"Roller-Coaster Critters" Narrative

The “balance of nature” is not always what it seems. The word “balance” implies a steady state of equilibrium between predators and prey, or between plants and the plant-eaters. This is not usually the case. Many hunters complain of a lack of game the season after a tough winter. Those same years, when game populations are low, predators such as cougars, wolves and bears seem to be numerous. They often hunt domestic animals near human populations.

Although simple logic would suggest that more predators equals less game, with the sightings and lack of hunting success to “prove” it, the real story can be summarized by two principles. (1) Predator populations grow and decline in response to prey populations and (2) the response of predator populations is often delayed by a year. During the year following a prey population crash, the “balance” between predators and prey is out of balance – sometimes quite a lot.

Population cycling, from high to low and back again, is the normal way that nature operates. For years, it was a big mystery why ruffed grouse seemed to peak every 9 – 13 years. Snowshoe hares peak every 10 or 11 years. Scientists looked at everything from internal hormones to predators for an answer, without any luck.

More recently, they began to apply the concept of “prey numbers determining predator numbers” to the prey (grouse and hare) and their food. In both cases, the answer to the cycles was their primary winter food, the aspen tree.

Ruffed grouse like to live in thick, pole-sized aspen thickets. The male grouse finds one or two drumming logs (sometimes a large rock) in thick cover. Every spring, male grouse in the aspen thickets “drum” on the logs, compressing air with their wings against their chest. They start slowly – thump, thump, thumping – and increase the speed until their wings are a blur. Drumming sounds like an old tractor starting up in the distance. The low pitch of the sound enables it to travel miles through thick brushy areas. The message is “stay out guys, come on in ladies."

Grouse have special extensions on their toes which act like a rubber stick-on gripper in your bathtub. They are for walking on icy branches high in aspen trees. There, all winter, they nibble on the male flower buds which only grow at the tops of aspens. (Aspens are dioecious, which means they are either a male tree or a female tree.) Male aspen buds just happen to be the buds highest in protein. It turns out that male aspen buds have a cycle of their own… You will hear about it later during this activity.

Snowshoe hares have huge feet for running on deep snow. They change color in the autumn from brown to white, for obvious camouflage reasons. Interestingly, male hares, late in the winter, also thump, thump, thump for female attention. They drum their large hind feet on the ground in small openings by moonlight. Hares spend the winter in evergreens, but move into aspen groves to feed during the spring, summer and fall. Hares can reproduce like, well, bunnies, having up to four litters every year and 4 – 5 young per litter. If the average female hare has 15 or more young per year, and those 15 have 15 young each the following year, you can see how much reproductive potential hares have. This kind of growth is called exponential growth, doubling, tripling, or quadrupling every year! In a single square mile, hare populations can go from 1 hare to 4000 over a few years!

When populations get very high, aspen bark and buds are constantly browsed, and the aspens say “Enough!” The stress on aspens (willows, too) causes them to produce new shoots high in resins – resins which prevent hares from digesting protein. Suddenly, when hare populations are at their highest, their main winter foods can’t be eaten.

Ruffed grouse are eaten by many predators, including us, but the goshawk specializes in eating grouse. Hares, too, are eaten by many animals, but lynx (who also have huge feet to chase them in the snow) find them especially tasty. In this activity, you will examine the causes of population cycles in grouse, goshawks, hares, and lynx. As it turns out, it’s all about aspens…

Last updated: February 24, 2015

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