Model estimated annual over wintering population size for the Eastern, migratory population of monarch butterflies (Danaus plexippus), 1993-2014. The red line is the median of posterior estimates, and the gray shaded area shows 95 percent credible intervals. The x symbols define overwintering habitat area data from Mexico, and the e symbols represent observations of annual egg production in the Midwest scaled to match the magnitude of the overwintering data. The inset depicts the data and model results on a log-scale. Graphic: Semmens, et al., 2016 / Nature Scientific Reports

WASHINGTON, 21 March 2016 (Center for Biological Diversity) – The eastern migratory population of the monarch butterfly — which includes 99 percent of the world’s monarchs — is at high risk of extinction within two decades unless the population rebounds dramatically, according to a new study published today by Nature Scientific Reports.

The study from the U.S. Geological Survey and other scientists predicts an 11 percent to 57 percent chance of extinction for the monarch migration in the next 20 years. The study reports that well-documented declines in milkweed, the monarch caterpillar’s only food source, are highly correlated with increased use of herbicide-resistant, genetically engineered corn and soybeans, which now make up about 90 percent of all corn and soy grown in the United States.

“This new study confirms that GE crops are the driving cause of monarchs’ precipitous decline, as we have warned for years. Monarchs need protection under the Endangered Species Act or face extinction,” said George Kimbrell, senior attorney at the Center for Food Safety.

“We need to protect monarchs under the Endangered Species Act and increase protections for their summer breeding habitat, or the next generation of children may never see a monarch butterfly,” said Tierra Curry, a senior scientist at the Center for Biological Diversity.

The vast majority of the world’s monarchs are found in the eastern United States and undertake an annual multigenerational migration from Mexico to Canada. A smaller population of around 260,000 butterflies is found west of the Rocky Mountains and overwinters on the California coast. Today’s study estimates that the eastern population must increase to at least 225 million butterflies to cut the extinction risk by half. This year’s population was estimated at 150 million butterflies, though up to half of those may have been killed by a severe winter storm.

Earlier this month the Center for Food Safety and the Center for Biological Diversity filed a lawsuit against the U.S. Fish and Wildlife Service over the agency’s failure to protect monarch butterflies under the Endangered Species Act. The two groups and allies formally petitioned the Service in August 2014 to protect the monarch as a “threatened” species following a 90 percent population decline over the preceding two decades. In December 2014 the Service determined that protection may be warranted, triggering an official review of the butterfly’s status. The lawsuit requests that the court set a deadline for that decision.

The same day the most recent lawsuit was filed, the overwintering colonies in Mexico were struck by a severe winter storm. Scientists are still tallying the damage, but early estimates predict that 30 percent to 50 percent of the overwintering monarchs may have been killed. This could set the population back to the record lows of the previous two years before this year’s small rebound.

Though monarch numbers increased slightly this past year due to favorable weather conditions, the long-term outlook remains bleak, especially in light of the recent storm. Monarchs require a very large population size to be resilient to severe weather events and other threats. A single winter storm in 2002 killed an estimated 500 million monarchs — more than three times the size of the entire current population.

Study: World's Largest Monarch Population Could Disappear in 20 Years

ABSTRACT: The Eastern, migratory population of monarch butterflies (Danaus plexippus), an iconic North American insect, has declined by ~80% over the last decade. The monarch’s multi-generational migration between overwintering grounds in central Mexico and the summer breeding grounds in the northern U.S. and southern Canada is celebrated in all three countries and creates shared management responsibilities across North America. Here we present a novel Bayesian multivariate auto-regressive state-space model to assess quasi-extinction risk and aid in the establishment of a target population size for monarch conservation planning. We find that, given a range of plausible quasi-extinction thresholds, the population has a substantial probability of quasi-extinction, from 11–57% over 20 years, although uncertainty in these estimates is large. Exceptionally high population stochasticity, declining numbers, and a small current population size act in concert to drive this risk. An approximately 5-fold increase of the monarch population size (relative to the winter of 2014–15) is necessary to halve the current risk of quasi-extinction across all thresholds considered. Conserving the monarch migration thus requires active management to reverse population declines, and the establishment of an ambitious target population size goal to buffer against future environmentally driven variability.

Quasi-extinction risk and population targets for the Eastern, migratory population of monarch butterflies (Danaus plexippus)



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