Heterogeneity of riparian habitats mediates responses of terrestrial arthropods to a subsidy of Pacific salmon carcasses

Abstract

The transfer of nutrients and energy across habitat or ecosystem boundaries, or resource subsidies, is important in structuring ecological communities. Variation in biotic and abiotic characteristics of patches receiving resource subsidies may mediate their effects, including responses by consumers, however few studies have investigated this explicitly or the potential effects at broader spatial scales. To test the role recipient patches may play in mediating effects of a subsidy and consequences at larger reach scales, we conducted an experiment to evaluate changes in arthropod community composition and abundance in response to a subsidy of salmon carcasses in vegetated and un‐vegetated riparian patches, where carcasses are frequently deposited by animals, and how these patterns in patches mediate responses at reach scales. Arthropod community composition differed between habitats, and salmon additions yielded strong positive changes in arthropod abundance among select families. Four dipteran families responded positively to additions of carcasses to riparian habitats, and effects were generally stronger in vegetated habitats. Salmon carcasses in un‐vegetated habitats desiccated whereas in vegetated habitats they remained moist which likely facilitated rapid consumption. Of the five predatory arthropod families observed, only the coleopteran, Staphylinidae, increased in response to the salmon carcass subsidy, and only in vegetated habitats. Differences in effect size between habitats suggest selection of salmon carcasses by riparian arthropods changes with habitat context. Additionally, we detected significant increases in adult Calliphoridae biomass at reach scales, with the largest increases occurring at streams with more carcasses in vegetated patches. Our results show that the responses to the subsidy of salmon carcasses are not ubiquitous among riparian arthropods, nor are they spatially homogenous. Rather, our findings demonstrate that spatial variation in recipient habitat can mediate the responses of primary and secondary consumers to a subsidy, with effects manifesting at broader spatial extents, highlighting an additional means by which landscape heterogeneity influences the dynamics of food webs.