Practice a College Board-style free response question on The Living World: Biodiversity. Write your response, then reveal the model answer to see exactly what earns each point.
A continuous 10,000-hectare forest is divided by new road construction into four separate patches of 2,500 hectares each, with no remaining habitat corridors connecting them. Wildlife researchers track species richness in the largest remaining patch over the following 20 years and compare it to historical records from when the forest was continuous. They also note that the region's only large predator, a wolf species, has disappeared from all four patches following fragmentation.
| Time period | Forest condition | Species richness (large patch) |
|---|---|---|
| Before fragmentation | Continuous, 10,000 ha | 142 species |
| 20 years after fragmentation | Isolated patch, 2,500 ha | 97 species |
Island biogeography theory predicts that smaller habitat area supports fewer species because there is less total habitat and fewer available resources to sustain diverse populations. Fragmentation reduced the patch from 10,000 to 2,500 hectares, and since the patches are now isolated with no corridors, the colonization rate of new species from other patches dropped while local extinction risk increased — both factors that island biogeography theory predicts will lower equilibrium species richness on a smaller, more isolated "island."
The wolves likely functioned as a keystone species — a species whose impact on community structure is disproportionately large relative to its abundance. As top predators, wolves control populations of herbivores (such as deer); without wolves, herbivore populations are likely to grow unchecked, leading to overgrazing of vegetation, which in turn reduces plant diversity and the habitat available for other species, compounding the species richness decline.
Constructing wildlife corridors connecting the four forest patches would allow species to move between patches, effectively increasing colonization rates and reducing isolation — directly addressing the island biogeography mechanism identified in Part A. By reconnecting the patches, the corridors would functionally increase the available habitat area and allow species (including predators like wolves) to recolonize patches where they had gone locally extinct, helping restore species richness over time. (Other valid answers: establishing a protected buffer zone around each patch, or reintroducing the wolf population directly.)