The influence of land use, microhabitat and aquatic insect subsidies on predatory riparian arthropod distributions

Agricultural land use effects on aquatic and riparian communities are complex and multifaceted, resulting in habitat degradation and biodiversity loss in riparian and instream ecosystems. This study correlated predatory arthropod densities and species richness to abundance of potential prey (emerged aquatic insects and terrestrial dipterans) along an agricultural to forested land use gradient. Pearson's correlation coefficient was used as an effect size to measure attraction to prey, and the ratio of predator to prey was calculated to indicate potential consumption capacity of prey. Results revealed that gradients in land use and microhabitat condition, distance from the stream and season were important explanatory factors. Positive correlations between predatory arthropods and aquatic insect abundance were more apparent than those with terrestrial dipteran abundance, suggesting an overall preference of aquatic prey. However, positive correlations between predatory arthropods and adult aquatic insect subsidies were strongest in microhabitats with characteristics associated with higher moisture (e.g., greater soil organic matter and shade), particularly with increasing agricultural land use. In September, there was an indication of reduced confinement to microhabitats, likely as an effect of elevated seasonal precipitation. Overall results of this study suggest that a limited tolerance to desiccation in predatory arthropods increases spatial confinement with agricultural land use and ultimately restricts access to adult aquatic insect subsidies. The findings of this study have implications for the pathways that adult aquatic insect subsidies use to enter into riparian food webs with consequences that could cascade across trophic levels and larger spatial scales.