From: Allison Winter, ENN
Published June 5, 2014 09:01 AM

Milkweed loss to blame for declining Monarch populations

Populations of the popular Monarch butterfly have been declining in recent years and a new study is citing habitat loss on US breeding grounds as the main culprit.

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The eastern North American monarch population is known not only for its iconic orange and black colors, but also for its late summer migration from the United States to Mexico, a migration covering thousands of miles. And despite the long-held belief that monarch butterflies are most vulnerable to disturbances on wintering grounds in Mexico, new research from the University of Guelph shows lack of milkweed in the US which provides breeding grounds for the species is playing more of a role for species decline.

"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, a professor in Guelph's Department of Integrative Biology.

During the winter months, monarch butterflies congregate in Mexico. Scientists thought factors on those wintering grounds, such as climate change or deforestation, were the greatest threat to the population. So as a result, many efforts have been made to protect butterfly overwintering habitats and to curb illegal deforestation.

"The protection of overwintering habitat has no doubt gone a long way towards conserving monarchs that breed throughout eastern North America. However, our results provide evidence that there is now another imminent threat," said Tyler Flockhart, lead author of the study.

Milkweed is the only group of plants that monarch caterpillars feed upon before they develop into butterflies. Industrial farming contributed to a 21-per-cent decline in milkweed plants between 1995 and 2013, and much of this loss occurred in the central breeding region, the study said.

More than 70 per cent of milkweed in this region is located in agricultural-intensive landscapes where genetically modified crops are increasing, the study said.

"The rapid loss of milkweed projected for this region, attributable to land cover changes and shifts in agricultural practices, is a very large concern," said Flockhart. Left unchecked, milkweed loss will cause the monarch population to decline by at least another 14 per cent, the study said.

"Reducing the negative effects of milkweed loss in the breeding grounds should be the top conservation priority to slow or halt future population declines of the monarch in North America," Flockhart said.

Norris added: "Planting milkweed in the south and central United States would provide the largest immediate benefit."

The study is published today in the Journal of Animal Ecology.

Read more at the University of Guelph.

Monarch butterfly image via Shutterstock.

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