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."

The Story

Charley Krebs sits at the back of the sled, tired and happy to be pulled along by the noisy snowmobile. Its high-pitched whine breaks the serenity of the frozen lake in Canada’s North. But Krebs loves it, the crisp cold, the wide-open whiteness. The wind sprays snow in his face as the snowmobile plows through another drift.

Krebs thinks about the morning as the sled swooshes along over bumps and cracks in the ice. With him are four ecology students from the University of British Columbia, two riding in front of him on the sled loaded with research equipment, two up front on the snowmobile. They are returning from a six-hour session of tagging snowshoe hares on a remote island in Kluane Lake in the southwest Yukon Territory. Working in teams, they have just finished checking live-hare traps placed throughout the island. They have removed many hares from the traps, taken notes on weight, sex and health and clipped id tags on their ears before letting them go. They have piles of notebooks to show for their work.

Krebs is satisfied. He’s thinking about how all this new information will fit into a paper he’s working on, when suddenly the student driving the snowmobile yells, “Hold on, guys!” and revs the engine to jump across a crack in the ice. It’s early May 1980. The lake is starting to break up with the spring thaw and they have crossed a few cracks already. The snowmobile makes it across, but it opens the crack too much. Before they know it, the sled, the equipment and three people are sinking fast in icy water. It doesn’t help that they’re all wearing winter parkas and heavy snow boots. Krebs treads water with one hand and holds the bundle of notebooks up with the other, yelling, “Save the data! Save the data!”

They did save the data in the notebooks, and themselves, fortunately. But his students never let Krebs forget that day. Data are hard to collect when you are a wildlife biologist like Krebs. He doesn’t work in a laboratory. His lab is the great outdoors. Since animal life cycles take years, you need decades of observation and data collection to understand a particular animal.

One of Krebs’ students tells a similar story. Once, with a different group of students, Krebs became trapped on an island in Kluane Lake. They had been flown in for the annual hare-counting and tagging session, but weather conditions became so bad that no plane could return to pick them up when they were done. The stormy weather went on and on. After they had been there two weeks longer than planned, the food ran out and the students suggested they eat some of the hares. Krebs refused, because the hares were his experimental subjects. The students were frustrated. They had all these traps and there were plenty of animals. Were they going to starve to death for the benefit of science? After a few more days, when people were getting really ravenous, Krebs finally relented and said they could catch and eat hares — but only if they didn’t have ear tags.

As A Young Scientist...

Charles “Charley” Krebs grew up in a small Illinois town near St. Louis, across the state line in Missouri. He remembers as a kid fishing for catfish in local rivers with his grandfather. Charley admired his grandfather and his tales of natural adventures and wildlife. In particular Krebs was drawn to the Canadian Arctic. At eight years of age he wanted to be a forest ranger. Even then, he was reading books about basic ecology and the science of wild animals. He was fascinated by the big mysteries of the North — for instance, did lemmings really commit mass suicide by jumping off cliffs?

All through his high school years, Krebs had an unusual summer job. He worked for a St. Louis fur-trading company harvesting seals in the Bering Sea. Each summer he travelled by train for three days to Seattle, then by boat for seven days up the west coast of Canada to the northern islands. Krebs was curious about all the wildlife on the islands.

After getting his bachelor of science (BSc) degree, Krebs moved to Vancouver to study at the University of British Columbia with Dennis Chitty, who was (and still is) the world expert on lemmings. Krebs obtained a master of arts (MA) and a doctorate (PhD) in zoology, then after a two-year fellowship at Berkeley, California, he went back east to teach zoology at Indiana University. In 1970 Krebs returned to Vancouver and he has been there ever since as a professor of zoology at UBC.

In 2002 Krebs retired from teaching and began spending part of the winter working with the Rodent Research Group at the Commonwealth Scientific and Industrial Research Organisation in Canberra, Australia, to help them figure out why house mice in Australia reach very high populations at irregular intervals and cause extensive damage to grain crops. He also continues to work on snowshoe hares and other animals of the boreal forest in southwestern Yukon, Canada.

Krebs’ textbook, Ecology, is the standard teaching text for ecology courses worldwide. His interests have become a family affair. His wife is a research associate in ecology at UBC and often goes on field trips with him. His son works for the B. C. Ministry of the Environment, and his daughter is an expert on birds.

The Science

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.



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.

Career Advice

So You Want to Be an Ecologist

A wildlife ecologist never has a typical day. A university professor will spend several hours a week lecturing, many hours talking to students, several hours going to research seminars and many hours reading and writing scientific papers. Krebs typically spends about three months of each year in the field doing physical work: live-trapping hares and other animals, measuring trees and vegetation and doing other field work.

The thing he likes most about being an ecologist is the freedom to explore important intellectual issues and questions whenever he likes, for as long as he wants. He says, “In short, I have the ability to think on my own schedule.” What he dislikes is dealing with uninformed bureaucracies, both in universities and governments.

Wildlife ecologists can work for universities, the government or private consulting companies. Many mining and forestry companies contract out their environmental studies to private consulting firms. If you want the freedom to work on anything you like, then a university career will offer the best opportunities, but if you want to make money you would be better off working for an ecological consulting company.

To become a wildlife ecologist you will need at least a BSc degree (four years) and preferably an MSc (two or three more years). The best jobs with the most freedom require a PhD (three to five more years). Most ecologists are 27 to 30 years old before they complete their training.

The typical salary range in Canada for an ecologist with a bachelor’s degree is about $30,000 to $35,000 a year. A master’s degree raises that to $40,000 to $45,000, and an ecologist with a PhD can expect to earn $40,000 to $60,000. “No one should become a wildlife ecologist if they desire to be wealthy,” says Krebs.

To succeed as an ecologist you will need good skills in all the natural sciences from mathematics to chemistry, physics and, of course, biology, as well as computer skills. You must also learn how to write effectively.

The Person

September 17, 1936
St. Louis, Missouri
Mayne Island, British Columbia
Family Members
  • Father: Lawrence Krebs
  • Mother: Jeanette Krebs
  • Spouse: Alice J. Kenney
  • Children: John and Elsie
Patient, makes lots of wise cracks, efficient, direct, organized, fair, confident, intimidating, even keeled, fun
Favorite Music
Beethoven’s 6th symphony (2nd movement)
Other Interests
Photography, classical music, skiing, hiking
Emeritus Professor of Zoology
Department of Zoology, University of BC, Vancouver, BC
  • BSc (Biology) University of Minnesota, 1957
  • MA (Animal Ecology) UBC, 1959
  • PhD (Animal Ecology) UBC, 1962
  • Wildlife Society, Terrestrial Publication of the Year award, 1965
  • Fellow, Royal Society of Canada, 1979
  • Killam Senior Fellowship, 1985
  • President’s Medal, University of Helsinki, 1986
  • Honorary doctorate, University of Lund, 1988
  • Sir Frederick McMaster Senior Fellowship, CSIRO, Australia, 1992
  • C. Hart Merriam Award, American Society of Mammalogists, 1994
  • Fry Medal, Canadian Society of Zoologists, 1996
Dennis Chitty, professor at UBC and world expert on lemming cycles
Last Updated
March 28, 2011

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Hares and voles. Two animals you can learn alot from if you try these two simple outdoor observation experiments.

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