Wildlife Population Cycles
I have have just finished reading the book Wildlife's Ten-Year Cycle by Lloyd Keith, and it strikes me that there are probably few areas in scientific investigation that are as hard to pin down as the question of why certain animal populations undergo the wild fluctuations which are observed.
The most visible manifestation, of course, is that of the snowshoe hare. If you see several squished rabbits on a mile of road, you know they're at a peak; if you don't see any in a hundred-mile drive, you know they're at a low.
The all-too-popular consensus is the seemingly obvious one that when the population density becomes too high, the food supply is insufficient to support them all, and they die from starvation. Keith's book makes the point that, while this may be a contributing factor, it is a minor one at most.
The primary cause of the "crash mortality" rates of the snowshoe hare at roughly ten-year intervals seems to be an epidemic disease--and a weird one at that. Even though it's not transmittable, it occurs during periods of overcrowding, seems to affect mainly the liver, and has been labeled the "shock disease." Symptoms occur suddenly, and are characterized by a sudden leap into the air, followed by collapse and convulsions including running movements of the legs, fixed eyes and retraction of the head and neck. I'll spare us the details on the autopsies that have been performed on some of the victims, except to say that the liver and spleen weren't all that they should be.
The cause? It's unknown, but appears almost certainly to be due to social pressures and stresses in an overpopulated environment.
Although the snowshoe hare cycle is the most visible and widely recognized of this phenomenon, it has far-ranging influences on other wildlife that few of us ever notice. For instance, the lynx population varies in direct proportion to their favorite prey, the hare. It's not because they die from starvation, it's because they have fewer and smaller litters. Another example is the grouse family. The grouse population declines shortly after a low in the number of hares available in the food chain. They are now prime targets not only of four-footed predators, but of hawks and owls as well.
Similar examples of cyclic fluctuations in the animal kingdom, such as that between the fox and the lemming abound. But the so-called ten-year cycle remains a mystery. Some researchers have even tried to relate it to the eleven-year (more or less) sunspot cycle, but this is almost certainly an exercise in futility.
What does come through loud and clear is that even a minor disruption in the animal food chain can have an immense impact throughout the entire wild community.
