Context-dependence of a cross-system trophic cascade in Gwaii Haanas, British Columbia.

Increasing evidence suggests that the occurrence and magnitude of trophic cascades are highly context-dependent, yet the mechanisms mediating these indirect effects remain difficult to detect and predict. We examined the strength of evidence for a cross-system trophic cascade whereby invasive terrestrial predators (Norway rats, Rattus norvegicus) were hypothesized to directly reduce avian shoreline predators and indirectly magnify intertidal macroinvertebrates, thereby reducing macroalgal biomass. Simultaneously, we quantified the extent to which wave intensity mediated these indirect effects. We found that densities of successful American Black Oystercatcher (Haematopus bachmani) breeding pairs were, on average, 50% lower on rat-invaded than rat-free islands. Furthermore, we detected evidence for an effect of rats on grazer biomass (Wi=0.996) and this effect was reduced on wave exposed islands. We found no consistent pattern however, in grazer density between rat-free and rat-infested islands, regardless of wave exposure intensity or quantification method. Sheltered islands with rats had 74% less macroalgal biomass than those without, whereas exposed rat-invaded islands had 39% less macroalgal biomass than their rat free counterparts. Consequently, we found strong evidence that the mediating effect of wave exposure on invasion status appears to drive intertidal macroalgal biomass (Wi=0.998). Identifying the conditions that promote or inhibit the cascading effects of introduced and/or recovering natural predators will allow managers to better anticipate where these indirect effects might occur and tailor their conservation and management actions.