Density estimates of monarch butterflies overwintering in central Mexico

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 US 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.

Quantifying the Contribution of Habitats and Pathways to a Spatially Structured Population Facing Environmental Change

“…For instance, nearly all levels of perturbation indicate that the central breeding region for monarch butterflies contributes most to population growth. This finding is concordant with recent declines in this population, which dropped by 80% between 1993 and 2014 (Vidal and Rendon-Salinas 2014) due in part to the loss of milkweed in agricultural habitat of the midwestern United States (Pleasants and Oberhauser 2012;Flockhart et al 2015;Thogmartin et al 2017). However, if the overwintering area in Mexico were to undergo a similar degree of degradation, its importance to population growth would approach that of the central breeding region, validating concerns about loss of overwintering habitat for this species (Oberhauser and Peterson 2003;Ramirez et al 2015;Flores-Martínez et al 2019) Lemoine (2015) suggested that under both moderate and severe emission scenarios and associated atmospheric warming, much of the south and central regions will become less suited to monarchs, whereas the north region is expected to expand in suitability.…”

Size of the Canadian Breeding Population of Monarch Butterflies Is Driven by Factors Acting During Spring Migration and Recolonization

“…Estimates of the overwintering density of monarchs (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) were obtained from Thogmartin et al (2017a), which provides the raw observed hectares collected by the World Wildlife Fund Mexico, fitted hectares as estimated by Semmens et al (2016), and associated predicted population size in millions of individuals (see Thogmartin et al, 2017a for details).…”

“…We use monarch overwintering density estimates (Semmens et al, 2016;Thogmartin et al, 2017a) and butterfly survey data collected in the form of checklists by the Ontario Butterfly Atlas (2003Macnaughton et al, 2017), and eButterfly Prudic et al, 2017) to test four competing hypotheses on whether monarchs breeding in southern Canada are most strongly limited by factors acting during fall migration, winter, spring migration and recolonization, or summer breeding ( Table 1). We predicted that if monarchs are limited by factors acting during fall migration ("fall limitation hypothesis"), there will be weak correlation between relative abundance on the breeding grounds and subsequent wintering density; and if monarchs are limited by factors acting on their overwintering grounds ("winter limitation hypothesis"), there should be a strong correlation between previous winter density and relative abundance on the breeding grounds during the following summer.…”

An Evaluation of Studies on the Potential Threats Contributing to the Decline of Eastern Migratory North American Monarch Butterflies (Danaus plexippus)

“…Changes in suitable environmental conditions (Barve et al, 2012;Thogmartin et al, 2017b) and habitat at both breeding (Pleasants, 2017;Thogmartin et al, 2017a) and overwintering sites, as well as contaminant exposure (Oberhauser et al, 2006Pecenka and Lundgren, 2015), are thought to be foremost threats to monarch butterfly populations. Suboptimal environmental conditions during the overwintering period, such as unseasonably warm temperatures (Hunt and Tongen, 2017) or cold and wet microclimates that pose a risk of freezing (Anderson and Brower, 1996), can accelerate lipolysis that quickly depletes lipid stores needed for overwinter survival (Alonso-Mejía et al, 1997).…”