Monarch butterfly populations may be in danger due to the loss of milkweeds, a new study has found. 

Biologists have recognized that populations of monarch butterflies have been declining throughout the eastern half of North America. It has been more than 20 years since the first declines were noted in scientific journals. 

Researchers at the University of Guelph in Ontario, Canada investigated different factors that may play a role in the disappearance. After eliminating several other possibilities as the primary cause for the vanishing insects, investigators determined the loss of milkweeds were primarily responsible for the population decline. 

"Three hypotheses have been posed to explain the decline: habitat loss on the overwintering grounds in Mexico, habitat loss on the breeding grounds in the United States and Canada, and extreme weather events," researchers wrote in an article detailing their study.

Ryan Norris, co-author of the study, told the press that this report contains the first evidence that the long-term health of Monarch butterflies depends on access to milkweeds. These insects lay their eggs on the weeds, making the plant invaluable in supporting the species. 

Farmers in the corn-belt region of the United States are using vast quantities of herbicides, in an effort to wipe out milkweed. Numbers of the plant have declined 21 percent between 1995 and 2013. Most of these losses were in areas used as breeding grounds by the bright orange butterflies. 

Investigators used a mathematical model to predict Monarch butterfly loses in the future, and found the species will likely decline 14 percent over the next 100 years, if conditions continue as they are now. The species also have a five percent chance of going extinct during that time, based on the new study. 

Many biologists believed the decline of the distinctive insects may have been caused by environmental changes in Mexico. This theory, tested by researchers proved to not be a primary factor. Extreme weather events were also shown not to be a leading reason for the loss of Monarch butterfly populations. 

"Simulations that considered only forest loss or climate change in Mexico predicted higher population sizes compared to milkweed declines on the breeding grounds," the team wrote in their journal article. 

The effects of genetically-modified crops also play a role. The team recommends reducing the presence of genetically-modified crops to assist recovery of Monarch populations. 

Investigation of monarch butterflies and how the loss of their milkweed habitats is affecting populations is detailed in the Journal of Animal Ecology.

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