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

Background/Question/Methods

The effects of resource subsidies on consumers in recipient habitats may be influenced by demographic or behavioral traits of consumers relative to the timing and duration of a subsidy. In addition, ecosystems are typically spatially heterogeneous, such that subsidy input occurs to habitats of different character.  Heterogeneity of biotic (e.g., community composition, life-histories) and/or abiotic (e.g., temperature, boundary structure) characteristics may influence how a subsidy is received or utilized.  There have been few investigations that explicitly examine the role of such spatial heterogeneity in mediating effects of subsidies on consumers.  We conducted a field experiment to test the hypothesis that the responses of riparian arthropod communities to subsidies of salmon carcasses are mediated by habitat character, in this case vegetated (grasses) versus un-vegetated (cobble-bar) plots.  In addition, because salmon carcasses are an ephemeral subsidy, we expected the strongest responses by taxa that are rapid colonizers and/or exhibit fast rates of reproduction.  Over a 3-week period that encompassed the time required for salmon carcasses to decompose, we used pitfall traps to quantify responses by riparian arthropods (composition and abundance) to salmon carcasses, which are typically transported to land by bears, but that we experimentally placed in vegetated and un-vegetated habitats.

Results/Conclusions

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 either riparian habitat, with stronger demographic effects in vegetated plots for Drosophilidae, Sphaeroceridae, and Calliphoridae larvae.  Salmon carcasses in un-vegetated plots desiccated, whereas they remained moist in vegetated plots, which likely facilitated rapid consumption by these saprophages.  This was evident for Calliphoridae larvae, which were 48X more abundant in vegetated habitats.  One of five predatory arthropods, the coleopteran Staphylinidae, aggregated near salmon carcasses (presumably to feed upon dipteran larvae), but only in vegetated plots.  Our results demonstrate that spatial variation in recipient habitat can mediate the effects of subsidies by influencing (1) the physical character of the subsidy itself and (2) the presence of consumers that can directly or indirectly capitalize on the subsidy.  With respect to the latter, we observed strongest responses by r-strategist (rapid colonizers and reproducers) arthropods, which could respond to the short duration of subsidy availability.  Subsequently, we observed only weak responses by predatory arthropods, suggesting effects of prey increases were either too diffuse or ephemeral to be detected.