Density estimates of monarch butterflies overwintering in central Mexico

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1 Upper Midwest Environmental Sciences Center, United States Geological Survey, La Crosse, Wisconsin, United States
2 Geosciences and Environmental Change Science Center, United States Geological Survey, Lakewood, Colorado, United States
3 School of Natural Resources and the Environment and Udall Center for Studies in Public Policy, University of Arizona, Tucson, Arizona, United States
4 Department of Fisheries, Wildlife and Conservation Biology, University of Minnesota, St. Paul, Minnesota, United States
5 Department of Ecology, Evolution, and Organismal Biology, Iowa State University, Ames, Iowa, United States
6 Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, United States
7 Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas, United States
8 Everglades Foundation, Palmetto Bay, FL, United States
DOI
10.7287/peerj.preprints.2832v1
Published
Accepted
Subject Areas
Conservation Biology, Ecology, Entomology, Mathematical Biology
Keywords
Mixture distribution, Monarch butterfly, Uncertainty modeling, Danaus plexxipus, Density estimation
Copyright
© 2017 Thogmartin et al.
Licence
This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, reproduction and adaptation in any medium and for any purpose provided that it is properly attributed. For attribution, the original author(s), title, publication source (PeerJ Preprints) and either DOI or URL of the article must be cited.
Cite this article
Thogmartin W, Diffendorfer JE, Lopez-Hoffman L, Oberhauser K, Pleasants J, Semmens BX, Semmens D, Taylor OR, Wiederholt R. 2017. Density estimates of monarch butterflies overwintering in central Mexico. PeerJ Preprints 5:e2832v1

Abstract

Given the rapid population decline and recent petition for listing of the monarch butterfly (Danaus plexippus L.) under the Endangered Species Act, an accurate estimate of the Eastern, migratory population size is needed. Because of difficulty in counting individual monarchs, the number of hectares occupied by monarchs in the overwintering area is commonly used as a proxy for population size, which is then multiplied by the density of individuals per hectare to estimate population size. There is, however, considerable variation in published estimates of overwintering density, ranging from 6.9–60.9 million ha-1. We develop a probability distribution for overwinter density of monarch butterflies from six published density estimates. The mean density among the mixture of the six published estimates was ~27.9 million butterflies ha-1 (95% CI: 2.4–80.7 million ha-1); the mixture distribution is approximately log-normal, and as such is better represented by the median (21.1 million butterflies ha-1). Based upon assumptions regarding the number of milkweed needed to support monarchs, the amount of milkweed (Asclepias spp.) lost (0.86 billion stems) in the northern U.S. plus the amount of milkweed remaining (1.34 billion stems), we estimate >1.8 billion stems is needed to return monarchs to an average population size of 6 ha. Considerable uncertainty exists in this required amount of milkweed because of the considerable uncertainty occurring in overwinter density estimates. Nevertheless, the estimate is on the same order as other published estimates. The studies included in our synthesis differ substantially by year, location, method, and measures of precision. A better understanding of the factors influencing overwintering density across space and time would be valuable for increasing the precision of conservation recommendations.

Author Comment

This is a submission to PeerJ for review.

Supplemental Information

Supplemental Information: Code for calculations

DOI: 10.7287/peerj.preprints.2832v1/supp-3