weather. A single severe winter is not enough to keep a herd below K in all years; without predators or hunting, the herd may grow again and repeat the boom–crash cycle. The manager's wolf proposal could reduce deer density and allow shrubs to recover, but wolves may also change deer behavior and kill rates in ways that are hard to predict. Before deciding, managers should monitor shrub regrowth, deer body condition, fawn survival, and wolf predation after any introduction; compare fenced plots with and without wolves; and check whether hunting could achieve the same goal with less ecological disruption. The decision depends on whether the goal is stable deer numbers, ecosystem recovery, or both.