An Experimental Test of Corridor Effects on Butterfly Densities

In a large-scale experiment, we found that three habitat-restricted butterfly species reached higher densities in patches connected by corridors than in similar, isolated patches. We conducted our study in 27 equal-sized (1.64-ha) patches that varied in whether or not they were connected to another patch by a corridor. Patches and corridors were open, early-successional habitat that contrasted strongly with the surrounding pine forest. Of four butterfly species studied, three open-habitat specialists (Junonia coenia, Euptoieta claudia, and Phoebis sennae) reached higher densities in patches connected by corridors than in isolated patches. A fourth species, Papilio troilus, showed no preference for open habitat or pine forest, and its density did not differ in connected or isolated patches. Although butterfly densities were often significantly, positively associated with densities of host plant or flower resources, plant densities did not confound corridor effects on butterfly densities. Higher densities in patches connected by corridors may have been caused by three different factors. First, species with higher densities in connected patches also moved more frequently between connected patches, and higher movement rates may have helped to sustain higher population sizes. Second, species with higher densities in connected patches also had higher densities farther from the forest edge. Corridors increased the area of a patch that was farther from the forest edge, which increased the “effective area” of connected patches and may have increased butterfly densities. Third, corridors may have acted as “drift fences,” intercepting dispersers from the surrounding forest and directing them to connected patches. We could not determine the relative contribution of each factor, and it is possible that all three contributed to higher densities of habitat-restricted butterflies in patches connected by corridors.