Understanding spring and summer climate impacts on monarch butterflies across the Midwestern United States
Understanding climate impacts on migratory species is complex because such species travel through several climates that may be differentially changing throughout their migratory range. Most studies do not tease out the direct and indirect effects of climate at various stages along the migration route, typically due to sparse spatial data or difficulties in defining appropriate fine-scale climate metrics. We assess the impacts of spring and summer climate conditions on breeding monarch butterflies using data from two state-wide butterfly monitoring networks in Ohio and Illinois. Because no single, broad-scale climate metric can explain summer breeding phenology or the substantial year-to-year fluctuations observed in monarch population abundances, we built a Poisson regression model to explain and compare annual arrival times and abundances in two distinct areas of the summer breeding range. We incorporated climate conditions experienced during a shared spring migration/breeding phase in Texas as well as during subsequent arrival and breeding during separate recruitment periods in Ohio and Illinois.
Spring conditions (e.g. precipitation and temperature) in Texas were strongly associated with annual population abundances in Ohio. Once in Ohio for summer breeding, precipitation had minimal impacts on overall monarch abundances, whereas cooler than average summer temperatures were generally associated with the highest expected abundances. We explored these seasonal relationships for monarchs breeding in Illinois to evaluate range-wide influences on observed abundances. Often, data available to explain factors influencing the dynamics of a given migratory population are limited to a fraction of the range used throughout the annual cycle. Extrapolating results from such studies to other geographic areas with different environmental conditions can lead to inaccurate predictions of range shifts and habitat suitability under future climate scenarios. By investigating the similarities and differences of climate impacts on breeding monarch butterflies in the eastern versus western portions of their breeding range, we demonstrate the importance of incorporating extensive spatial data to fully understand climatic drivers of abundance in a migratory species.