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Article 3/9/05 |
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| Blue
Planet: The flight of the monarch
By Dan Whipple, UPI
March 9, 2005 |
Except
for Jiminy Cricket, the monarch butterfly is probably
the most popular insect in the world.
Danaus plexippus has been studied
by professional entomologists for generations. It is a
favorite learning tool in high school biology classes.
People release hordes of them in celebration at weddings
or simply observe them in the wild for their delicate
beauty.
Monarchs and their close relatives
are found on every continent except Antarctica. They measure
about 3 inches across the wingspread. Their four wings
compose a field of gold or orange, with veins of black
running through and a border of black dappled with white
spots. They spend much of their life cycle in pursuit
of milkweed, whose wide distribution is at least partly
responsible for the monarch's success.
In the eastern United States,
monarch populations undertake incredibly long seasonal
migrations -- as far as 3,000 miles -- from Canada to
their wintering grounds in Mexico. According to a recent
paper by two researchers at Emory University in Atlanta,
this journey may do much to ensure the evolutionary hardiness
of the species.
The reason: Monarchs are parasitized
by Ophryocystis elektroscirrha, a protozoan in the same
phylum as Plasmodium falciparum, the malaria parasite
that affects humans.
"Parasite prevalence is lower
among migratory (populations) relative to non-migratory
populations," wrote Sonia Altizer, assistant professor
of environmental studies, and her co-author, Catherine
Bradley, in the journal Ecology Letters.
The migration appears to weed
out animals infected with the parasite, reducing its effect
on the population and its transmission between generations
of butterflies.
The migration phenomenon and its
ecological impact are engaging the attention of biologists.
Many long-distance movements of mammals in North America
have been cut off by fences, cities, roads or population
decline. The American bison, for instance, used to migrate
hundreds of miles, but no longer. Now, bison that try
to leave Yellowstone National Park are shot for attempting
to follow their migratory instincts.
Altizer's work indicates that
for butterflies, at least, there may be important evolutionary
benefits to the migrations. In monarch populations that
do not migrate, Altizer found the parasite more virulent
and aggressive and the butterflies generally less healthy.
"The results of this experiment,
then, point to one reason why the migratory populations
are healthier," Altizer told UPI's Blue Planet, "and
we think that this example of small effects of parasites
being amplified by migration probably occurs in other
migratory species, too."
She cited the many bird and insect
species that migrate long distances.
"I think that even though
we're concerned about monarchs and protecting their migration,
this points to a possible general role of migration in
other species as well," she said.
Butterflies that carry the parasites
do not appear to be any different from other individuals
before they start the trip south, Altizer said. They weigh
the same, are the same size and cannot be separated from
non-parasitized animals without a microscope.
After the insects start to fly,
however, which Altizer and Bradley test using a little
butterfly treadmill -- they prefer to call it a "flymill"
-- the differences between the two begin to show up. Butterflies
that carry O. elektroscirrha lose weight faster, fly slower
and go for shorter distances. By the end of the 3,000
miles, presumably, most of the infected insects have dropped
out of the population, along with their parasites and
their genes.
Evolutionary success often is
viewed as predator vs. prey, but the kind of effective
resistance to disease and parasites exhibited by the monarch
is probably equally or more important to the success of
an individual. Many African-Americans, for instance, carry
a genetic variation that causes sickle cell anemia in
their blood. This is a disadvantageous condition in northern
climates, but the variation actually confers some immunity
to malaria in regions where that disease is prevalent
-- it is a survival advantage.
The arms race among microbes,
parasites and their hosts' defenses may be more important
to the evolution of species than Mother Nature's reliance
on tooth and claw vs. speed and agility.
Altizer said it remains unclear
whether O. elektroscirrha is a recent hitchhiker on monarchs,
though she suspects it is not. The parasite was discovered
on the insects about 30 years ago, but because it is found
on individuals in populations all around the world, it
may have evolved with the butterflies long before that.
Because of its wide distribution,
reliance on abundant milkweed and strategies to deal with
parasites, monarch butterflies probably are not in any
immediate peril -- but they do face challenges.
For example, recently, their population
in Mexico crashed because of an unseasonable freeze. This
is potentially dangerous for the monarchs because "their
overwintering roost is determined by a narrow range of
climate tolerance met only by this one area," Camille
Parmesan, a University of Texas biologist and one of the
world's leading experts on butterfly ecology, told Blue
Planet.
The most recent census in the
Mexico wintering grounds showed only about 2 hectares
of forest were covered by monarchs, which is the smallest
population since the area was discovered in the 1970s.
Altizer said they have covered
as many as 20 hectares in the past.
In her work, Parmesan has documented
changes in habitat and migration resulting from climate
change, which eventually could affect the butterfly's
migration pattern. Indeed, any such change could affect
the insect's fitness.
"Loss of migration will affect
the parasite," Altizer said. "The butterflies
may suffer more from the resident parasites."
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