Conservation biologists have long emphasized the potential benefits of connecting fragmented pieces of habitat with wildlife corridors. But a lack of empirical evidence regarding corridors has often stopped land managers and community planners from recommending their use. Now two studies, published in the most recent edition of Ecological Applications, show that corridors work -- at least for certain species.
The first study, conducted by Nick Haddad at the University of Georgia (now at the University of Minnesota), examined how two butterfly species, the variegated fritillary (Euptoieta claudia) and the buckeye (Junonia coenia), moved between several isolated open spaces at the Savannah River Site in South Carolina. The spaces were created by the sustainable harvest of several acres worth of loblolly (Pinus taeda) and slash pine (Pinus elliotii) trees from the forest.
The open spaces left behind after the harvest provided favorable habitat for the butterflies, but were surrounded by dense areas of remaining pine plantation. Densely wooded areas are not hospitable to the two species, in part because they often lack the plants that the creatures need for food.
Haddad and his team studied butterfly movement within 27 patches of open space. Each square patch measured 128 meters (about 140 yards) on a side. Several patches were connected by open habitat corridors of varying length, measuring about 32 meters (about 35 yards) across. The others remained isolated and unconnected to open areas.
Once the cleared areas were open to sunlight, their plant communities changed. Dormant seeds germinated and nectar plants developed. Existing tree stumps sprouted new growth. Soon, butterflies began to colonize these areas to take advantage of the new sources of food.
Haddad tracked the movements of these butterflies by capturing them and marking their wings. Each butterfly was assigned a number and then released. Capture and release records were then kept to track the butterflies' visits to different open patches within the study area.
After two years, the researcher and his team concluded that the butterflies readily moved between patches that were close together, even when they did not have access to a corridor of open space. But the greater the distance between the unconnected patches, the less likely the butterflies were to move successfully between them.
"The butterflies may be capable of visually detecting the light environment of a nearby patch," says Haddad. "But at larger interpatch distances, movement rates were always greater between connected patches."
For both species of butterfly, males showed stronger positive responses to corridors than females. In addition, Haddad found that interpatch movement of the buckeye was very dependent upon the presence of blue toadflax (Linaria canadensis), an important food source for the buckeye caterpillar.
The second study, conducted by Haddad and his colleague Kristen Baum (now at Texas A&M University), found that corridors also increased the sheer number of open-habitat butterflies found at the same research site. The buckeye, the variegated fritillary, and cloudless sulphur (Phoebis sennae), all open-habitat species, reached higher densities in patches connected by corridors than in isolated patches. By contrast, a fourth species in the study, the spicebush swallowtail (Papilio troilus), showed no preference for open habitat or pine forest, and its density did not differ in connected or isolated patches.
The results of these studies may help scientists and land managers implement better, more effective corridor designs for the future.
"Corridors are often designed with the thought that they benefit all species living in a given habitat," says the researcher. "But one general implication of our work is that the greatest potential of corridors is for habitat-restricted species. For example, roadside prairies will be most effective as corridors for prairie species, and forest corridors will most benefit forest species. Because habitat restricted species are most often threatened by fragmentation, corridors should be effective tools in conservation."
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