Climate Change, Predator-Prey Interactions, and Population Dynamics of the Ranchman’s Tiger Moth (Arctia virginalis)
In my dissertation, I examined how a changing environment affects Ranchman’s tiger moth (Arctia virginalis) population dynamics via predator-prey and host-pathogen interactions. I also examined the complex ways in which biotic drivers of population dynamics interact with climate forcing.In Chapter 1, I examined how body size mediates the effect of warming on the interaction between Arctia virginalis and an ant predator (Formica lasioides). I also developed a general framework for understanding warming effects on stage or size-dependent predator-prey interactions. Specifically, I showed through experiments and modelling that A. virginalis is only vulnerable to predation by F. lasioides when small. This window of vulnerability narrows as the development rate of A. virginalis increases with warming. However, the attack rate of F. lasioides also increases with warming and overcompensates for the decreasing window of prey vulnerability. We therefore project warming to favor the predator in this case, in contrast to the results of previous related work in other systems. In Chapter 2, I expanded upon Chapter 1 by examining the effects of warming on predator-prey interactions in multi-predator communities. In Chapter 1, I showed that asymmetries in predator-prey thermal response rates can result in changing interactions, constituting an important indirect effect of warming. In demographic models using simulated and empirically informed parameters from a natural community of ant predators, I showed that greater predator diversity paired with sufficient thermal niche diversity attenuates these indirect effects of warming, leaving only direct, physiological effects. This result depends on predator diversity and thermal niche diversity and complementarity, which has significant implications for how we might expect warming to affect predator-prey interactions in different communities dependent on traits of the community thermal niche. In Chapter 3, I examined viral disease as a mechanism for delayed density-dependent dynamics in A. virginalis populations and the role of ultraviolet radiation in reducing viral infection through attenuating virions persisting in the environment. I censused 18 populations across 9° of latitude and a gradient of ultraviolet radiation intensity for two years and measured viral infection rate, severity, and survival of lab-reared caterpillars. I found that caterpillar density in the previous year led to a higher infection rate and severity and lower survival and that ultraviolet radiation led to lower infection severity and higher survival. This suggested that virus is the main mechanism for cyclic dynamics in this species, which has been proposed theoretically but rarely shown empirically. I also found a population-level effect of ultraviolet radiation on viral infection severity, which had not been shown previously. In Chapter 4, I analyzed long-term A. virginalis census data, finding that changing precipitation patterns due to changes in large-scale climate led to qualitative changes in population dynamics. Using change-point analysis and state-space models, I showed that caterpillar dynamics transitioned from short to long-period dynamics concurrent with a change in precipitation dynamics. Using deterministic simulations, I showed that shifting dynamics were likely due to resonance: precipitation patterns interfered with cycles initially and later amplified cycles.