Why are Monarch Butterflies Declining in the West? Understanding the Importance of Multiple Correlated Drivers

Abstract

Study summaryMonarch butterflies in the western United States have declined more than the larger eastern population. However, less is known about causes of their declines. We evaluated climate and land use as drivers of monarch abundance using partial least squares regression (PLSR). Land development near overwintering sites and herbicide and insecticide use were stronger predictors of declines than climate variables. This result offers a positive message for restoration, in the sense that land use is more directly under local management control than climate. It also points to the need for further research to discriminate effects of these correlated variables.