UC Santa Barbara

Influences of species introductions are context dependent and can extend from nutrient pools and fluxes to predator populations and behavior. Trout stocking is a classic example of a globally pervasive introduction where a single species can cause whole system changes and cross-boundary effects. Omnivorous trout consume the larvae of aquatic insects, reducing insect emergence that otherwise provides a nutrient and energy subsidy to terrestrial systems. In this dissertation I first ask whether multiple stressors (trout presence and lake temperature) have interactive effects on aquatic insect emergence. I then ask how the presence of trout affects nitrogen patterns in near-shore terrestrial zones. As a testing site for these questions I use a set of lakes in California’s Sierra Nevada where trout have been both introduced and removed over the past century, establishing a natural experiment. Across paired lakes with and without fish I examine the effects of trout presence on within-lake communities and across the aquatic-terrestrial ecotone. I conducted multiple years of field surveys in which I sampled insect communities, soil and lake sediment characteristics, and stable isotope signatures of terrestrial soils and plants, and quantified algal biomass in the littoral benthic zone of each lake. I find that insect emergence is significantly lower from lakes with trout, as expected, but also from warmer lakes – and that the impact of trout is lessened in warmer lakes. My data confirm that trout are associated with reduced insect deposition to terrestrial near-shore areas. Despite this reduction in insect-vectored nutrients, I also find a positive relationship between trout presence and nitrogen content of both soils and plants. By producing a detailed record of trout impacts 1) across climatic conditions and 2) to multiple trophic levels, in this dissertation I highlight the context dependencies of introduced species effects and expand understanding of how trout alter lake and lakeside ecosystems.