Melting
Sea Ice Keeps Hungry Polar Bears on Land
Polar
bears, the iconic victims of climate change, are shifting their
migration patterns because of changes in sea ice. The bears are
arriving on land earlier and departing later, a new study found, and
it's threatening their access to food.
13
March, 2013
A
team of researchers studied the migration patterns of polar bears
(Ursus maritimus) in Hudson Bay, Canada, using satellite-tracking
data collected between 1991 and1997 and 2004 and 2009. They found
that the rate at which sea ice melts and re-freezes, as well as how
the ice is distributed around the bay, predicted when the bears
migrated onto or off of land. The findings are detailed today (March
19) in the Journal of Animal Ecology.
Animal
ecologists have been trying to figure out how seasonal conditions in
the environment affect animal migration patterns, and how climate
change can redistribute resources that affect those patterns.
"Keeping
track of when polar bears move on and off the ice is an important
aspect of monitoring the risks to the population associated with
climate change," study leader Seth Cherry, a graduate student in
ecology at the University of Alberta, Canada, told LiveScience in an
email.
Polar
bears hunt their main food source, seals, primarily while on sea ice.
Changes in the ice are driving the bears to spend more time on land,
where they have to go longer without eating and rely on fat reserves
to tide them over.
"Climate-induced
changes that cause sea ice to melt earlier, form later, or both,
likely affect the overall health of polar bears in the area,"
Cherry said.
Cherry
and colleagues fitted 109 female polar bears with tracking collars
(males can't wear collars because their necks are wider than their
heads). Putting a tracking collar on a polar bear is quite a feat.
The researchers located the bears from a helicopter and flew in close
to dart the bears. With the animals immobilized, the researchers
attached the collars, which were equipped with GPS transmitters that
beamed their location.
The
scientists also took measurements in the form of blood samples and
fat biopsies, which told them about the bears' diet and nutrition.
"When
we look at the migration patterns of the collared bears, it appears
as though bears in recent years are arriving on shore earlier in the
summer and leaving later in the autumn," Cherry said. "These
are precisely the kind of changes one would expect to see as a result
of a warming climate."
The
findings suggest that it's not only the distribution of sea ice that
affects the bears' migration, but how quickly that ice melts or
forms. When the ice melts in Hudson Bay, the bears spend longer on
land, where they are essentially fasting. Pregnant females are
particularly vulnerable, Cherry said, because when the other bears
resume hunting in late November or early December, these females stay
in dens on land to give birth to cubs and nurse them
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