Animals regulate their numbers by own population density

November 22, 2000

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.
This study was funded by the Natural Sciences and Engineering Research Council of Canada.


Janet Wong
U of T Department of Public Affairs
(416) 978-6974

Rudy Boonstra
U of T at Scarborough Division of Life Sciences
(416) 287-7419

Tim Karels
University of Alberta Department of Biological Sciences
(780) 492-9878

University of Toronto

Related Predators Articles from Brightsurf:

Boo! How do mexican cavefish escape predators?
When startled, do all fish respond the same way? A few fish, like Mexican cavefish, have evolved in unique environments without any predators.

Herbivores, not predators, most at risk of extinction
One million years ago, the extinction of large-bodied plant-eaters changed the trajectory of life on Earth.

Bugs resort to several colours to protect themselves from predators
New research has revealed for the first time that shield bugs use a variety of colours throughout their lives to avoid predators.

Jellyfish contain no calories, so why do they still attract predators?
New study shows that jellyfish are an important food source for many animals.

'Matador' guppies trick predators
Trinidadian guppies behave like matadors, focusing a predator's point of attack before dodging away at the last moment, new research shows.

The European viper uses cloak-and-dazzle to escape predators
Research of the University of Jyväskylä demonstrates that the characteristic zig-zag pattern on a viper's back performs opposing functions during a predation event.

Predators help prey adapt to an uncertain future
What effect does extinction of species have on the evolution of surviving species?

To warn or to hide from predators?: New computer simulation provides answers
Some toxic animals are bright to warn predators from attacking them, and some hide the warning colors, showing them only at the very last moment when they are about to be attacked.

Dragonflies are efficient predators
A study led by the University of Turku, Finland, has found that small, fiercely predatory damselflies catch and eat hundreds of thousands of insects during a single summer -- in an area surrounding just a single pond.

Predators to spare
In 2014, a disease of epidemic proportions gripped the West Coast of the US.

Read More: Predators News and Predators Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to