Monarch Butterflies Threatened Due To North American Habitat Loss
June 5, 2014

Monarch Butterflies Threatened Due To North American Habitat Loss

Brett Smith for - Your Universe Online

Populations of monarch butterflies are projected to decline in eastern North America and a new study published today in the Journal of Animal Ecology concluded that habitat loss in North America would be to blame.

"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 study author Ryan Norris, a biology professor at the University of Guelph.

The results of the new study go against the predominant belief that monarch butterflies are most susceptible to disturbances on wintering grounds in Mexico. They also validate suspicions that recent reductions have actually been driven by mating factors.

During the winter, monarch butterflies gather in a small region at high densities in Mexico. Scientists believed aspects affecting those wintering grounds, such as global warming or deforestation, were the biggest threat to the butterflies. These theories led to a number of Mexican presidential decrees to safeguard butterfly overwintering habitats and attempts to restrain 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 study author Tyler Flockhart, a post-doctoral researcher in Norris’ lab.

The scientists created a model to forecast effects of habitat loss on both mating and wintering grounds, as well as the effects of global warming. They tried to explain the population drop and make predictions for the next century.

They found a boost in genetically modified, herbicide-resistant crops has contributed to the ongoing population drop of monarch butterflies in eastern North America. Most notably, farming techniques have led to a 21-percent drop in milkweed plants, mostly in the butterflies’ central breeding region, between 1995 and 2013.

Milkweed is the only group of plants that monarch caterpillars eat before they turn into butterflies. Shifts in milkweed abundance can impact everything from larval competition for food to egg-laying in adults.

"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.

“Planting milkweed in the south and central United States would provide the largest immediate benefit,” Norris said.

A study published in December based on survey of Mexico’s Monarch Butterfly Biosphere Reserve found that monarch butterflies are in “grave danger.”

“The monarch butterfly as a species is not endangered. What is endangered is its migratory phenomenon from Canada to Mexico and back,” Omar Vidal, director of WWF-Mexico, said in an email to National Geographic in January.

Researchers from that study said gardeners in North America can help conservation efforts by planting milkweed and making their gardens more butterfly-friendly.

“Given the conservation challenges facing monarchs, it’s vitally important that we mobilize as many people as possible,” Oberhauser said. “Through our collective efforts, monarch populations can rebound, so that their migrations may be appreciated by many generations to come.”