Friday, September 01, 2006

Form follows function, in nature as in art

Chris Lyons writes:

Regarding the debate over whether raptors have any controlling effect on rodent populations--this is from "The Mississippi Kite", by Eric G. Bolen and Dan Flores, University of Texas Press, 1993

"Among the earliest attempts at what was then considered 'game' management was a bounty system that often included hawks--and the generic and ill-founded designation 'chicken hawk' for just about any bird of prey certainly didn't help matters. In 1885, Pennsylvania enacted a hawk-and-owl bounty law--known at the time as the 'scalp act'--which provided for a payment of 50 cents per raptor scalp. Within two years, 180,000 hawk and owl scalps were bountied, for which the taxpayers of Pennsyvania parted with $90,000--big money for the day. Yet one estimate of the program indicated that farmers saved just about $1 for each $1,205 paid out in bounties! But the folly of the bounty on raptors was not limited to direct payments. Fields and orchards were overrun with mice and other vermin to the point that farmers requested repeal of the bounty law, which happened two years later, but not before crop damage had added another $2 million to the program's cost. Even so, the idiocy of bounties continued in other areas well into the current century. Maryland paid bounties on nearly 90,000 hawks between 1925 and 1930, and Alaska's territorial government authorized a $2 bounty on Bald Eagles in 1917. By 1952, when the bounty ended in Alaska, more than 128,000 Bald Eagles had been cashiered."

There are clearly many factors involved in maintaining a healthy population of our native rodents, which after all are as valuable a part of their respective ecosystems as birds of prey. I would tend to think that the effect of diurnal and nocturnal raptors on rodents has an effect far beyond the number of individuals that raptors prey upon. Rodents need to spend a considerable part of their day foraging, and as is well known, the only real check on their population growth is the availability of food. Unlimited food means unlimited rodents, no matter how many predators they have. Agriculture opens up new and abundant food sources for them, which can lead to their numbers increasing far beyond their normal limits, which can mean ruin for the farmer. However, the presence of food is not enough--rodents must also be able to access the food, and this generally means venturing from the safety of burrows and tree cavities, out into the open, where they must expose themselves to potential predation. Millions of years of evading a host of hunters has encoded wariness into these small creatures--when they sense the presence of their natural enemies, they conduct themselves with far greater discretion, as any New York apartment dweller who has lived with and without a cat knows full well.

Foxes, coyotes, bobcats, snakes, and many other predators are a check on rodents, and a healthy ecosystem will have all of the above predators, and more--but raptors are unique in their ability to strike from above, with little or no warning. Rodents foraging in the open during daylight hours must maintain a constant look-out for hawks, particularly buteos, whether circling overhead, or perched in a tree. Owls take the night shift. If Red-Tails and other such raptors are eliminated from agricultural areas by shooting, trapping, and poisoning (methods that work far more effectively against hawks than against the legendarily fecund microtus and their numberless relations), then vigilance can be considerably relaxed, and the little mammals can venture more boldly from their redoubts, with fewer of the precautions they normally take. The ultimate result is that they can devote more of their time to food-gathering--and reproducing, in ever greater numbers.

John Blakeman is quite right--you can't just view it as an exercise in simple subtraction--it's more about multiplication. And without hawks and other predators to worry about, it seems probable that rodents and other abundant small mammals, can multiply far faster, and the only limit on their populations will be the supply of available food--unless the farmer resorts to poisons, which have a devastating effect on the environment, and are ultimately ineffectual, because they have no deterrent effect--they merely subtract from an unlimited population, which replaces its losses faster than they are incurred. Mice may die from poison, but they don't fear it. Not even if they learn to recognize and avoid it, in which case they don't even die from it. Though many beneficial animals will.

In many cases, it may be that the presence or absence of Red-Tails and other diurnal raptors makes little difference to the numbers of rodents in a given area. In other cases, there is circumstantial evidence that they do, in fact, serve as an important controlling factor on agricultural pests. A sky devoid of circling buteos is a sky under which the vole and the field mouse can move more securely, and breed more prolifically.

Ornamental our raptors surely are, but form follows function.