From: "Adam Weissman, Wetlands Preserve"
University of Toronto -
Animals regulate their numbers by own population
density. Zoologists from the University of Toronto have cracked the
ecological puzzle of how animals - in this case the arctic ground
squirrel - manage to control their own population in the northern boreal
forest of Canada.
In a study to be published in the Nov. 23  issue
of Nature, the researchers found that when arctic ground squirrel
populations reached the maximum limit the environment could support, the
females severely reduced reproduction and most died over winter during
hibernation, thus controlling the population.
"No population of organisms increases without limit. The
central question in population ecology is what regulates their numbers.
And the answer often is: the actions of the populations themselves,"
says Rudy Boonstra, a professor of zoology in the Division of Life
Sciences at the University of Toronto at
Scarborough and co-author of the paper. "The populations themselves are
critical to preventing unlimited growth. There are obviously other
processes going on - predators and things like that - but the regulation
that occurs in arctic ground squirrels is mainly dictated by the number
of fellow squirrels that are around it."
"Animals can change their reproductive output depending
on certain environmental conditions. And one of those environmental
conditions is population density," notes Tim Karels, lead author of the
paper who conducted the research as part of his PhD thesis at U of T.
"So if you have lots of neighbors and you're competing for the same
food, it can lower reproduction. And that's what we saw. At very high
population densities, female ground squirrels basically shut down their
reproduction, and that was done in order to sustain their own survival.
When conditions were better, they would start reproducing again."
The arctic ground squirrel lives in the tundra, alpine
and forested regions of the Northwest Territories, the Yukon and Alaska
and hibernates over winter. Karels conducted the research between the
spring of 1996 and spring of 1998 at the Arctic Institute Base at Kluane
Lake, about 200 km west of Whitehorse.
Karels and Boonstra took groups of arctic ground
squirrels that lived under certain conditions - one group was protected
from predators via an electric fence, another was provided with food in
the form of rabbit pellets, a third group was both protected from
predators and given food, and the last served as the control group. In
the spring of 1996, the food and protection were cut off to see how the
squirrel populations from these experimental groups would respond.
"In high density populations - which resulted when the
squirrels had both protection and food - the first thing we noticed is
that females stopped reproducing. They got pregnant but terminated
reproduction somewhere between pregnancy and when the babies should have
appeared above ground after weaning," says Karels.
The researchers believe the female squirrels shut down
reproduction in order to increase their own chance of survival. The cost
of reproduction is extraordinarily high, they say, since the squirrel
must provide nutrients for itself as well as a litter. Without food
provided by the researchers, the squirrels had to forage as they would
in their natural habitat.
Karels and Boonstra found that certain types of plants
that normally feed the squirrels were completely consumed in 1996.
Although the squirrels looked relatively healthy as winter came, the
researchers were surprised to find that 93 per cent in the highest
density population died that first winter. They
believe that the types of food needed to sustain certain types of body
fat throughout the winter were insufficient for the dense populations.
This study was funded by the Natural Sciences and
Engineering Research Council of Canada.
CONTACT: Janet Wong
U of T Department of Public Affairs
U of T at Scarborough Division of Life Sciences
University of Alberta Department of Biological Sciences
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