Individual study: Effects of experimentally cleared woodland patches on the dispersal of the buckeye butterfly Junonia coenia at 'The River Site', South Carolina, USA
Haddad N. (2000) Corridor length and patch colonization by a butterfly, Junonia coenia. Conservation Biology, 14, 738-745
Increasing habitat destruction has led to habitat fragmentation and isolation. There has been much debate as to the benefits of habitat corridors and the extent to which they promote dispersal and population survival by linking up fragmented habitats. It is thought that provision of habitat corridors may increase movement between habitat patches, thus increasing survival and population persistence. The extent to which species use corridors will ultimately depend on animal behaviour.
The buckeye butterfly Junonia coenia was used to test the extent to which corridors increase patch colonization and dispersal. This butterfly inhabits areas of open habitat, and, as opposed to much of the previous work that has taken place on corridors with forested areas acting as habitat refuges, this study used experimentally cleared patches and created corridors within a managed pine forest to investigate their effects on butterfly colonization.
Study site: Experimentally created patches and corridors of open habitat were located in the pine Pinus spp. forest plantations at 'The River Site', west-central South Carolina. In total, thirteen patches, located in three forest stands were used. The open, cleared patches were each 1.64 ha in size and separated from other patches by 256-384 m of woodland. Some of the patches were connected by 32 m wide corridors within which trees had been felled and removed. These patches and corridors were created by clearing areas of forest during pine harvesting in 1994 and 1995.
The open, early successional habitat created in the patches and corridors contained the following plant species (which did not occur in forested areas) used by buckeyes:
larval food plants - blue toadflax Linaria canadensis, purple foxglove Gerardia purpurea,
nectar plants - Rubus sp., butterfly milkweed Asclepias tuberosa and Liatris sp.
Releases: Individual buckeye butterflies were collected at distances more than 5 km from the study patches, and stored in a cooler for up to two hours prior to release. A total of 736 buckeyes were released in two groups of four individuals at 92 points. The first group were released between 14 June-3 July 1997, and the second between 13 July-6 August. Releases took place at set points along transects placed in the cleared corridors and in strips of forest of the same size as corridors (32 m wide). Forest strips were located on the opposite side of patches to corridors, with the cleared patches in the centre of the two. Both corridor transects and forest transects were placed at right angles to the edge of experimental patches.
Release sites were established 16, 34, 62, 128 and 192 m away from patches, along 10 replicated transects. At each point, individually marked butterflies were released, one each facing north, east, south and west.
Monitoring: For four days following releases, both patches and corridors were searched for marked butterflies, forest was not searched as this species rarely enters wooded areas. The initial three days were spent searching each patch in eight 128 m transects 16 m apart. On the remaining day, two transects, 16 m apart, were used to search corridors. Transects were walked in 6-minute periods, and when found, butterflies were captured in order to prevent recording the same individual twice, and their identity and location recorded.
Of the 736 released buckeye butterflies, 29% (212) were recaptured. Of these 60% (127) were located in open patches adjacent to release sites, 32% (68) were in more distant patches, and 8% (16) were found in the corridors cleared of trees.
Data analysis showed that neither release distance from patches nor corridors affected patch colonization by released butterflies. However, the number of butterflies colonizing patches from corridors was lower (mean ± SE: 1.5 ± 0.2) than those colonizing from forest (2.5 ± 0.2) at distances of 16 to 62 m. At distances of 128 and 192 m, double the number of butterflies colonized patches released in corridors than in forest (mean ± SE: corridor = 1.6 ± 0.4, forest = 0.8 ± 0.2). As release distance increased, the number of butterflies colonizing patches, which were released in forest, decreased.
Conclusions: The presence or absence of corridors, together with distance, did not have an effect on patch colonization, however, there were interactions between corridor presence and distance. Buckeyes were twice as likely to colonize patches by the use of corridors than through forest, when released at long distances. Patches separated by small distances (< 100 m apart) acted as stepping stones for butterfly movement thus acting as 'discontinuous' corridors for movement themselves. The recapture of butterflies within corridors and the presence of suitable larval food and nectar plants within them, indicates that they acted as suitable habitat, thus the creation of corridors would not only increase patch colonization, but also increase habitat availability.
This study suggests that the effective use of corridors to enhance dispersal in fragmented landscapes, depends on the distance between remnant patches, and movement abilities of the species in question. When the distance an animal can travel through or over unsuitable habitat is less than the distance between patches, corridors may be effective in promoting movement and hence survival.