Findings refute idea of monarchs' migration mortality as major cause of population decline

Date:
August 17, 2020
Source:
University of Kansas
Summary:
Research shows that the decline in the monarchs' overwintering
numbers is not due to an increase in the deaths of monarchs during
the migration.

The main determinant of yearly variation in overwintering population
size is the size of the summer monarch butterfly population.



FULL STORY ==========================================================================
In a new study, Monarch Watch Director Chip Taylor and colleagues have
shown that speculation regarding the declining monarch population,
despite having received much attention, is unsupported.


========================================================================== Published Aug. 7 in the journal Frontiers in Ecology and Evolution,
the researchers show that the decline in the monarchs' overwintering
numbers is not due to an increase in the deaths of monarchs during the migration -- the "migration mortality hypothesis." The main determinant
of yearly variation in overwintering population size, they found, is
the size of the summer population.

Taylor, a University of Kansas professor emeritus of ecology &
evolutionary biology, said the monarch butterfly populations have been declining for most of the last two decades. The numbers of monarchs
measured at the monarch overwintering sites in Mexico in the winter of 2013-2014 were an all-time low.

The progressive decline in prior years, and these low numbers, led to
the submission of a petition to the Department of the Interior to have
the monarch declared a threatened species. These concerns also increased
the search for an explanation for the decline.

The prevailing view was that the decline was due to habitat loss that
followed increased use of glyphosate herbicide on corn and soybean fields
in the Upper Midwest -- the "milkweed limitation hypothesis." However,
that view was challenged by a number of researchers who maintained that
the decline was likely due to increasingly high levels of mortality
during the butterflies' migration. This became known as the migration
mortality hypothesis.



========================================================================== Taylor said the migration mortality hypothesis, though unsupported
by data, has received substantial coverage in Science and Scientific
American.

"Monarch Watch has been collecting recovery data for tagged monarchs since 1992, and we knew that those advocating the migration mortality hypothesis
were on the wrong track from the outset and told them so," he said.

In this recently published study, Taylor and co-authors summarized the
results of tagging almost 1.4 million monarchs that resulted in nearly
14,000 recoveries of tagged butterflies in Mexico.

"Showing the migration mortality hypothesis advocates their assumptions
were wrong took awhile since that required a significant effort to vet
our monarch tagging database for accuracy and to analyze the data,"
Taylor said. "Dealing with 1.4 million records is no simple task."
In contrast to the predictions of the migration mortality advocates,
the tagging recoveries -- a measure of migration success -- did not
decrease over time, the researchers found.

In addition, the number tagged each year was correlated with the size
of the overwintering population in Mexico, consistent with the milkweed limitation hypothesis. The tagging also confirmed that the majority
of monarchs reaching the overwintering sites originated from the Upper
Midwest.

These findings support the conclusion reached by a team of experts that sustaining the monarch migration will require the restoration of over
a billion milkweed stems in the Upper Midwest in the coming years.

The co-authors on the paper were John Pleasants of Iowa State University;
Ralph Grundel and Samuel Pecoraro, both of the U.S. Geological Survey
Great Lakes Science Center; and James Lovett and Ann Ryan of Monarch
Watch and the Kansas Biological Survey.


========================================================================== Story Source: Materials provided by University_of_Kansas. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Orley R. Taylor, John M. Pleasants, Ralph Grundel, Samuel
D. Pecoraro,
James P. Lovett, Ann Ryan. Evaluating the Migration Mortality
Hypothesis Using Monarch Tagging Data. Frontiers in Ecology and
Evolution, 2020; 8 DOI: 10.3389/fevo.2020.00264 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200817191741.htm

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