Wednesday, August 30, 2006

Blakeman: Vole clarification

A Fifth Ave. offspring with full crop of rodent or bird prey [but no voles in CP]
Photo [long ago] by Charles Kennedy


Ben Cacace points out quite accurately that red-tails aren't the only predators consuming voles. Indeed, foxes, coyotes, weasels, minks, owls, and a few others also thrive on these abundant small rodents.
But there aren't many more of these than red-tails. Foxes and coyotes commonly hunt in many square miles of territory each night, not just in two (the typical size of a Midwestern or Eastern red-tailed hawk territory). Owls probably take as many voles as red-tails, although the smaller owl species tend to hunt in forests and woods, where there are few or no voles.
I didn't make my point clear. Ben contends, accurately, that if red-tails actually consumed a third of the vole population each year, that’s not insignificant. That would seem to lop off a major portion of a local vole population, and in conjunction with all the other vole predators, the cumulative effects of vole predation should keep them in check.
Except: – our cute little voles, as I mentioned, don't just sit there during the year. They breed – like – mice. Let’s add to the static numbers of voles the low-range numbers of new offspring produced each year.
If a red-tail’s territory began a year with about 10,000 voles, and a pair of hawks ate about 3600, there would be only 6400 left at the end of the year, presuming that there was no vole reproduction. But reproduction is the more important factor, the one that tips the scale in favor of the voles, regardless of the cumulative predation pressure by all local predators.
In typical periods, each acre typically has 8 to 10 voles.(At populations “highs,” every four or five years or so, there can be 50 or more voles per acre. But I'll use the low-end numbers throughout, to make the point.) That’s 4 or 5 pairs in normal years. Each pair produces an average of 4 or 5 pups from 5 to 10 times a year. Again, let’s use the low numbers, just 5 broods of 4 pups for each breeding pair. How many voles, then, are produced for all the predators to consume in a year’s time? It’s not the static 10,000 I mentioned.
Four pairs in each acre, reproducing only 5 times in a year with only 4 pups, yields 80 voles available for local predators to consume (4 pairs x 5 litters x 4 pups in each). That’s on just one acre. A typical red-tail’s 2-square mile territory has 1280 acres. 80 x 1280 = 102,400.
So, at any one time only about 10,000 voles are available for a hawk to capture. But over an entire year, the population is in the range of 100,000. If the local pair of hawks ate only voles (and they don't, they prefer to vary their diet), they could consume no more than about 3600. That’s a bit less than 4% of all the voles produced in the territory.
Here’s an interesting quantum. How many pounds of vole flesh are annually available to all the vole predators in a two-square mile red-tail territory? A vole weighs about 2.5 ounces. A hundred thousand of these weigh 250,00 ounces, or 15,625 pounds. That’s about 7.8 tons of vole flesh produced each year. For a 2.5 lb feathered predator, that’s a big pile of vole flesh to eat in any useful numbers.
The ecological fact is the reverse of the “hawks control rodents” dictum. It’s the other way around. Vole (and other rodent numbers) actually control hawk populations. That’s why red-tails in California have much smaller territories, because there are abundant supplies of ground squirrels and other rodents. Where prey numbers are small, territories have to be large, to support a single hawk pair. Lots of prey, small territories. Few prey, large territories, and fewer hawks.
Readers can now understand my initial concern about Central Park harboring two pairs of red-tails. Would there be enough rats and other prey for them to subsist? Today, it’s not a question. And does anyone now think that Pale Male, Lola, and the south end pair of red-tails make any dent in either the rat, pigeon, or squirrel populations of Central Park? They don't. The prey reproduce faster than the hawks can eat them, period. It’s the other way around. The small animals of Central Park support two pairs of red-tails, very nicely.
Sorry, but the old conservation maxim that hawks and other predators control rodent populations just isn't so. They are controlled by internal food and territory factors. But that’s a separate story too big to dwell upon here.
–John Blakeman