Changes to spider community ecology mediated by altered forest understory vegetation
Date
2016
Authors
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Publisher
University of Delaware
Abstract
Despite the broad implications and the long history of research on the impacts of dense ungulate populations and invasive plant species on native vegetation, work involving indirect effects to the spider community is explicitly lacking. Forest spiders are dependent upon the abundance of palatable insect prey and habitat structure, both of which can be negatively affected by herbivory and invasive vegetation. To examine the indirect interactions between spiders and factors that may influence forest vegetation, I investigated comprehensive changes to spider communities resulting from altered habitat structure and prey availability. I sampled spider communities, insect prey, and vegetation in central Maryland within paired exclusion plots to prevent white-tailed deer herbivory. I used generalized linear models to analyze spider richness and abundance, and several multivariate models to assess overall effects to the spider community. I also measured stable isotopic ratios of an orb-weaving species (Tetragnathidae: Leucauge venusta) to examine potential physiological stress in altered habitats. Greater prey density increased the abundance and richness of all spider functional groups, and nearly all spider families. Increased habitat structure from deer exclusion influenced some spider families with responses discordant within each functional group. More complex habitat structure also decreased overall species richness. Habitat structural complexity and insect prey abundance both influenced the composition of the spider community. Isotopic nitrogen enrichment decreased in spatially complex habitats, suggesting physiological stress and/or altered diet in structurally homogeneous forests. In habitats dominated by an invasive C4 plant, spider isotopic carbon signatures closely resembled those of C3 plants, indicating avoidance of the exotic species by the herbivorous and saprophagous insect community. This work identifies the importance of both habitat structure and insect prey in defining the composition, abundance, and richness of forest spider communities. A history of heavy browsing pressure in the area has resulted in a local spider fauna consisting largely of species that are able to thrive in low-growing vegetation and an open forest understory. Such changes to vegetative structure resulting from dense deer populations and invasive plants have the potential to affect these important primary predators as well as araneophagic birds and the overall nutritional dynamics of the forest food web. Future work should focus on a finer scale and include further examination of changes in insect genera or families, potential changes in taxonomic composition of individual spider diet using stable isotope mixing models, and additional components of spider body condition such as individual biomass and fecundity.