Master of Science / Department of Biology / Keith B. Gido / Nonnative species represent a major threat to the continued persistence of native fishes globally, especially in the Colorado River Basin of western North America, where there are now more nonnative than native fishes. In the upper Gila River, a tributary of the Colorado, numerous nonnative fishes have established populations, and predation by these nonnatives has been linked to extirpation of native fishes under low-flow conditions at some locations. Historically, the upper Gila lacked a top piscivore, and it is unclear what mechanisms have allowed the establishment of nonnative piscivores and resultant extension in food chain length. To investigate the phenomenon of increased food chain length through nonnative introductions we explored the influence of autochthonous energy availability on nonnative predator abundance, food chain length, and abundance of other trophic levels. Predictions were that increased basal energy availability would lead to increased nonnative predator abundance and thus increased food chain length, based upon predictions from food web theory. Annual production and biomass of four trophic levels measured across six longitudinally-positioned sites were calculated between June 2008 and June 2009 to test these predictions. In addition, energy demand of trophic levels relative to energy supply was compared across sites using a quantitative food web approach, to evaluate energy limitation across trophic levels. Primary production was found to vary considerably across the upper Gila (1,677-16,276 kcal m-2 yr-1), but production and biomass of other trophic levels was not related to this gradient as predicted. In addition, food chain length demonstrated a marginally-significant negative relationship with primary production (R[superscript]2=0.42, d.f.=5, p=0.16), which was in contrast with predicted responses. These results suggest that energy availability does not appear to be a limiting factor to the production or biomass of consumers. The influence of other mechanisms on food chain length in the upper Gila River, in particular disturbance frequency and intensity, deserve further investigation.

We are currently faced with the global challenge of conserving biological diversity while also increasing food production to meet the demands of a growing human population. Land-use change, primarily resulting from conversion to production land, is currently the leading cause of biodiversity loss. This occurs through habitat loss, fragmentation of remaining natural habitats, and resulting edge effects. Land-sparing and land-sharing approaches have been discussed as alternative ways to engineer landscapes to mitigate biodiversity loss while meeting production objectives. However, these represent extremes on a continuum of real-world landscapes, and it will be important to understand the mechanisms by which adjacent land use affects natural remnant ecosystems in order to make local land-management decisions that achieve conservation, as well as production, objectives. This thesis investigates the impact of juxtaposing production and natural forest on the community-wide interactions between lepidopteran herbivores and their parasitoids, as mediated by parasitoid spillover between habitats. The first and overarching objective was to determine whether herbivore productivity drives asymmetrical spillover of predators and parasitoids, primarily from managed to natural habitats, and whether this spillover alters trophic interactions in the recipient habitat. The study of trophic interactions at a community level requires understanding of both direct and indirect interactions. However, community-level indirect interactions are generally difficult to predict and measure, and these have therefore remained understudied. Apparent competition is an indirect interaction mechanism thought to be very important in structuring host-parasitoid assemblages. However, this is known primarily from studies of single species pairs, and its community-wide impacts are less clear. Therefore, my second objective was to determine whether apparent competition could be predicted for all species pairs within an herbivore assemblage, based on a measure of parasitoid overlap. My third objective was to determine whether certain host or parasitoid species traits can predict the involvement of those species in apparent competition. My key findings were that there is a net spillover of generalist predators and parasitoids from plantation to native forest, and that for generalists, this depends on herbivore abundance in the plantation forest. Herbivore populations across the edge were linked by shared parasitoids in apparent competition. Consequently, an experimental reduction of herbivore density in the plantation forest changed parasitism rates in the natural forest, as predicted based on parasitoid overlap. Finally, several host and parasitoid traits were identified that can predict the degree to which host or parasitoid species will be involved in apparent competition, a finding which may have extensive application in biological control, as well as in predicting spillover edge effects. Overall, this work suggests that asymmetrical spillover between production and natural habitats occurs in relation to productivity differences, with greater movement of predators and parasitoids in the managed-to-natural forest direction. The degree to which this affected species interactions has implications for landscape design to achieve conservation objectives in production landscapes.