Population Trends of Monarch Butterflies (Lepidoptera: Nymphalidae) Migrating From the Core of Canada’s Eastern Breeding Population

Abstract

Abstract The Monarch butterfly (Danaus plexippus) (Linnaeus 1758) is being considered for up-listing to Endangered under the Canadian federal Species-at-Risk Act due to population declines recorded throughout the annual life cycle. Understanding local population dynamics is therefore necessary to assess the effect of environmental and human induced stressors, and to establish a benchmark from which management success can be measured through time. Using fall count data collected along the Lake Erie shoreline, which captures migrants moving south from the core eastern breeding population in Canada, monarch abundance trends are quantified. Count data from three migration count sites and one roost site were analyzed following similar methods as Crewe and McCracken (2015) to make findings comparable. Two of these datasets are newly compiled and are analyzed here for the first time. Results suggest that during the past 10 yr, the number of migrating monarchs along the north shore of Lake Erie has been stable [mean: −3.05% per year, credibility interval (CI): −13.15, 9.97], which is consistent with changes being observed on the wintering grounds. Only migration counts collected between 1995 and 2018 at the Long Point sites demonstrated significant abundance declines (5.25% per year, CI: −8.60, −1.39), which is a similar results to previous analysis of this dataset. Opportunities for future research are discussed within the context of using monarch count data for future conservation efforts.