19 September 2007

Lions, Wolves and Horses

Colorado Confidential ran a couple of stories about wild mustangs today, one here, with a link to the first story within the second story. This got me thinking about what kind of predator population it would take to control the wild horse population. The data I could find revealed that wolves and mountain lions are the only natural predators that make much sense for horse population control, and I found the data below about how much they eat.

How much do wolves eat?

15. What do wolves eat?

Gray wolves prey primarily on large, hoofed mammals such as white-tailed deer, mule deer, moose, elk, caribou, bison, Dall sheep, musk oxen, and mountain goat. Medium sized mammals, such as beaver and snowshoe hare, can be an important secondary food source. Occasional wolves will prey on birds or small mammals.

Red wolves primarily prey on white-tailed deer, raccoons, rabbits and rodents.

16. How much do wolves eat?

Gray wolves can survive on about 2 1/2 pounds of food per wolf per day, but they require about 5 pounds per wolf per day to reproduce successfully. The most a gray wolf can eat in one sitting is about 22.5 pounds.

Red wolves eat an average of 5 pounds of food per day, but have been known to eat up to 12 pounds in one sitting.

17. How many prey do gray wolves kill per year?

In Minnesota, wolves kill the average equivalent of 15 to 20 adult-sized deer per wolf per year. Given the 1997-98 estimate of 2,450 wolves in Minnesota, that would equal about 36,750 to 49,000 deer killed by wolves. In comparison, from 1995-1999 hunters killed between 32,300 to 78,200 deer each year in Minnesota's wolf range. In addition, several thousand deer are killed during collisions with vehicles each year.

The deer population of Minnesota is roughly 800,000, as of 1997. Thus, it takes roughly 200 deer to support one gray wolf, and given that they tend to operate in packs of 5-6 wolves, it takes roughly 1,000-1,200 deer to support a wolf pack. Multiple wolf packs are necessary to maintain a viable gene pool.

How much do mountain lion's eat?

Mountain lions eat primarily deer throughout their range. If there are no deer, there are few lions. Secondary prey can include bighorn, javelina, and even porcupines. A puma generally kills one deer per week. The lion caches the carcass under a shrub or buries it under leaves, and may return to feed nightly for several days. Pumas in the desert kill more often then those in the mountain woodlands, because the cached carcasses decay faster in the hot desert.

If the kill to total population ratio of deer is similar for mountain lions and wolves, then it takes about 500 deer to support a single mountain lion. They live in solitary environments most of the time, but it still takes many mountain lions to support a viable gene pool.

Converting to horses

Of course, a horse is much larger than a deer. Adult horses are 850-2000 pounds. An adult white tailed deer usually weighs 90-220 pounds, although they can get to be up to 350 pounds. Thus, a horse has about ten times as much meat as a deer.

This suggests that solitary mountain lions would be poor horse predators. A dead horse doesn't last long in the wild, and a mountain lion can only eat so much at once. Most of the meat on a dead horse would be wasted by a single mountain lion whose usual weekly meal is a deer.

On the other hand, a pack of twenty wolves that took down 40 horses a year, could probably eat a comparable share of the meat from the horses to what they take from deer prey. This suggests that a herd of about 400 horses could support a single large wolf pack.

Moreover, in reality, the wolves would probably eat some prey other than horses, so a large wolf pack could probably do well managing a somewhat smaller herd of horses, if it also had other prey available.

Given the number of wild horses in Colorado, about 100-160 wolves could probably fully control wild horse population growth in the state. This many wolves would also probably provide something close to a sufficient genetic diversity to maintain a sustainable wild wolf population in the state. Add additional wolves to control deer, elk and moose populations and those numbers could be increased considerably, to several hundred wolves, or even thousands of wolves.

Of course, ranchers, residents of rural communities and exurbanites might not appreciate a state with little or no human hunting, and a thousand wolves roaming the state in large packs in the wild.

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