Long-term effects of salmon subsidies on terrestrial and aquatic invertebrate communities
Pacific salmon (Oncorhynchus spp.) provide a flux of nutrients into terrestrial and aquatic food webs during the spawning season which has been shown to positively increase the abundance and biomass of terrestrial and aquatic invertebrates. Previous research has shown that salmon subsidies cause two-fold increases in the abundance and biomass of terrestrial invertebrates, in the immediate post-spawning period. We quantify terrestrial and aquatic invertebrate abundance, biomass, and composition to determine if the salmon subsidy has a lasting effect on invertebrates into the pre-spawning period the following year. We hypothesize that if a lasting effect is present, the abundance and biomass of invertebrates collected at the salmon-bearing reaches will be greater when compared to those collected in the salmon-absent reaches. Terrestrial and aquatic invertebrates were collected and identified from above and below a salmon barrier in the pre-spawning season at two streams: Sugsaw Creek and Sarita Falls on the West Coast of Vancouver Island, BC. Abundance and biomass was negatively and significantly related for both terrestrial and aquatic invertebrates, demonstrating a bottom-heavy trophic level community. Abundance and biomass for terrestrial and aquatic invertebrates was slightly greater below the falls with the exception of terrestrial invertebrates at Sugsaw Creek. Outliers were noted for Orders such as Diptera and Coleoptera that appeared to escape the size spectrum. There was little difference in invertebrate diversity across any of the locations. Our results indicate that there is no yearly legacy effect of salmon subsidies on terrestrial and aquatic invertebrates into the pre-spawning season.