Each fall, volunteers in New Jersey and Michigan keep a careful count of the colorful monarch butterflies they see fluttering south on their annual migration. Now, an analysis of those counts suggests that monarch populations have held steady over the last few decades – in marked contrast to a recent study that documented a worrying decline in wintering colonies in Mexico. The discrepancy has researchers struggling to understand exactly how the species is doing.
“The status of the eastern North American monarch butterfly population is a highly sensitive issue, given that winter and breeding habitats are being lost at an alarming rate each year,” Andrew Davis of the University of Georgia in Athens writes in Insect Conservation and Diversity. “Because of this, most believe the population to be declining, although there has been little empirical data to support this idea.”
In March, however, researchers published a study in the same journal (see Monarch Decline, Conservation Magazine, Summer 2011) reporting that Mexico’s wintering colonies had experienced an overall decline over the past 17 years, including a record low census in 2009-2010. That study, Davis notes, suggested the decline was “the first sign of impending collapse” of Monarch populations.
But Davis wondered how the Mexican numbers compared with the migration counts conducted each fall at monitoring stations in Cape May, New Jersey – which had data for 15 years — and Peninsula Point, Michigan, which had 19 years of numbers. After analyzing the counts, he found no overall decline: “At both locations there was no significant linear trend in average monarch numbers counted over time.” That suggests “the population remains stable for now,” he concluded, “probably because of the high fecundity of the species and its ability to rebound from small winter numbers.”
But why are the U.S. and Mexican numbers so different? It’s surprising, he notes, “that these three data sets are not more consistent. In other words, large numbers of fall migrants should lead to large wintering colonies and vice versa.”
One possibility is that existing data is simply too crude to “estimate the number of individuals in a population that has a massive range and numbers in the millions.” Another is that many of the migrating butterflies die before they reach Mexico – or that they are finding new places to spend the winter. “Indeed, alternative wintering sites are not unheard of for this population, with known areas being in Cuba, southern Florida and increasing reports of locations along the US Gulf coast,” he writes. So far, however, none of these new overwintering sites seem to hold the millions of “missing monarchs” from Mexico.
Another explanation may be that the butterflies have high enough reproductive rates to bounce back from bad years. “Even though the cohort in Mexico may be shrinking, there still could be enough monarchs each year that survive to re-colonize the breeding range in the United States and Canada,” he writes.
The take-home message, he says, is that the monarch “picture seems
more complicated than what is portrayed… So even though the decline in suitable breeding and wintering habitat makes it a foregone conclusion that this unique and well-studied population of monarchs may someday collapse, I contend that it does not appear to be doing so just yet.” – David Malakoff | July 9, 2011
DAVIS, A. (2011). Are migratory monarchs really declining in eastern North America? Examining evidence from two fall census programs. Insect Conservation and Diversity DOI: 10.1111/j.1752-4598.2011.00158.x
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