Western Monarch Population Viability Analysis

Description

Monarch butterflies (Danaus plexippus plexippus L. Lepidoptera: Danainae) across North America are in decline. Recent studies, largely based on trends in monarch overwintering abundance in Mexico and coastal California, indicate that over the past two decades populations across eastern North American have declined by as much as 80% and populations in the west have declined by more than 70% (Pelton et al. 2016; Semmens et al. 2016). Analyses by Semmens et al (2016) predict a quasi-extinction probability for eastern monarchs of 3⿿42% over 10 years and 16⿿62% over 20 years, and a need to increase the population five-fold to halve the extinction risk. No similar analyses have been conducted for monarchs in the west. Washington State University (Recipient) will conduct a parallel population viability analysis for western monarchs, modified based on current knowledge and structure of the overwintering population in coastal California. As a preliminary evaluation of trend, we estimated the rate of change through time using multivariate autoregressive state space models (following the analysis used by Semmens et al 2016). Our preliminary analysis of western monarchs was based on Western Monarch Thanksgiving Count data from 1997-2014. The estimated population growth rate of western monarchs in this analysis was u = -0.077 (roughly equivalent to λ = 0.93). For comparison, Semmens et al⿿s (2016) estimated population growth rate for eastern monarchs is u = -0.061 (extended data Table 2, roughly equivalent to λ = 0.94). The Recipient will conduct additional analyses in order to: 1) provide a robust estimate of the probability of quasi-extinction for western monarchs over the next 20 years, and 2) estimate the magnitude of change in population size needed to halve the extinction risk for the western monarch. In addition, we will explore ways to extend the model to include covariates that 3) estimate changes in population trend over time to investigate if the rate of change over the last 20 years has been constant or not, and 4) assess the influence of co-variates such as drought and pesticides on extinction risk for western monarchs. These analyses will help guide assessment of the status of western monarchs and direct future priorities for research and conservation. This award is entered into under the U.S. Fish and Wildlife Service's CFDA 15.658 under the authority of Fish and Wildlife Coordination Act of 1958, 16 U.S.C.661; Land and Water Conservation Fund Act of 1965, as amended (16 U.S.C. 4601-4 through 11).

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