WHY DO ANIMALS COME AND GO?

by Whit Gibbons

April 5, 2009

With the advent of spring and the reappearance of birds and other animals in backyards, I receive many questions like the following one from a woman in Alabama.

Q. I have just realized that I no longer see thrashers, blue jays, or the once-plentiful woodpeckers in our yard. Could this have anything to do with the loss of chipmunks due to the rat poison (only in boxes with small openings) put out after the happy rats sat on our air conditioner enjoying the nearby bird seed? Or is it due to cats--none right around us, except maybe a wild one. Or is it a more widespread problem?

A. Rat poison and cats can certainly be indicted for some changes in species composition that might be observed in a backyard, but change in the abundance of certain animal species at a given location over time can also be a natural phenomenon. Almost everywhere, some species are more abundant than in previous years whereas other species seem to be on the decline or even absent entirely. These so-called wildlife cycles are not just perceptions, they are real. And what causes them has puzzled and intrigued fur trappers, farmers, and ecologists for well over a century.

One wildlife cycle was documented many years ago by the Hudson's Bay Company, which recorded the number of furs of different species, including snowshoe hares and lynx, the northern equivalent of bobcats. The simplistic explanation is that hares are a staple in the diet of lynx, and the number of snowshoe hares declines precipitously presumably because their primary predator eats them, thus reducing the size of their population. Then, presumably because the lynx's target prey species is no longer abundant, lynx populations begin to decline because of lower reproduction rates and possibly even starvation. This is followed by an increase in hare population size because the lynx predator is no longer common enough to control the number of hares. Thus at approximately 10-year intervals snowshoe hares and lynx reach high numbers, but in different years.

The continuous cycle was first reported for the period from 1820 to the early 1900s. Ecologists and wildlife biologists are not unlike economists in offering different explanations for what causes or influences certain observed patterns. Just as not all economists agree on what causes fluctuations in the stock market, not all ecologists agree on why some animal numbers fluctuate in apparent patterns. To be sure, observed changes in animal population sizes can have multilayered explanations, but natural variation in numbers is real, albeit often inexplicable, in many species.

The lynx-snowshoe hare model may explain how a single predator and prey species can influence each other. But how do we explain fluctuations in populations of blue jays, thrashers, and woodpeckers, which do not fit the single predator-prey pattern? Despite their absence in at least one area of Alabama, all of these species continue to be common in some areas of the Southeast. All of these species eat insects, so can their absence in a particular area be explained by something (such as a restricted pesticide application) that has reduced insect prey locally? Or could a new bird-eating predator, such as a barred owl or Cooper's hawk have moved into the neighborhood and thinned out certain bird species? If the latter, the predators will soon have fewer birds to eat and will move on to more productive areas, whereupon the birds will again become abundant.

The decline of a species locally might be a natural occurrence, as with the snowshoe hare and the lynx. For one reason or another, such species often return to their former population levels after a time. But the absence of a species could be cause for concern. What we must be alert for is the steady decline of a once-common species throughout its range. Natural fluctuations might be the explanation, but the decline could be caused or exacerbated by human environmental abuse. And when a decline becomes a repeating trend rather than a cycle, the affected species might never recover, which is not natural at all.

If you have an environmental question or comment, email