Effects of spawning salmon on the size spectra of stream insect communities

Spawning Pacific salmon disturb stream ecosystems through their spawning behaviour and subsidize them via nutrients from carcasses. To further understand the net effects of spawning salmon in streams, I examined size distributions (size-spectra) of stream insect communities across a salmon density gradient. This size-based method offers a novel, ecologically meaningful way to understand the net effects of salmon in stream ecosystems. I used a Before-(during)-After, Control-Impact observational approach in 15 streams along the central coast of British Columbia to determine how slopes and intercepts of stream insect biomass size-spectra change before, during, and after a fall salmon run. I found no effect of salmon density on size spectra intercepts in any season and steeper size-spectra slopes (attributed to greater numbers of smaller insects and fewer larger insects) with increasing salmon density in the fall. These data may indicate differential effects of disturbance and subsidization on stream insect size classes.