Diversity, Community Structure and Phenology of Spiders on Orchid Island Forest Ecosystem. (NSC 89-2621-Z-029-006)


In this proposed study, we would like to investigate the insecticidal effect of the giant crab spiders (Araneae: Heteropodidae) with uneven distribution of Lanyu scops owl as the theme question. Lanyu scops owl, a nocturnal raptor, is the top predator in Orchid Island forest ecosystem. The density of Lanyu scops owl on Orchis Island varies from habitat to habitat, which may be affected by the availability of tree holes as well as the abundance of prey insects. Spiders, the most abundant invertebrate predators in various ecosystems, were shown to play a significant role in controlling community structure and abundance of insects. Therefore, spiders on Orchid Island may potentially be a major competitor of Lanyu scops owl by affecting the distribution and abundance of prey insects. In this study we compared the density, microhabitat preference and height of perching of giant crab spiders and large orthopteran insects among breeding and non-breeding areas of Lanyu scops owl. In each area we assessed the aforementioned quantities in three types of habitats: forest interior, forest margin and meadow. Preliminary results indicated that in both breeding and non-breeding areas the density of giant crab spiders in three habitat types differed significantly. The densities in forest habitats were always significantly higher than those in meadows. Among three habitat types, forest interior in breeding areas exhibited much higher density of spiders than those in non-breeding areas. However, no such difference was found in the other two habitat types. Most giant crab spiders were found on shrubs in forest understory with the height lower than 1m and were found to prey upon ground insects no larger than 15mm. The density and biomass of large orthopterans also differed significantly among three habitat types, with forest interior exhibited the highest density. However, in all three habitat types there was no significant difference in density or biomass of orthopterans between breeding and non-breeding areas. Similar to giant crab spiders, orthopteran insects also inhabited low understory shrubs. These preliminary results suggest that given giant crab spiders’ large size and abundance, they do not seem to compete directly with Lanyu scopes owl by catching large orthopterans. Our data also demonstrated that although no statistical difference was found in orthopteran abundance between breeding and non-breeding areas, the latter seemed to exhibit higher mean orthopteran abundance. Whether or not the lower food abundance in breeding areas was responsible for the dispersal of some Lanyu scopes owl to non-breeding areas after breeding season await further study.

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Updated: Jan-24-2014 11:34:24 (Taiwan, GMT+08:00).