The Influence of Mosquito Predators on Population Dynamics of Endemic and Exotic Mosquitoes
The presence of predators can shape the population dynamics of prey. Here, I evaluated the influence of predators on mosquito populations by direct effects through predation, and indirect effects through sub-lethal responses. The predator under investigation was the backswimmer Anisops wakefieldi. I first quantified the relationship between mosquito and predator populations in animal drinking water troughs, and correlated their abundance with water volume and environmental factors. Logistic regression indicated that the presence of mosquitoes was primarily affected by three factors; predator numbers, week of observation, and water volume. A translocation experiment was established to understand the pre- and post-treatment effects on mosquito survival after exposure to the predator Anisops wakefieldi. The presence of these predators in water troughs significantly decreased subsequent survival of mosquito prey within two days posttranslocation. A major hypothesis in invasion ecology suggests that native predators have less impact on exotic species relative to native prey species, enabling exotic species to establish and thrive in novel environments. This is the "escape from natural enemies" hypothesis. Contrary to this hypothesis A. wakefieldi, a native New Zealand predator, showed a greater preference, and consumed more of the exotic mosquito Aedes (Ochleratatus) notoscriptus compared to the native (Culex pervigilans) mosquito larvae. Anisops wakefieldi exhibited a decelerating Type II functional response for both prey species, but consumed more exotic mosquito species at high prey densities and displayed higher attack rates. The effects of the preference of this predator on mosquito behaviour were examined. In the presence of predators the exotic species, Ae. notoscriptus, demonstrated significantly higher levels of "thrashing" behaviours, apparently making itself more obvious to the predators. In contrast Cx. pervigilans fulfilled the "threat sensitivity" hypothesis by altering its behaviour towards "resting", known to be the best strategy for avoiding predators. In addition to lethal effects, predators can substantially alter prey populations by means of sub-lethal influences. To further our understanding of how predators limit mosquito abundance, I developed an experiment based on adult mosquito oviposition. I predicted that the New Zealand native mosquito, Cx. pervigilans would likely avoid containers with A. wakefieldi or their kairomones. Contrary to our predictions, Cx. pervigilans appeared to ignore the presence of predators and their kairomones when choosing oviposition habitats and the number of egg rafts was not significantly affected by the density of predators. Culex pervigilans eggs from the oviposition experiment were reared in two different laboratory conditions: (A) in clean water without any traces of predators, or (B) in water with the same treatments as in field. This experiment was established in order to have better understand on what happens to the next generation after exposure to A. wakefieldi predators or their kairomones. Sub-lethal effects were still apparent in the developing larvae. I observed a significant reduction in the survival rate of Cx. pervigilans in the presence of predators and their kairomones, even when the eggs were only exposed briefly to water containing either predators or kairomones in the field, and were then reared in clean water without any traces of the predator. No effect was observed on the sex ratio of Cx. pervigilans, or the development times of each life stage. Overall, this thesis has highlighted the importance of predators in influencing mosquito populations, not only through direct predation, but also in indirect and sub-lethal ways. This study may have implications for the dynamics of other predator-prey systems. Despite this, we are only beginning to understand the complex interactions between predators and prey.