Animals regulate their numbers by own population density

d 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.


Contact: Janet Wong
University of Toronto

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