Saturday, February 16, 2008

Deer Populations and Ecological Illiteracy

The New York Times this week has an article about deer overpopulation in New Jersey (although the problem is just as bad throughout the forested northeast) and efforts to control it through culls.

The problem, and reactions to it, demonstrate the deep ecological illiteracy informing both sides of the debate. Groups that value the deer see them as a natural and beautiful part of the landscape. They are beautiful, and individually they are natural, but their populations are much higher than they would ever have been naturally. This is partially because of removal of predators (deer evolved to breed faster than the wolves and mountain lions could eat them) and partially because we have created so much high quality feeding habitat for them. Deer find the easiest and best food at forest edges, and because of human land use there is so much more forest edge than there has ever been before. Somebody or other, maybe Audubon, said that a squirrel could travel from Maine to Louisiana without ever having to come down from the trees. These were big trees with their leaves way up where deer can't reach, and very few breaks. The forests of the east are now young and fragmented; basically an all you can eat for the deer. And we can no longer rely on long snowy winters to cull deer populations every year. As a result, the effects that deer are having on the forest is very far from natural.

Experiments with deer exclosures (basically large fenced areas, chosen randomly in the forest to see what happens without deer) are fairly unambiguous. I've seen these experiments first hand in the old growth forest of upper Michigan. Working for ecologist Dr. Kerry Woods the summer after I graduated from college, one of my tasks was to revisit exclosures he'd built three years earlier. Outside the exclosures, where the deer can get, extremely few seedlings make it. The forest has an open, park like feeling, no understory. The deer population is so dense everything gets mowed. Inside the exclosures, dense hedges of rapidly growing saplings compete for light, but any branch that reaches outside the enclosure is quickly nipped off. This forest that had never been cut by humans was being kept from replacing itself by the deer overpopulation. The same pattern is being found all over the north-east.

But if the people who see the deer as wonderful parts of nature are ignoring ecology, they are not alone. Those who see deer overpopulation as a problem generally ignore the same ecological facts. Governments organize huge one time culls in limited areas, and are then surprised when deer from next door are attracted by the unexploited greenery. Deer populations with plenty of food and few predators can double every two years or so. Shoot 1500 out of 2000 deer in your county and four years later you are back where you started, even if you don't have deer moving in from adjoining counties.

Basic arithmetic tells us that until White Tailed Deer mortality rates exceed birth rates throughout the northeast, the regional population will not decline. Birthrates are unlikely to decline, unless the structure of the current forest changes so much that the deer are nutritionally limited. Not likely. And plans to control the wild regional population through some sort of sterilization program strike me as ludicrously expensive and incredibly unlikely to work.

So what causes mortality? Disease, winter, cars, predators, hunters. Disease we don't want to encourage, especially considering that many diseases that affect deer can also fell horses, sheep, cows and such. Winters are tending weaker. Sever storms are increasing, but to really knock back the deer population, one needs a winter that is very cold and snowy for months over a large area. Not likely. Few would advocate increased car-deer collisions. Hunters are limited in where they can hunt by the danger of bullets carrying to nearby houses, roads and such. And hunting seasons are limited, in part to make forests safe for other uses the rest of the year. There is also mounting evidence that deer hunters regularly dose themselves and their families with lead when they bring the carcass home. Natural predators are few and far between in the northeast, although coyotes are becoming both more common and larger, making it easier for them to bring deer down. The traditional predators, wolves and mountain lions, are unlikely to be restored to populations in the northeast sufficient to keep the deer in check.

So if reducing deer populations is important and none of our current strategies seem promising, what do I suggest? Integrated pest management. Look at the problem on a broad geographic scale, understand the demography in as much detail as we can, and figure out what the vulnerable point in the demographic cycle is. As an example, hunting seasons were traditionally in the fall and winter, exactly because hunting then reduces the deer herd the least. Fewer winter deaths, no pregnant does, few dependant young. If we really want to keep deer populations down, hunting should be done in the spring. Hunters have traditionally hoped for a buck. But killing a buck does basically nothing to reduce the population in the long term. Some other buck will be happy to inseminate all the does. One buck and a hundred does will produce just as many fawns as one hundred bucks and one hundred does. If our goal is to bring the population down, we need to target does, and let hunters take as many as they please, perhaps requiring the donation of excess meat to charitable food pantries. And we should do it with non-lead ammunition, so we don't poisoning ourselves in the process. And we should be honest enough to admit that we are going to have to do this every year in perpetuity, until the ecology of the northeast changes enough that deer populations are regulated naturally.

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