What Determines the Local Distributions of the Two Orb-Web Spider Species? : Inferences from the Field Experiment

Abstract

The local distributions of the two common large orb-web spiders Nephila clavata and Argiope bruennichii are somewhat different; the former lives in woodlands and their surroundings, while the latter inhabits grasslands and their periphery. To explore factors limiting local distributions of these spiders, three plots (in the forest edge, and in the grassland) with the same physical support were established, into which spiders were released. The decreasing patterns of numbers at the three plots were quite different between the two species; Nephila showed the highest decreasing rate in the grassland and lowest in the forest, while Argiope showed the opposite tendency. The body size of Nephila tended to be larger in the grassland. Matured females of Argiope appeared to be larger in the grassland, though the sample size was too small to test. Potential prey, estimated by using a trap for capturing flying insects, was most abundant in the grassland and least in the forest. Thus, the availability of web support could not explain the difference in local distribution between the species. Food supply was not a critical factor for the distribution of Nephila either. Possible factors concerning the differences in the decreasing patterns among habitats and between species were discussed.