Responses of orb-weaving spider aggregations to spatiotemporal variation in lake-to-land subsidies at Lake Mývatn, Iceland

Abstract

Tundra ecosystems are often unproductive, and peaks in resource availability are highly variable in space and time. Therefore, the success of organisms inhabiting these biomes likely depends on their ability to efficiently exploit heterogeneously distributed resources. We assessed how orb-weaving spider (Larinioides patagiatus) aggregations near Lake Mývatn, Iceland, respond to large midge (Diptera: Chironomidae) emergences that subsidize ecological communities in the surrounding landscape. The emergences occur for only a few weeks each summer, and the subsidy declines with distance from the lakeshore, producing large spatiotemporal variation in prey availability that might drive orb-weaver foraging behavior. We conducted three surveys during different phases of the summer emergence along a distance gradient to quantify variation in spider aggregation, web size, and habitat use in response to prey abundance. We found that aggregation size increased with the abundance of aerial prey, with the highest orb-weaver densities occurring at peak midge emergence. In contrast, changes in web size did not vary with midge abundance, but rather were determined by physical habitat structure. Our results illustrate how orb-weaver aggregations and web building can respond to spatiotemporal variation in resource subsidies across ecosystem boundaries.