Diversity and conservation of forest-floor arthropods on a small Seychelles island
Cousine Island, Seychelles, is of major conservation significance as it is in a biodiversity hotspot. Furthermore, it is relatively pristine, and is apparently the only tropical island over 20 ha with no alien invasive mammals. This study focuses on the island's log and litter arthropods, which were sampled by extraction methods from the dominant species, Pisonia grandis, Ficus spp. and Cocos nucifera. Stage of decomposition, and forest type in which the logs occurred, both significantly influenced the composition and structure of the arthropod assemblages. Young logs were significantly richer in species than older logs, possibly because they had the most resources and microhabitats. There were some significant changes in arthropod species richness, composition and abundance between species of young logs, but not old ones, because as logs decomposed, arthropod assemblages converged. Nevertheless, each old log species had some arthropod species not present in other log species, which has important implications for conservation. Arthropod assemblages in woody litter varied according to the forest type in which they occurred, and were different from those in logs in the same forest type. Cousine Island arthropod species richness, both in logs and litter, was comparable to figures from other tropical areas. As the logs, especially P. grandis, are home to many Seychelles endemic species, their conservation is essential. Furthermore, as the arthropods are also the main food of certain threatened Seychelles vertebrates, their conservation also underpins a food chain on this unique tropical island.