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Dr. Charles J. Krebs: Seven Questions about the 10-year Cycle of Snowshoe Hares in Canada’s Yukon

By Bradley Udell, Masters student, Wildlife Ecology & Conservation Department 


For the last 40 years, Dr. Charles Krebs has been studying the 10-year snowshoe hare and lynx population cycles in the Yukon, trying to understand what makes them tick. He recently traveled to UF to give a seminar on these famous cycles. This article will highlight Dr. Krebs, one of the most influential ecologists of his time, and the population cycles for which he is known. These should not to be mistaken for the Krebs cycle (aka citric acid cycle) of cellular biology… the topic of this post is much more interesting!


Dr. Charles Krebs is a professor emeritus of population ecology at the University of British Columbia, a Fellow of the Canadian Royal Society, and a Thinker-in-residence at the Institute for Applied Ecology at the University of Canberra Australia. He has authored over 300 publications, including one of the most widely used textbooks on ecology. He also happens to be one of the nicest guys you will ever meet, and his candor, humor, and knowledge make for memorable and enlightening conversations. Additionally, he maintains a strong presence online, with his ecological rants blog. He describes blogging as a great medium for presenting scientific opinions, rather than eating up valuable space in scientific journals.

The snowshoe hare and Canada lynx (shown above) data sets are two of the oldest and arguably the most famous ecological data sets ever, dating back to the early-1800s from fur trapping records gathered by the Hudson Bay Company. The 10-year snowshoe hare populations cycles (see the figure below) span at least 3 trophic levels and are nested in a complex food web. Over the years, researchers attempting to tackle this phenomenon proposed a variety of possible driving mechanisms. From a bottom-up perspective, the environment or food availably may drive hare density, which would then drive lynx density (and the density of other predators). From the hare’s perspective, when the hare population is at high densities the hares might overexploit their food resources, resulting in a crash of the hare population followed by a crash in the lynx populations. From a top-down perspective, a high density of predators may reduce the density of prey, which would ultimately cause the lynx density to crash after they exhausted their prey.

Many other plausible models have been proposed to explain these population cycles. As Dr. Krebs put it, hundreds of beautiful and sometimes crazy models were developed to explain the 10-year cycle… and every one of them was wrong. Truly unraveling their mysteries necessitates: 1) testing and rejecting hypotheses with data, and 2) conducting manipulative experiments that are properly controlled and replicated. This brings us to the content of Dr. Krebs’ seminar. Below, I briefly detail some of the highlights of each of the seven questions presented by Dr. Krebs.


Q1) Are snowshoe hares limited by winter food supplies? Dr. Krebs and others conducted manipulative experiments on hare populations by increasing food, excluding predators, and doing both, while also monitoring controls. They found that treatments with both predator and food manipulations lead to a synergy between them, yielding many more hares than would be expected by adding total abundance of the single treatments together.

Q2) Which demographic rates drive hare cycles? In other words, is it changes in reproduction or in mortality that drive the population cycles? Krebs and others did not find evidence for changes in mortality rates (the rate that hares are killed by predators) but did find a trend of declining reproduction that lagged behind the lynx density cycle by 2 years. Studies also found that hares did not migrate very far, indicating that migration was not an important driver of the population cycles.

Q3) What drives reproductive changes? It could be as simple as a food shortage or the length of the breeding season, but there was no evidence for either. The collapse in hare reproduction occurs in the summer when food is abundant. Dr. Tony Sinclair found that juvenile hares collected at different times of the population cycle, and afterwards raised in controlled environments in captivity, showed differences in reproductive output later in life. This led to the hypothesis that maternal effects or effects inherited by offspring that are influenced by the conditions experienced by the mother, were a mechanism for the change in reproductive rates, perhaps due to predation stress.

Q4) Could predators drive changes in reproduction? How could predators affect reproduction? When hunting snowshoe hares, lynx are only successful in 15-20% of hunts. It was suggested that predation attempts by lynx lead to stress in maternal hares, and that stress can affect reproduction. For this to be true, 3 things had to also be true: 1) stress was caused by predation risk, 2) predation risk increased maternal stress, and 3) maternal stress is inherited. After more research, all 3 turned out to be true! This showed top-down predator control of both hare survival and reproduction.

Q5) What causes predator cycles? Radio and GPS tracking of lynx revealed that lynx likely either starve in the area, or disperse and probably starve elsewhere after the hare population falls. So crashes in lynx populations are direct results of crashes in hare populations.

Q6) What causes spatial synchrony? An amazing pattern arises in space at the scale of 500km in these population cycles. Populations within this distance of each other rise and fall together, while those farther away do not. In fact, lynx population crashes can occur up to four years apart between different populations. What could possibly cause this? Regional weather, hare dispersal, and predator dispersal were all considered as possibilities. The best hypothesis, however, in population density, which moves across Canada, perhaps driven by the movement of lynx populations as they search for food.

Q7) What causes amplitude variation? Or, put another way, what determines how high a population will peak in a cycle? Maybe regional weather, food quality, or predator dispersal can contribute? The most favored explanation is the predator hypothesis, which predicts that amplitude depends on starting condition of predator population. The higher it starts, the lower the peak of the prey.

In conclusion, these population cycles are still being researched in order to divine their remaining mysteries. From what is currently known, an interaction between food and predation drives the population cycles, with top-down pressures on hare survival and reproduction from lynx (and other predators). This emphasizes that often there are multiple factors affecting ecological phenomena. Thus, in closing, I point out a final piece of advice from Dr. Krebs: it’s often more useful to ask what is the relative importance of multiple factors on ecological phenomena rather than focus exclusively on any single factor.