Do agricultural pesticides in streams influence riparian spiders?

Freshwater ecosystems are coupled with their riparian area. Emerging insects are prey for predators in the riparian zone, enriching the terrestrial ecosystem with energy and nutrients. Stressors associated with agriculture can alter insect communities in water and on land, resulting in complex response patterns of terrestrial predators relying on prey from both systems. Examining the effects from individual agricultural stressors such as pesticides is hampered in landscapes with intensive agriculture where multiple stressors like habitat degradation and typically co-occur. In rural regions of Eastern Europe, traditional low intensity agriculture based on working animals and human labour prevails alongside intensive, mechanised agriculture. Assuming that low-intensity agriculture relies on no or limited pesticide use, whereas fertilizer use is similar across different agricultural intensities, such regions may allow to study in-stream pesticide effects independent from other stressors, such as nutrient input or habitat degradation. We examined the taxonomic and trait response of riparian spider communities to gradients of agricultural stressors and environmental variables in the region around Cluj-Napoca, Romania. Pesticide sampling was done using passive samplers in the streams adjacent to spider sampling sites. To capture spiders with different traits and survival strategies, we used multiple collection methods. Community composition was best explained by in-stream pesticide toxicity and shading of the stream bank, a proxy for the quality of the habitat. Species richness and the number of spider individuals were negatively associated with in-stream pesticide toxicity. In contrast, mean body size and shading preference of spider communities responded strongest to shading, whereas mean niche width (habitat preference for moisture and shading) responded strongest to the other environmental variables. Our study suggests that in-stream pesticide toxicity can influence riparian communities. The identification of mechanisms requires further studies targeting the potential contributions of direct toxicity and indirect effects from reduced aquatic and terrestrial prey availability.