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Conservation Status and Ecology of the Monarch Butterfly in the United States
Conservation Status and Ecology of the Monarch Butterfly in the United States

The monarch (Danaus plexippus) is widespread in the conterminous United States and in southern Canada. Scientists have documented recent 90% drop from the 20-year average population that overwinters in Mexico. The western monarch population had declined by an estimated 50% from its long term average. These declines are so severe that a group of biologists has petitioned the U.S. Fish and Wildlife Service to list the North American monarch as a threatened species under the U.S. Endangered Species Act.

Using population abundance, trend, and threat data, we employ the NatureServe conservation sta¬tus assessment methodology (Master et al. 2012) to determine the level of imperilment for the eastern and western monarch populations, the entire subspecies (Danaus plexippus plexippus) that includes these two populations, as well as the global monarch population (Danaus plexippus). As a species, the monarch is apparently secure (Table 1). Across the range of the species, populations in many places where it is not strongly migratory or is nonnative remain apparently stable such that the species is not in immediate danger of extinction. However, the subspecies occurring in North America and the two North America populations are threatened. The recent, rapid decline and widespread threats to the eastern monarch population qualify it as critically imperiled. The western population, with a slightly slower rate of decline and less widespread threats is categorized as vulnerable to imperiled. Thus despite the species as a whole being apparently secure, the two major populations at the heart of the range are now threatened with extinction. The subspecies that includes these two populations, Danaus plexippus plexippus, is also vulnerable to extinction.

Three factors appear most important to explain the decline of eastern monarchs: loss of milkweed breeding habitat due to increased use of herbicides on genetically modified herbicide-resistant cropland and land conversion, logging at overwintering sites, and climate change and extreme weather. In addi¬tion, natural enemies such as diseases, predators, and parasites, as well as insecticides used in agricul¬tural areas may also contribute to the decline.

In this report, we briefly summarize the monarch’s North American distribution, life history, pop-ulation, current conservation status, and potential causes of decline. In addition, we include a set of breeding and overwintering habitat management recommendations. This report aims to inform gov¬ernment agencies charged with biodiversity protection, as well as conservation organizations and the public in general about the threats to and current conservation status of this much-loved, iconic insect.

Jepsen, S., D. F. Schweitzer, B. Young, N. Sears, M. Ormes, and S. H. Black. 2015. Conservation Status and Ecology of Monarchs in the United States. NatureServe, Arlington, Virginia, and the Xerces Society for Invertebrate Conservation, Portland, Oregon.