Charles J. Krebs

Zoology, Animals, Physiology, Metabolism

Famous for writing Ecology: The Experimental Analysis of Distribution and Abundance (now in its fifth edition), a textbook used worldwide to teach ecology, and for his work on the Fence Effect.

"We should be conservative in the ways we deal with natural systems."

Zoology is the study of animals. Krebs is an ecologist, a person who studies natural systems of plants and animals. His specialty is animal ecology, a combination of physiology — the study of the workings of an animal’s body — genetics, evolution and behaviour. One way animal ecologists conduct experiments is to mark off a section of wild country with a grid; the markers might be stakes in the ground, string or coloured ribbons. By keeping logs of the numbers and behaviour of animals in different sections of the grid, animal ecologists uncover facts about animals that help us understand more about the mysteries of nature.

Krebs is still trying to unravel the mystery of lemmings and other small northern mammals whose populations rise slowly and then fall suddenly, for no apparent reason, every four to 10 years. Hudson’s Bay Company fur-trading records show these fluctuations stretch back for hundreds of years. By accident, Krebs discovered something that might help explain what’s been happening. In 1965 he tried a simple experiment in an Indiana pasture: He fenced in an area of grassland the size of a soccer field to see what would happen to the population of voles living inside the fence. Voles are like mice, but they have shorter legs and heavier bodies. The fence extended down into the soil for several centimetres to stop tunnelling.

Amazingly, within a year he found that the population of voles had increased by about five times, much more than it would have had the field been left unfenced. The population changes that resulted were called the Fence Effect. It is now also called the Krebs Effect, since Krebs was the first to study animals this way. He has spent his working life trying to explain the Krebs Effect. He says, “You just put a fence up. You don’t do anything to the animals. So what is the fence doing?” In nature you find that populations of animal species on islands are much higher than similar populations on the mainland. Is this an example of a natural Krebs Effect?

Krebs says you have to learn a lot of details about the natural history of any animal you are trying to understand. He is driven by pure curiosity, a desire to learn more and more about the wonders of the natural world. His findings could ultimately be used to help manage wildlife or to design better, more sensitive methods for using natural resources.

Krebs’ largest research project spans 20 years, studying the 10-year population cycle of snowshoe hares and their predators in the Yukon, in collaboration with eight other scientists from three Canadian universities. The group has discovered that the snowshoe hare is the dominant herbivore in the Yukon boreal forest and that the changing size of its population is caused by predation by lynx, coyotes, great-horned owls and goshawks. Nearly 90 percent die because a predator kills them; almost no snowshoe hares die of starvation or disease.

In 1999 Krebs was one of 31 biologists who took part in the Swedish “Tundra Northwest” project, sailing on the Canadian icebreaker Louis St. Laurent. The group visited 17 sites during three months in the Canadian Arctic Archipelago, from Baffin Island to the north Yukon and as far as the north magnetic pole on Ellef Ringnes Island. The data collected on the expedition demonstrated food chain interactions between the plants, herbivores and predators of the Arctic.

Krebs says, “Ecology has become concentrated on two of the world’s most serious problems: conservation of endangered species and the impact of climate change on ecosystems. Both of these problems have been ignored since September 11, 2001 [when the attack on the World Trade Center in New York turned nations’ attention to global terrorism], and yet both are more serious in the long run than the problem of terrorism.” Ecologists work hard to find out how human impacts affect threatened species and how we can design parks and protected areas to conserve our natural heritage. Like many ecologists, Krebs has an almost religious drive to conserve the natural world for future generations.

The Fence Effect, also known as the Krebs Effect, is demonstrated here on Westham Island in Delta, British Columbia, with voles, a type of small rodent. A population explosion has occurred on the left side, the fenced-in area, and the voles have eaten everything except the thistles. After the population explosion there’s a population crash, in which almost all the voles die. What interested Krebs is that these population explosions and crashes occur in lemmings and other wild animals in nature.

This graph shows the population explosion and crash caused by the fence. The grey line is for voles in a similar but unfenced area. The black line shows the number of voles over time in the fenced area.

Lemmings are small rodents that look like guinea pigs. They live throughout the world in northern latitudes. Every four years, their population increases up to 500 times and then crashes to almost nothing. Even after 50 years of research, experts still don’t know exactly why populations of lemmings in the North seem to disappear like this. But one thing is for sure: they do not jump off cliffs.

Over the years Krebs has systematically eliminated possible reasons for the Fence Effect. Food is not a factor: you can supply the fenced-in area with unlimited food and the explosion-and-crash cycle is still observed. Predators are not the answer: You can make the fence low enough to let predators in but high enough to keep the voles from escaping; the Fence Effect still happens. The fence is never a deterrent for birds of prey like hawks, yet you still see the Fence Effect. Krebs now believes that the effect might be due to social behaviour among the voles and animals like them. For instance, male voles would naturally migrate — that is, move to other areas — but the fence stops them.

Also, as living conditions become more crowded within the fence, aggressive voles can’t leave or be kicked out, so they have more impact on other voles. According to Krebs, with voles the final crash seems to be caused by an increased tendency for mothers to kill all the babies in neighbouring mothers’ nests.



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Krebs still has not entirely figured out the mystery of the Fence Effect. One thing he would like to know is the following: The Fence Effect only happens if you put up a fence. If you don’t fence an area, you won’t necessarily see a population explosion and crash in that area. Yet you always see it if you fence in an area. Krebs wonders, How big does a fenced-in area have to be before the Fence Effect disappears?

Explore Further

Charles Krebs, Ecology: The Experimental Analysis of Distribution and Abundance, fifth edition, Benjamin Cummings, 2001.

Charles Krebs, S. Boutin and R. Boonstra (editors), Ecosystem Dynamics of the Boreal Forest: The Kluane Project, University of Oxford Press, 2001.

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