Back to voles -- Lyons responds to Blakeman

Pale Male with rodent prey -Metropolitan Museum -8/6/06
Photo by Bruce Yolton http://urbanhawks.blogs.com



The final salvo in the vole debate--by Chris Lyons
[This is in response to John Blakeman's post on 9/02/06] :


After John Blakeman's last response on this subject, I figured I'd better do some more careful research, to see if in fact there was a strong scientific consensus on the question of what role (if any) predation plays in the life cycles of rodents, particularly voles--the answer seems to be that biologists have held a variety of opinions on the matter of vole populations, and what makes them periodically rise and fall. There certainly was a strong current of opinion in the latter half of the 20th century that microtine (vole and lemming) populations are self-regulating, and primarily affected by overpopulation pressures, disease, available food, etc--and there is no doubt all those factors are of tremendous importance. At one point, the pure cyclical explanations do seem to have been dominant over theories involving wild predators. But scientific opinions on the role of predation in rodent populations have themselves experienced a certain cyclic ebb and flow, and there have been a large number of recent studies that indicate predators may play a very important role in regulating voles, and other prolific potential pests (all of which have their place in nature, of course).

As microtus populations increase, their predators likewise can breed more abundantly, and more mobile predators will seek out especially large concentrations in winter, when territoriality is not an issue. Any serious local birder knows there are fields a short drive from New York City where multiple species of owls and hawks can be seen in amazing numbers in winter, all of them hunting small rodents--and then drifting away to other hunting grounds as prey numbers become too low to sustain them. A rodent 'hot-spot' is quickly identified and exploited by birds of prey during the fall and winter months.

Some predators that don't normally specialize in microtus will feed heavily on them as their numbers climb, including coyotes, wolves, and even bears. Everybody joins in the feast. Many scientists believe that the cumulative impact of this steadily increasing multi-predator onslaught does, in fact, seriously diminish vole and lemming populations at certain points in their population cycles--perhaps only hastening their natural cyclic decline to a greater or lesser extent. Others believe predators have more of an effect in terms of slowing the upswing of vole populations. Overall, there is a substantial body of opinion that even if microtine populations would fluctuate with or without predators, predation does limit the severity of their regular rises and falls--it both preserves and controls natural cyclicity. There is speculation that one reason for voles and lemmings having evolved in such a way as to guarantee regular crashes in their numbers may be to discourage more predators from becoming specialized to hunt them. Whether by design or not, their population cycles also have the effect of temporarily decreasing the numbers of their most dangerous enemies. It's well known that when lemming populations crash, the populations of those lemming-hunters who can't readily switch to other prey crash along with them--these crashes frequently trigger mass southward irruptions of birds of prey such as Snowy Owls, most of whom probably do not survive their exile. The Red-Tailed Hawk is too much of a generalist to be critically affected by sharp declines in the numbers of one of its prey animals. Others are not so fortunate.

Getting back to the issue of the Pennsylvania "Scalp Law" of 1895, I found primary sources hard to come by, but I did manage to learn a bit more. The law was enacted to encourage the shooting and trapping of all predators perceived to be threats to livestock (primarily poultry, since large predators were nearly eradicated in Pennsylvania by this time). "For the benefit of agriculture and the protection of game", it attached a price of 50 cents per scalp for ANY bird of prey other than the "Acadian" (Saw-Whet), Screech, and Barn Owl. It also included "Wild Cats, Foxes, Minks, and Weasels." With 50 cents being a significant sum of money at the time, the law created a powerful added incentive to hunt down animals that were already being ruthlessly destroyed by man--most of which were important rodent predators; most particularly the weasels, voracious animals, well-adapted to 'ferret out' and slaughter rodents in their own burrows, known to sometimes kill more individuals than they can eat--and which have been observed to prey with particular success on female voles with young.

It also put a big drain on the public coffers, and created a lot of work for the government officals administering it, which quickly made it unpopular with them. Proto-environmentalists of the time were vocal in their criticism of the "Fool Hawk Law" (while still insisting that it was perfectly all right to kill any accipiter you saw). However, raids on chicken yards did apparently decrease, and the law might have remained on the books a lot longer than it did, had Pennsylvania farmers not begun to report a great increase in agricultural pests, primarily rodents--no specific information on what species of rodents, or what crops were most affected, or in what areas--there were dollar-estimates put out as to how much the law saved agriculture in terms of chickens, as opposed to how much it lost in terms of damage caused by vermin, but these were educated guesses at best. Probably a great deal of the destruction was caused by rats (the imported variety), which as John Blakeman has many times pointed out, are ideal Red-Tail food. While fast breeders, rats are less staggeringly prolific than voles, and are usually found in the immediate vicinity of human habitations--where no hawk in a non-suicidal frame of mind would have been found in 1896.

Could populations of rats and other medium-sized rodents have been regulated by raptors, including Red-Tails? I'll leave that question to the scientists, but I can relate a personal observation. In the short time since Hawkeye and Rose, the Fordham Red-Tails, took up residence at the Rose Hill campus, everyone here has noted a substantial drop in the numbers of squirrels. Only a few years ago, you couldn't leave the library at certain times of the year without seeing 30-40 squirrels foraging on the lawn outside--and their numbers on the campus as a whole were extremely dense, and somewhat isolated from surrounding areas, due to the campus being entirely surrounded by wide roads with heavy traffic. Though likable animals, who add much to the atmosphere of the campus, the squirrels were so numerous and bold as to frighten some people--they startled me more than once. While watching the hawk nest, I've had quite a few people mention to me that they are seeing a lot fewer squirrels around. This may be partly because the squirrels are being more careful about exposing themselves to potential predation, but I still see them out in the open--just not nearly as many. The change has been remarkably rapid and obvious. And squirrels, of course, are far harder for Red-Tails to catch and kill than rats. So if one breeding pair of Red-Tails, not feeding exclusively on squirrels, could have this great an effect in so short a time on so challenging a quarry, it's not hard to imagine that removing nearly all potential rat predators from a farming area could have the opposite effect.

Back to 1896--if voles did increase during this time, it is hard to believe that farmers who had spent their whole lives dealing with rodent pests would not have been able to recognize a perfectly normal 4-5 year upswing in vole numbers (however they explained it). So whatever did happen in this period, presumably it was something both immediately noticeable and out of the ordinary--and rather glaringly coincident with the dramatically increased slaughter of vole predators. It would be nice to know more, but there just isn't enough available data to definitively support any particular point of view on the cause of this phenomenon.

What we do know is that the 1895 bounty law was repealed in 1897, but persecution of most of these wild hunters continued for many subsequent decades, albeit with an increasing attempt to spare those deemed useful to man (so long as they weren't seen to be overly interested in his chickens). To argue that reports of increased rodent problems were inspired by emotion-driven conservationist fantasies is to ignore the fact that none of these creatures was viewed sentimentally at the time by the great majority of the population--they were almost universally reviled as murdering thieves and worthless vermin.

The passage of the Scalp Law was viewed positively by around 90% of Pennsylvanians, until its less fortunate consequences became apparent. No 19th century farmer was going to change his mind about these bloodthirsty chicken-stealing varmints. He might, however, come to realize that he was paying a higher price for their extermination, in both cash and crops, than the loss of a few chickens warranted. The decision to repeal the bounty was strictly a matter of dollars and sense.

Interestingly, a similar situation occurred in Scandinavia, not long after the Pennsylvania bounty was repealed--.

"In the beginning of last century a massive countrywide predator extermination program was started in Norway. The government of Norway paid a bounty to hunters for all medium-sized mammalian predators, owls and raptors. Bounty program and propaganda against all predators led to a large-scale reduction in the numbers of these predators. Steen et al. (1990) reported the apparent consequences of this 'unplanned experiment' influencing small mammal populations. They observed loss of cyclicity for approximately 20 years, in addition to the loss of large-scale regional synchrony in voles and lemmings "
[ Source: Vole Population dynamics: experiments on predation
Janne Sundell, Academic Dissertation, May 2002.
University of Helsinki, Faculty of Science, Department of Ecology and Systematics, Division of Population Biology .]

If I have made this subject sound overly complex and involved, believe me, I've actually oversimplified it quite a lot--for my own sake as much as the reader's. I realize this discussion has strayed pretty far afield of its origins (somebody found a dead vole in his backyard, killed by a species of hawk that isn't even supposed to be preying on voles). Perhaps the most important thing is that we all agree that birds of prey are worthy of survival, that they enhance our lives, and that their lives are valuable in their own right, not merely as ornaments to our skies, or as unpaid pest-controllers.

I remain convinced that raptors, and many other predators play a vital role in regulating the populations of their prey, often to the benefit of man, although many other regulating factors exist. Whatever the facts may be, they must be determined by scientific inquiry and analysis. Since the scientific community is notorious for taking a long time to make its mind up, I'm not holding my breath for its final answer.