Urbanization increases the proportion of aquatic insects in the diets of riparian spiders

The exchange of nutrients and organic material that occurs along the riparian zones of stream ecosystems is vital to providing energetic subsidies for both aquatic and terrestrial food webs. Orb-weaver spiders are major consumers of emerging aquatic insects, so impacts to the stream ecosystem and the riparian zone around it can have significant effects on riparian spider assemblages. Aquatic insects can represent anywhere from similar to 50-100% of the diet of riparian orb-weavers, generally depending on factors such as spider taxa and prey diversity. However, most of the studies that examine riparian orb-weaver diets have been done in non-urban areas. Thus, little is known about how urbanization affects the transfer of energy and nutrients between aquatic and terrestrial ecosystems. The objective of our study was to determine if the proportion of aquatic insects in the diets of riparian orb-weaver spiders is altered by urbanization. We used stable isotope analyses and Bayesian mixing models to assess trophic transfers between primary energy sources, insects, and orb-weaver spiders along the riparian zone of an urban gradient in San Juan, Puerto Rico. We found that both delta C-13 and delta N-15 values varied with the amount of urbanization in aquatic insects, terrestrial insects, and 2 genera of riparian orb-weaver spiders. Spiders closely reflected changes in the isotopic values observed in aquatic insects along the urban gradient, with a noticeable decrease in delta N-15 values at the most urbanized sites. In addition, we found that the proportion of aquatic insects in the diets of the orb-weavers was similar to 30% greater in heavily urbanized areas than non-urbanized areas. This study represents one of the few efforts to determine how urbanization can significantly alter riparian food webs and influence the exchange of subsidies between aquatic and terrestrial ecosystems.