Spatiotemporal Patterns of Population Decline in Oarisma poweshiek (Hesperiidae) in Michigan and Minnesota Between 1990 and 2013

The recent contraction of the geographic range of Oarisma poweshiek (Poweshiek skipperling) (Lepidoptera: Hesperiidae) resulted in its listing as an endangered species in 2014 (USFWS 2014). However, changes in population density within sites associated with this contraction are still not understood. This range-wide contraction of Poweshiek skipperling appears to indicate a regional cause. Using population data from multiple sites in Michigan and Minnesota, we investigated the temporal and spatial patterns of population decline to determine if regional or site-specific factors likely contributed to the decline. We hypothesized that if regional factors caused the density declines, then the declines would be temporally synchronous and spatially correlated across sites. We found decline patterns to be temporally asynchronous and spatially uncorrelated, except for sites in west-central Minnesota. These results imply that regional factors may not be the main causes for the population density declines, and understanding multiple site-specific factors consistent with observed patterns of decline will be important for recovering Poweshiek skipperling populations.