Lethal and non-lethal effects of predators on Culex mosquitoes: implications for pathogen transmission

Abstract

Biodiversity loss, especially at higher trophic levels, can degrade critical ecosystem services such as disease regulation. Predators play pivotal roles in regulating ecosystem services by impacting the abundance, distribution, physiology, condition, and behavior of their prey. Previous work shows the ecological consequences arising from single-predator nonconsumptive effects (NCEs) can be just as significant as those resulting from actual predation, yet the study of NCEs within naturally-diverse communities is relatively underdeveloped. This dissertation seeks to examine how larval-stage predator community structure influences mosquito (Culex pipiens) traits that are relevant to pathogen transmission. When developing mosquito larvae are exposed to predation risk, this stress may produce lasting effects throughout mosquito ontogeny that can influence such key traits as longevity and immune function. The work presented here provides several key findings. First, an examination of how naturally-diverse larval-stage predator communities influence mosquito condition throughout their ontogeny revealed that the presence of Aeshna dragonfly naiads, which were limited to the most diverse field predator communities, significantly muted the NCEs evident in mosquitoes. Mosquitoes emerging from Aeshna communities experienced increased survival through development, but decreased adult longevity compared to no-Aeshna communities. This trend occurred across natural variation in predator community composition, density, and evenness. We designed a follow-up experiment that exposed developing mosquitoes to factorial manipulations of predator diversity and tested adult mosquitoes for variation in phenoloxidase investment (an important component of arthropod innate immunity) and for susceptibility to the fungal pathogen Beauveria bassiana. We found that when presented with Aeshna cues, mosquitoes had lower phenoloxidase levels than mosquitoes reared in the presence of other predators; however, these negative impacts on mosquito immune investment were offset when mosquitoes were exposed to Aeshna in combination with other predators. These results imply that diverse predator communities, in addition to their capacity to suppress vector densities through predation, may lower disease risk by offsetting negative NCEs imposed by less-diverse communities that would otherwise increase vector competence. This dissertation ends with a review and synthesis paper that broadly explores the role of arthropod and mammalian predators in regulating other critical ecosystem services such as biodiversity maintenance.