Photo: Monarch butterflies
The principal cause of the shrinking population of monarch butterflies is loss of habitat in their U.S. breeding grounds, Canadian scientists say in a study published in the Journal of Animal Ecology.
“Our work provides the first evidence that monarch butterfly numbers in eastern North America are most sensitive to changes in the availability of milkweed on breeding grounds, particularly in the Corn Belt region of the United States,” said Ryan Norris, professor in the University of Guelph’s Department of Integrative Biology.
The research contradicts the view that monarch populations are most threatened by ecological degradation in western Mexico, where the butterflies spend the winter.
Successive Mexican administrations have taken steps to protest the monarchs’ winter quarters, including more aggressive action against illegal logging.
“The protection of over-wintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America,” study lead author Tyler Flockhart said. “However, our results provide evidence that there is now another imminent threat.”
Monarch caterpillars feed exclusively on milkweed before they develop into butterflies.
Milkweed declined 21 percent in the U.S. Corn Belt between 1995 and 2013.
More than 70 percent of the milkweed in that region lies in areas of intensive, mechanized agriculture where the use of genetically modified crops is increasing, according to the study.
Without action to stop the loss of milkweed, the monarch population will decline by at least an additional 14 per cent, the researchers said.
“Planting milkweed in the south and central United States would provide the largest immediate benefit,” Norris said.