The linkage between riparian predators and aquatic insects across a stream-resource spectrum

1. Spatial subsidies, defined as the flow of energy, nutrients, organisms or pollutants from one habitat to another, have been shown to affect the food-web dynamics in a wide range of ecosystems. An important subsidy to riparian communities is the contribution of adult stream insects to terrestrial predators such as birds, bats and lizards, but also invertebrates including ground and web-building spiders. 2. We surveyed 37 first- and second-order forest streams across differing environmental gradients in the Central South Island, New Zealand, to investigate the relationship between potential aquatic prey subsidies and predatory riparian arachnids. We anticipated that stream-insect biomass would be positively associated with riparian arachnids, as a result of emergent adult aquatic insect subsidies to the adjacent habitat. 3. We confirmed positive associations between stream-insect biomass as a predictor variable and riparian arachnid biomass (R-2 = 0.42, F-1,F-34 = 25.2, P < 0.001) and web densities (R-2 = 0.45, F-1,F-14 = 11.5, P < 0.01) respectively as dependent variables after adjusting for the confounding effects of environmental variables. Hierarchical partitioning confirmed the importance of stream insect biomass as a statistically significant contributor to the total explained variance in analyses calculated for arachnid biomass, abundance and web density. 4. A concurrent survey of spider-web density along 20-m transects from the stream edge into the forest indicated a strong decline in web-building spider density moving away from the stream (R-2 = 0.41, F-1,F-158 = 109, P < 0.001), with stream-insect biomass as a significant covariate (F-1,F-149 = 17.7, P < 0.001). 5. Our results suggest that productivity gradients present in the donor system affect the magnitude of the interaction between adjacent habitats. Productivity gradients may lead to increased reciprocal subsidies through a positive feedback loop involving the predation of spiders and other predatory terrestrial invertebrates by aquatic predators. However, terrestrial insectivores such as birds, bats and lizards that are not readily used as prey by aquatic predators may circumvent the feedback cycle by consuming a large proportion of emergent aquatic-insect biomass. This may lead to asymmetry in the strength of food-web linkages between aquatic and terrestrial habitats.