How Animals Regulate Their Own Numbers

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 neighbours 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.

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