Climatic and pathogenic impacts on spongy moth range expansion and contraction

Invasive species cause roughly $137.2 billion in damages annually in the United States, but the factors that govern their spread through new areas are not fully understood. As one of the primary forces that limit species’ distributions, climate can influence where and when an invasive species will spread aggressively. In this dissertation I evaluate the potential effects of climate on rates of range expansion (or contraction) in invasive species, using the spongy moth, one of the most destructive forest insects in North America, as my model study system. The climate can alter interactions between pests and their natural enemies, with possible consequences for invasive spread, especially if the natural enemy is a substantial source of mortality in the pest. Motivated by predictions that climate change will lead to rising air temperatures and more frequent summertime drought events, I conducted a manipulative field experiment to test the hypothesis that warmer, drier conditions decrease rates of infection of spongy moth larvae by its host-specific fungal pathogen Entomophaga maimaiga (Chapter 2). The number of infected larvae increased under elevated temperature treatments compared to the ambient temperature treatment, but there was no significant effect of supplemental precipitation or an interaction between temperature and precipitation. These results suggest that in colder portions of the spongy moth’s range, such as Minnesota where the experiment was conducted, increasing temperatures due to climate change may enhance the ability of E. maimaiga to help control populations of the spongy moth. In Chapters 3 and 4 I investigated whether fluctuations in weather and climate at different timescales influence spatiotemporal variability in rates of spread. For example, annual variability in weather may influence year-to-year fluctuations in spread rates, whereas at interannual timescales, regionally synchronized climatic conditions may produce similarly widespread fluctuations in spread. Much past work has focused on how spatial, as opposed to temporal, variability in climate may alter the distributional range limits of invasive species. To help close this knowledge gap, in Chapter 3 I analyzed how rates of spread and population growth of low-density populations responded to annual variability in temperature, precipitation, and snow depth in five ecoregions along the spongy moth’s range edge. I found that abiotic conditions strongly influenced spongy moth spread and growth rates, and that the effects varied by ecoregion. Additionally, the rates of population growth and spread usually responded differently to the same abiotic variable. These differences may stem from effects of weather on dispersal (e.g., wind-aided dispersal during storms) and density-dependent effects of abiotic conditions on population growth. In Chapter 4 I investigated whether synchrony in climate at multi-annual timescales would produce synchronized rates of range expansion and contraction at similar timescales. This is the first study to provide evidence of spatial synchrony in rates of range expansion and contraction, and that the drivers of this phenomenon can be similar to the drivers of population synchrony. Specifically, teleconnections led to multi-annual synchronized fluctuations in climate which drove synchrony in spread rates in the northernmost and southernmost ecoregions. Information on the strength and phase of teleconnections could inform managers about when and where spatially synchronized fluctuations in spread may occur. This dissertation broadens our current understanding of drivers of invasive spread and provides novel empirical evidence on the effects of temporal variability in climate on biological invasions. I also demonstrated that climate warming may affect interactions between pest insects and their pathogens, and in some locations these effects may facilitate biological control. The findings presented herein are also relevant for rare species, for which conservation efforts can facilitate range shifts to track a changing climate.