Undergraduate Honors Theses

Thesis Defended

Spring 2013

Document Type



Ecology and Evolutionary Biology

First Advisor

Dr. Pieter J. Johnson


The effects of invasive organisms on native ecosystems can be challenging to understand when their effects are moderated by environmental characteristics. Here, investigation evaluated the effects of western mosquitofish (Gambusia affinis) on native aquatic communities over a gradient of nutrient availability. Our study consisted of (1) a laboratory experiment that assessed how body size of three native amphibian larvae – Pacific chorus frogs (Pseudacris regilla), western toads (Anaxyrus boreas) and California newts (Taricha torosa) – influenced the risk of mosquitofish predation and (2) a two-month-long outdoor mesocosm experiment that evaluated the influence of mosquitofish simulating wetland communities across a gradient of nutrient availability. In the laboratory study, chorus frog larvae demonstrated a significant relationship between body size and survival. In the mesocosm experiment, mosquitofish significantly reduced overall amphibian survival compared to non-fish treatments, but the magnitude of these effects varied among amphibian species and nutrient condition. However, for both newts and toads, survival in the presence of fish occurred only (newts) or was higher (toads) in the high nutrient concentration, indicating an interaction between fish presence and nutrients. Correspondingly, amphibians in the high nutrient conditions grew more rapidly over the course of the experiment. Fish also sharply reduced zooplankton abundance and thereby enhanced primary production. Importantly, however, mosquitofish total survival of offspring also increased strongly (>300%) with elevated nutrient levels, presumably through bottom-up food web effects. These findings suggest that while increases in primary productivity can mitigate the direct effects of invasive fish on a sensitive native taxon (i.e., amphibians), nutrient increases may differentially benefit invasive fish and lead to a series of indirect effects throughout the community. Our study has practical implications for understanding how environmental variables influence interactions between native and nonnative species, which can inform freshwater ecosystem management and conservation efforts focused on mitigating effects of invasive species.