Why Do Lynx Numbers Rise and Fall? The Lynx-Hare Cycle

How the lynx-hare cycle works

The Canada lynx is a specialist hunter. Snowshoe hares make up roughly 35–97% of its diet, depending on the season and the year, and a lynx needs about one hare every two days. That makes lynx populations extremely sensitive to hare numbers.

A typical cycle in the boreal forests of Alaska and Canada runs like this:

1. Hare increase: Hare numbers climb over several years. Lynx eat well, more females breed, litters are larger and more kittens survive.
2. Hare peak: Hares reach very high densities. Lynx numbers keep rising.
3. Hare crash: Hare numbers collapse, often by ten times or more, within one or two years.
4. Lynx crash: Lynx numbers fall about one or two years after the hares. Females stop breeding, kitten survival drops close to zero and many adults die or leave.
5. Low phase: Hares stay scarce for a few years before beginning to recover, and the cycle starts again.

Over one cycle, lynx populations in a region can vary between about three and seventeen times their lowest level.

The Hudson’s Bay Company records

The cycle was first revealed by the fur trade. The Hudson’s Bay Company bought lynx and hare pelts from trappers across northern Canada and kept detailed records, some going back to the 18th century. When biologists studied these numbers, they found a remarkably regular rise and fall roughly every ten years.

In 1942 the British ecologist Charles Elton and his colleague Mary Nicholson published an analysis of lynx fur returns from 1821 to 1934, showing the ten-year cycle across much of Canada. A famous graph of hare and lynx pelts, based on records compiled by D. A. MacLulich in 1937, became one of the most widely reproduced figures in ecology textbooks. It is often used to illustrate the Lotka-Volterra equations, simple mathematical models in which predator and prey numbers chase each other up and down.

Fur records have limits. They reflect how hard trappers worked and the price of furs as well as the true number of animals. Modern field studies, however, have confirmed that the cycle is real.

What causes the hare cycle?

If lynx simply followed hares, the next question is why hares cycle at all. Ecologists have studied this for decades, especially at Kluane Lake in the southwestern Yukon, where a large experiment ran from 1986 to 1996.

The researchers fenced out ground predators from some forest blocks, added food in others and did both in a third set. Keeping predators out roughly doubled hare density, extra food roughly tripled it, and the combination increased it about elevenfold. The results, published in Science in 1995 by Charles Krebs and colleagues, showed that predation and food supply act together, with predation the main direct cause of hare deaths.

Snowshoe hares are eaten by many predators besides lynx, including the coyote, great horned owls, northern goshawks, red foxes and other hunters. During the decline, predators kill most hares before they can breed. Later research by Rudy Boonstra, Michael Sheriff and colleagues found that the constant threat of predators also stresses hares, lowering their breeding success. Stressed mothers seem to pass this effect on to their young, which may help explain why hare numbers stay low for several years after a crash.

Why lynx follow the hares

Lynx respond to hare numbers in several ways:

• Breeding: In good hare years, nearly all adult females breed and raise large litters. In poor years, many females skip breeding, and few kittens survive their first winter.
• Survival: Adult lynx starve or are killed more often when hares are scarce.
• Movement: When hares crash, many lynx travel long distances, sometimes more than 1,000 km (620 mi), in search of food, and some appear far south of their usual range.
• Diet switching: Lynx eat more red squirrels, grouse and carrion, but these rarely replace hares completely.

Because these responses take time, lynx numbers lag behind hare numbers by about one to two years. Lynx probably add to the hare decline at high densities, but studies suggest they are followers of the cycle more than its main cause.

Is the cycle the same everywhere?

The strongest cycles occur in the boreal forests of Alaska and central Canada. Cycles in different regions are often broadly synchronized, rising and falling at similar times across huge areas, possibly because of dispersing lynx and shared weather patterns. Some studies have linked differences between regions to climate.

At the southern edge of the range, including the northern United States, hares live at lower densities in patchier habitat, and cycles are weak or absent. Lynx there are scarce and depend on stable areas of dense young forest. Some researchers are also investigating whether warmer winters and changing snow conditions could weaken the cycle in the future.

Other animals with population cycles

The lynx-hare cycle is not unique. Lemmings and voles in the Arctic and northern Europe often cycle every three to five years, and their predators, such as Arctic foxes and snowy owls, follow them. Some grouse and ptarmigan populations also rise and fall in regular cycles. Hares are larger relatives of rabbits with very different lifestyles; see rabbit vs hare for how they compare, and why do lynx have ear tufts? for another lynx puzzle.