A recent study by the World Wildlife Fund has shown an increase in migratory monarch butterfly populations in the past year. The monarch famously migrates up to 3,000 miles from Canada and the United States to overwinter in the Oyamel Fir forests of Mexico, a journey that takes several generations to complete. Instead of counting individual butterflies to estimate population size, researchers measure the total forest area that is occupied by monarch colonies during the winter. This year the monarchs occupied 4.01 hectares, a 255% increase on last year’s 1.13 hectares.
Fewer and fewer monarchs have arrived annually in Mexico over the past two decades, reaching a historic low of 33 million individuals in 2013. (To put this in historical perspective, researchers estimated in 1996 that 1 billion monarchs successfully migrated to Mexico, covering approximately 18 hectares of forest.) A myriad of threats, including unfavorable weather patterns and illegal logging in Mexico, have contributed to this decline. Recent research has shown that the greatest threat to monarch populations is the loss of milkweed plants in crucial breeding grounds in the U.S. Milkweed is the monarch’s host plant – the only plant on which female monarchs will lay eggs, and an essential food source for adults and caterpillars.
Mounting concern over the monarch’s decline resulted in a White House-led initiative, the “National Strategy to Promote Pollinator Health”, launched in summer 2015. The Strategy promotes the establishment of a monarch migration corridor along 1,500 miles of interstate highway I-35, which runs north-south through the central United States. The initiative takes advantage of federal “rights-of-way” along the interstate to allow agencies such as the Department of Transportation and the U.S. Fish and Wildlife Service (USFWS) to improve habitat for monarchs, bees and other pollinators. The route stretches from southern Texas to northern Minnesota, following key spring and summer breeding habitats along the monarch’s migratory route. The federal government hopes that the I-35 corridor will increase monarch populations to 225 million by 2020.
In 2015 the USFWS pledged $3.2 million for monarch conservation. It is prioritizing habitat conservation efforts along the corridor, with the goal of identifying and restoring over 200,000 acres of prairie and other native vegetation, including milkweed. The money will also be used to create a conservation fund that will offer habitat conservation grants to farmers and other landowners. Concurrent to these initiatives, the USFWS is reviewing a petition to list the monarch butterfly as a threatened species under the Endangered Species Act.
Flockhart, D. T. T., J.-B. Pichancourt, D. R. Norris, and T. G. Martin. 2015. Unravelling the annual cycle in a migratory animal: breeding-season habitat loss drives population declines of monarch butterflies. Journal of Animal Ecology 84(1): 155-165.