Precipitation Drives the Abundance and Distribution of Arctia virginalis: A 40-Year Study

To understand processes that govern the abundance and distribution of species, ecologists typically collect either long time series without surveying potential drivers or perform short-term experiments that may not scale up. We characterized the annual population dynamics of Arctia virginalis for 40 years and conducted experiments to examine the relative roles of abiotic conditions, host plants, predation, parasitoids, and viral infection. Rather than finding a single limiting factor, these factors were all important at some times or places. Annual densities varied by a thousand times and showed evidence of a regime shift around 2002, coincident with changing precipitation patterns. Wet sites and wet years supported higher densities, and precipitation interacted with most of the factors considered. Population control was context dependent, but water availability was generally the relevant context. Precipitation seems to be important for other Lepidoptera in western North America. Studies that include experimental tests of population drivers are required to manage insect populations.