The effect of riparian buffer properties on spider communities and aquatic-terrestrial food-web lingages : using polyunsaturated fatty acids as biomarkers

Abstract: Riparian habitats are key habitat interfaces that regulate flows of resource subsidies between aquatic and terrestrial food webs, whilst supporting high biodiversity andproviding ecosystem services. However, frequently these habitats are highly degraded, especially in agricultural landscapes. In this study, the effect of riparianbuffer properties on the diversity and composition of spider communities along stream channels was investigated. Additionally, trophic connectivity was investigatedby analysing the polyunsaturated fatty acid (PUFA) content of the riparian spiders. PUFAs are physiologically essential for animals, and some are exclusively producedby algae in aquatic environments. These aquatic PUFAs can therefore be used as biomarkers to track the uptake of algal-derived aquatic subsidies into terrestrialfood webs. As different spider taxa rely on aquatic subsidies to varying degrees, the PUFA content of specific taxa was also examined. This to ascertain to what degreeeach taxon potentially contributes to the transfer of PUFAs into terrestrial food-webs.Spiders were collected in Uppland, Sweden, from 10 paired sites (each pair with one unbuffered and one buffered site) in an agricultural landscape, and five reference forest sites. The spiders were identified to family level, and then freeze-dried, pulverized and homogenised, and their fatty acid content then extracted and analysed. Spider diversity, community composition and PUFA data were then statistically analysed using multivariate methods in R to reveal differences and interactions between site types and spider families. I found that the abundances, community composition and biomass of the riparian spiders differed between site types. This result was largely due to differences in functional types of spiders, with web-building spiders dominating in buffered sites and free-living spiders more common in unbuffered sites. The differences can partly beexplained by trait-mediated habitat preferences and local habitat availability. Statistical analyses revealed differences in the PUFA profiles of the spiders, which werelargely driven by spider taxonomic identity, but also influenced by site type and an interaction between site type and spider family, as well as stream identity. OverallPUFA content was highest in forest site spiders, however, aquatically-derived PUFAs were similar between site types. Lycosidae spiders had consistently high levels of aquatic PUFAs. Thus, it seems that the assimilation and transfer of aquatic PUFAs from spiders further into terrestrial food-webs may be primarily routed through particular families. Understanding the factors that affect trophic connectivity and flow of resource subsidies is crucial for effective management and restoration of streamriparian networks. A more varied buffer design may be one mitigation strategy that could benefit both biodiversity and trophic connectivity.