New study on migration success reinforces need for monarch butterfly
milkweed habitat

Date:
September 2, 2020
Source:
Iowa State University
Summary:
A recent study presents evidence that the migration success of
monarchs hasn't declined in recent years and thus cannot explain
the steep decline in the monarch population over the last few
decades. The study drew on data collected on 1.4 million monarch
butterflies that were tagged in the United States Midwest from
1998 to 2015 and emphasizes the need for new monarch habitat.



FULL STORY ==========================================================================
A recently published analysis of data on tagged monarch butterflies
migrating from the United States to Mexico emphasizes the importance
of creating new habitat to ensure the future of the species' iconic
migratory pattern.


==========================================================================
The study, published this month in the peer-reviewed journal Frontiers
in Ecology and Evolution, drew on data collected on 1.4 million monarch butterflies that were tagged in the United States Midwest from 1998
to 2015.

The study presents evidence that the migration success of monarchs
hasn't declined and thus cannot explain the steep decline in the monarch population over the last few decades.

Several lines of evidence support the primary hypothesis for the monarch population decline, which is the loss of milkweed habitat. John Pleasants,
an adjunct associate professor in Iowa State University's Department
of Ecology, Evolution and Organismal Biology, said the analysis should
put to rest this persistent alternative explanation for the population
decline that posits monarch butterflies are experiencing increasing
mortality during their fall migration to Mexico, Pleasants said.

"If there was some problem with migration, we should have found fewer
tagged monarchs recovered in Mexico over time, but that was not the case,"
he said.

Monarch butterflies carry out a remarkable migration pattern year
after year.

Monarchs born in Midwestern states move south during the late summer and
fall and arrive in central Mexico for the winter. The monarchs then move northward again in the spring. But the monarch population has dwindled
to such an extent over the last two decades that scientists worried the migratory system could collapse forever.

Focus on milkweed In response, the Iowa Monarch Conservation Consortium,
a diverse partnership of 45 organizations supported by Iowa State
University, the Iowa Department of Agriculture and Land Stewardship,
and the Iowa Department of Natural Resources, is spearheading an effort
to plant between 480,000 and 830,000 acres of new habitat by 2038. This
effort focuses on milkweed, the only plant on which monarchs will
lay eggs.

Pleasants said some researchers looked at yearly surveys of monarch
adults and did not find a decline at the same time the overwintering
population in Mexico was falling and hypothesized that increasing
mortality during the southward migration may be driving the overall
population decline. Those researchers suggested increased parasite load
or declining nectar availability in Texas might contribute to migratory mortality. However, Pleasants said no reliable data show a decline in
those factors. Instead, Pleasants said the data show no trend in the
tag recovery rate, an indication the migratory journey hasn't become
more dangerous over the years.

Pleasants said the discrepancy between the surveys of the summer
populations and the overwintering population likely stems from the loss
of milkweed habitat on agricultural land in the Midwest. Milkweed was
once plentiful in farm fields, providing plenty of habitat for monarchs
in rural areas, until about 2006 when it had all but disappeared due
to herbicide use. Surveys done in the late 1990s and early 2000s did
not include field habitat and therefore missed the monarchs in fields, underestimating the true size of the population, Pleasants said. Since
2006, population estimates from those surveys have been highly correlated
with overwintering numbers.

"Our analysis points us back to the idea that the loss of milkweeds, particularly from agricultural fields, is most responsible for this
decline," he said. "If you want to bring the monarch butterfly back,
you need to bring the milkweeds back."

========================================================================== Story Source: Materials provided by Iowa_State_University. 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/2020/09/200902114349.htm

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