University of Guelph researchers say
disappearance of the plant in North American breeding grounds is most
important factor in butterflies’ decline — estimated at 90 per cent this
year.
Colin McConnell
/ Toronto Star file photo
A new study says the loss of milkweed in monarch butterfly breeding
grounds is the key factor in the butterflies' decline, rather than the
degradation of their wintering grounds in Mexico.
The dramatic decline in the monarch butterfly
population in eastern North America is due largely to the steady loss of
milkweed crops in U.S. breeding grounds, according to a new study that
researchers say provides the first proof of the critical connection.
Scientists at the University of Guelph say
they picked apart the various factors linked to the drop in migratory
monarch butterflies and confirmed theories that the lack of milkweed is
likely responsible for the plummeting butterfly population.
Ryan Norris, a professor in the university’s
department of integrative biology, co-wrote the report and said it
provides the first evidence that monarch butterfly numbers are most
sensitive to changes in the availability of milkweed in certain breeding
areas.
“We’re losing milkweed throughout eastern
North America, but what we found out is milkweed loss specifically in
the midwestern U.S. is likely contributing the most to monarch
declines,” he said upon the study’s publication Wednesday in the Journal of Animal Ecology.
Norris said the loss of the tall, leafy plant
is being felt most in the corn belt region and other high-intensity
agricultural areas that are using herbicides to wipe out milkweed.
Norris, lead author Tyler Flockhart and
members of Australia’s national science agency say industrial farming
contributed to a 21-per-cent decline in milkweed plants between 1995 and
2013, mostly in the butterflies’ central breeding region.
The plant is important for the distinct orange
and black butterfly species because the insect lays its eggs on it. It
is also the only group of plants that monarch caterpillars feed on
before developing into butterflies.
“The rapid loss of milkweed projected for this
region, attributable to land cover changes and shifts in agricultural
practices, is a very large concern,” Flockhart said, adding that the
continued loss of milkweed will cause the monarch population to decline
further.
“Reducing the negative effects of milkweed
loss in the breeding grounds should be the top conservation priority to
slow or halt future population declines of the monarch in North
America.”
It’s estimated that there was a 90-per-cent decline in monarchs this past year, Norris said.
The David Suzuki Foundation estimates that
monarch populations in Mexico plummeted to a record low of about 33.5
million this year from an annual average of about 350 million.
The study’s findings, which are based on a
mathematical model that includes all known factors linked to the decline
of the butterflies, challenge long-held beliefs that their population
drop was due to the degradation of their wintering grounds in Mexico.
Norris said that led to several Mexican presidential decrees to protect butterfly habitats and curb illegal deforestation.
“We have essentially ignored what’s been
happening in the breeding grounds and focusing our efforts on the
wintering grounds, and this is kind of the situation we’re in,” he said.
The authors say the loss of milkweed crops is
likely not the only cause of the butterflies’ falling numbers, but
they’re calling on governments to restore milkweed habitats.
Mexico, the U.S. and Canada agreed in February
to form a working group on the conservation of monarch butterflies,
with discussions expected to include milkweed restoration.
