In the face of scientific dogma that
faults the population decline of monarch butterflies on a lack of
milkweed, herbicides and genetically modified crops, a new Cornell
University study casts wider blame: sparse autumnal nectar sources,
weather and habitat fragmentation.
"Thanks to years of data collected by the World
Wildlife Fund and citizen-scientists across North America, we have
pieced together the monarch life cycle to make inferences about what is
impacting the butterflies," said Anurag Agrawal, Cornell University
professor of ecology and evolutionary biology and senior author on the
new paper, to be published April 25 in the journal Oikos.
The scientists did not find evidence supporting the
"milkweed-limitation hypothesis" during the monarch's summer breeding
season in the midwestern and northeastern United States. Rather, through
statistical analyses, the group found problems in the transition from
the U.S. and southern Canada to the overwintering grounds in Mexico.
Milkweed is only a food source for the caterpillars in summer, but not
as the butterflies leave for their epic southern migration in autumn.
The study finds that a "lack of milkweed, the only host plant for
monarch butterfly caterpillars, is unlikely to be driving the monarch's population decline, as the problem appears to occur after they take flight in the fall," said Agrawal.
In any given year, four generations of monarch butterflies
traverse much of North America over a 2,000-mile trek beginning in
early spring when they leave the Mexican wintering grounds. In the first
generation, millions of monarchs flow through Texas and Oklahoma, with
the subsequent generations moving into the Midwest and Northeast, until
the start of fall, when the fourth generation returns to the
mountainous, high-altitude Oyamel fir forests of central Mexico.
Despite the seemingly good news of annual population
bounce-back on the return from the south each year, the scientists were
clear that the monarch population has been dwindling. Yes, said
Agrawal: "The consistent decline at the overwintering sites in Mexico is
cause for concern. Nonetheless, the population is six times what it was
two years ago, when it was at its all-time low." Agrawal credits the
population rebound to improved weather and release from the severe
drought in Texas.
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Agrawal said that a persistent decline caused by lack of
nectar sources or other threats such as habitat loss or insecticide use
can conspire with large annual population fluctuations - mostly due to
weather - and may eventually push monarchs to dangerously low numbers.
"Given the intense interest in monarch conservation, the
blame being put on herbicide use and the national dialog about
potentially listing monarchs under the endangered species act, we have
to get the science right," said Agrawal.
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