DO ANIMALS COME AND GO?
April 5, 2009
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.
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.
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.
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.
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.
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