Linking communities across ecosystem boundaries: the influence of aquatic subsidies on terrestrial predators
Sanzone, Diane Mary
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Movement of nutrients and organisms between aquatic and terrestrial habitats may have a greater impact on assemblage structure and community dynamics than within-habitat inputs. In this study, I document the effect of aquatic prey subsidies on surrounding terrestrial predators in eight riparian zones using a stable isotopic 15N tracer experiment to quantify the flow of nitrogen from aquatic to terrestrial food webs via emerging aquatic insects. I continuously dripped 15N-NH4Cl for six weeks into four temperate forested streams (North Carolina, Tennessee, New Hampshire and Oregon), one tropical forest stream (Puerto Rico), a desert stream (Arizona), a grassland stream (Kansas), and one arctic stream (Iceland), and traced the flow of 15N from the streams into spiders living in the riparian zone. After correcting for background 15N values, I used mixing models to calculate the proportion of 15N tracer from emerging aquatic insects incorporated into spider biomass. In addition, I documented spider abundance, biomass and diversity along a transitional gradient at various distances from the stream bank (0-50m). Although sites were located in different biogeographic regions, varied greatly in temperature and precipitation, and were sampled in different seasons, consistent patterns in the spatial distribution of spiders among the eight sites were found. Lower canopy and ground-dwelling spiders were significantly more abundant in riparian zones than in upland habitats in at least four of the eight sites, whereas upland habitats never contained more spiders. Ground-dwelling and lower canopy spider richness was also higher along the stream edge in seven of the eight sites. Evidence from the eight different biogeographic regions suggests that aquatic insect emergence and microhabitat availability are important predictors of spider biomass, abundance and diversity. Stream-derived 15N tracer was incorporated into spider tissue in eight riparian zones located in eight different biogeographic regions. Stream-derived N was highest in riparian spiders in desert, arctic and tall-grass prairie, tropical and northern conifer forest sites and lowest in temperate deciduous forest sites. 15N labeling in spiders tracked that of emerging aquatic insects, indicating spiders are relying, at least in part, on aquatic resources. There was a direct positive relationship between incorporation of stream-derived N by spiders and biomass of emerging aquatic insects.