Outcomes of stream invertebrate mesopredator interactions to benthic food webs and ecosystem functions

Within the predator guild of ecological communities are smaller-bodied mesopredators that have the ability to directly impact community structure and indirectly alter ecosystem processes. Interactions between predators can result in combined predator outcomes to their prey, termed Multiple Predator Effects (MPEs), that can be additive, antagonistic, or synergistic with the strength and direction contingent on predator identity and guild diversity. In streams on the west coast of North America, there are two co-occurring predatory stonefly larvae, Calineuria californica and Hesperoperla pacifica, both of which can cause direct depletion of the prey community which can cascade to indirectly alter leaf litter decomposition and periphyton standing stock. My objective was to quantify the role and outcome of intra- and interspecific interactions of C. californica and H. pacifica on stream food webs. I manipulated stonefly larvae identity and density, as well as richness of the predator guild, then compared larval growth and survival, prey community composition, and measures of basal resources to determine the MPE outcomes. I hypothesized that due to the high degree of niche overlap that competition would lead to antagonistic MPEs. I also hypothesized that increasing density and the presence of the more aggressive H. pacifica would lead to higher competition and stronger antagonistic MPEs. I found that stonefly larvae presence caused a 46% reduction in the prey community, and that increasing biomass of either species significantly caused this decline. For interspecific and intraspecific C. californica pairings there was density-dependent competition resulting in a switch from additive to antagonistic MPEs, while intraspecific H. pacifica pairings had antagonistic MPEs at all densities. I also found that the mechanism of predator impacts varied between the two species, with H. pacifica decreasing the prey community through consumption and C. californica through non-consumptive behavioural modification of prey. However, the effects to the prey community did not cascade to impact the basal resources. My results demonstrate that the type and strength of predator interactions may be contingent on the composition of the predator guild and may result in unexpected, non-linear outcomes to the community.Forestry, Faculty ofGraduat