Importance of Aquatic Prey Subsidies and Habitat Structure to Riparian Spider Communities Along a Stream Size Gradient

Elaine Langone Center, Terrace Room

Abundance and distribution of riparian predators are strongly affected by trophic subsidies from aquatic ecosystems (prey availability) and habitat structure within the riparian zone. As suggested by the river continuum concept (RCC), biological communities change as river size increases from small headwater streams to large rivers due to differences in river size and food resources. During the summer of 2014, we investigated how changes in aquatic communities affected riparian predators, specifically orb-weaving spiders, preying on emerging aquatic insects along river size gradients. We deployed standardized wooden tree structures to control for varying habitats in order to focus solely on the influence of trophic subsidies. These structures (catering to both horizontal and vertical orb-weaving spiders) were placed along riparian zones of 1st, 3rd, 5th, and 7th order sites of three local rivers (the North and West Branches of the Susquehanna River and the Juniata River). We hypothesized that if trophic subsidies are more influential than habitat structure, then riparian predator communities on standardized habitats will be similar to natural habitats but will vary along stream size gradients. Conversely, if physical habitat structure has more influence, then riparian predator communities will be similar on standardized habitats regardless of stream size but different from surveys of predators in natural riparian zones. Additionally, we hypothesized that, as stream order increases, spider abundance, biomass, and diversity will increase due to higher aquatic insect availability. Preliminary data show that tree structures attracted fewer and smaller spiders than adjacent riparian plots, which could be caused by limited exposure time for colonization or colonization by younger spiders avoiding competition with larger spiders for prime web sites. As a result, our results are inconclusive at this point regarding the relative importance of habitat structure to riparian spider communities.