Salmon subsidize an escape from a size spectrum

Background/Question/Methods

A general rule in ecology is that the abundance (N) of species or individuals in communities sharing a common energy source scales negatively with body size (M). However, external energy inputs in the form of resource subsidies may modify the N ~ M size spectrum relationship. Here we provide the first test of how a resource subsidy can affect the size spectra, based on energy derived from Pacific salmon (Oncorhynchus spp.) carcasses affecting a forest soil community beside streams in Western Canada. We test if salmon subsidies that vary across spatial and seasonal contrasts result in system-wide increases in abundance across all size classes in the forest soil community (a shift in the N ~ M intercept). We also test whether salmon subsidize an escape from this size spectrum by specific species (outliers in the N ~ M relationship), whether organisms in smaller versus larger size classes are differentially affected (shifts in the N ~ M slope), and whether salmon subsidies are transient (seasonal) or longer-lasting.

Results/Conclusions

Using both species and individual approaches, we found size structuring in this forest soil community, and transient community-wide doubling of standing biomass in response to energy pulses from spawning salmon. One group of species were clear outliers in the size spectrum relationship, larval Calliphorid and Dryomyzid flies, which specialize on salmon carcasses. The superabundance of larval flies resulted in a 10-fold increase in biomass in their size class when salmon were available. Thus, salmon subsidize their escape from the size spectrum. These results suggest that using a size-based perspective of resource subsidies can provide new insight into the structure and functioning of food webs.