Abstract
Efforts to conserve the migratory phenomenon of monarch butterflies in eastern North America have increased since a 2013-2014 monitoring report documenting a historical population low at the Monarch Butterfly Biosphere Reserve in Mexico. Surprisingly, there have been few systematic attempts to develop predictive models of monarch butterfly distributions along their migratory route between Mexico and the United States and Canada. Here we produced monthly habitat suitability models for monarch butterflies along their migratory route to identify potential areas for resting, feeding, and reproduction of the population. We compiled a point occurrence database of monarch butterflies for Mexico, USA, and Canada, including georeferenced records from GBIF, the Naturalista platform in Mexico, Correo Real initiative, and the Mexican governmental monitoring network for the monarch butterfly. We produced monthly habitat suitability models (HSM), using the R language and environment for statistical computing, abiotic (WorldClim), edaphic, and topographic variables. A total of 95 HSM were produced for each month. June to September, corresponding to the reproduction months in North America showed the highest geographic extent with suitable habitats; April, corresponding to the reproduction of the first post-migration generation, showed the smallest area. September, October and November, correspond to the movement of the monarch butterflies southwards, showed typical recognized distribution of the phenomenon and the overwintering months. December to February showed the smallest geographic extent in habitat suitability. Edaphic variables ranked high in importance in HSM for 11 of 12 months, indicating the relevance of vegetation and floral resources in the monarch butterfly migration route. Identifying such regions contribute to establish concrete conservation programs accordingly, as reduction of the use of pesticides and herbicides, decrease in the speed of cars in roads, and planting species with high nectary value, among others.. Our study provides a first predictive spatio-temporal approximation of the monarch butterfly migratory route annual cycle..
Highlights
The eastern North American monarch butterfly (Danaus plexippus) population undertakes the longest documented insect migration in the world (Agrawal, 2017; Sarkar, 2017)
Model predictive performance, estimated by the AUC based on the test set and true skill statistic (TSS) are shown in Table 2. tAUC scores had a range from 0.63 to 0.90 while TSS ranged from 0.40 to 0.69
Thousands of individuals across North America participate annually in different initiatives to monitor the presence of monarch butterflies and these efforts allow, year after year, the delineation of a sketch of the movements of monarch butterflies
Summary
The eastern North American monarch butterfly (Danaus plexippus) population undertakes the longest documented insect migration in the world (Agrawal, 2017; Sarkar, 2017). Different threats have been proposed to negatively affect monarch butterflies along their migratory route including (a) the lack of availability of milkweed (Asclepias ssp.) in their breeding areas due to pesticide use along their migratory route, contributing to loss of vegetation (the milkweed limitation hypothesis; Brower et al, 2012); (b) individual mortality during the fall migration (the migration survival hypothesis; Agrawal, 2017); (c) decline in the size of the winter habitat for roosting in Mexico (the winter habitat loss hypothesis; Brower et al, 2012); (d) extreme climatic events in their overwintering area (the climate hypothesis; Brower et al, 2012), and the loss of nectar resources (Malcolm, 2018). The relative role of each of the proposed mechanisms will require further research
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