Wild Turkeys in Kansas: Landscape change and Allee effects

The Journal World points out how turkey populations are changing in Kansas:

In the 1950s the only places where turkeys could be found in Kansas were along the southern border where they had crossed over from Oklahoma, [small game coordinator for the Kansas Department of Wildlife and Parks] Pitman said.

“It’s my understanding that before we had regulated hunting seasons, they were just shot to extinction,” he said.

Kansas began bringing in turkeys from other states, but it wasn’t until the 1990s that the turkey population showed a dramatic increase, Pitman said.

“It just takes a while to reach critical mass,” he said. “It finally gets to the point where there are enough birds out there. Then it (the population) just takes off.”

Wild turkeys are more prevalent in eastern Kansas because there is more timber, and turkeys roost in trees. Their populations, however, are still growing in central Kansas, Pitman said.

There is more timber in eastern Kansas now than there was 50 years ago, also, and I suspect that plays into this change, too. Western Kansas is simply too dry to sustain forests, and regular prairie fires help keep low shrubs out. In the northeast corner of the state, rainfall is adequate to support habitat characteristic of eastern woodlands, an ecoregion characterized by deciduous trees and by animals like the wild turkey.

Fire played a huge role in maintaining the prairie in eastern Kansas. There's a piece of land the University owns just north of Lawrence that was given to the school in the 1940s. The idea was that it would be used to study conservation, so the deed forbids any manipulations of the land. That means no mowing, no trapping (other than live-trapping) and no burning.

There are photographs of the land just after its last harvest, when the University took control. The ground is bare. Sixty years later, the entire plot is covered by a thick forest of trees a foot wide or more. As more and more land is taken out of use for farming, trees have invaded further and further into this former prairie, giving turkeys ever more land to live on.

What Pitman seems to be arguing for is a different process. He's saying that population levels were too low to allow population growth, a phenomenon called the Allee effect.

In general, a population that isn't limited by predation or resources ought to grow exponentially, doubling in size on a regular basis. That should continue until the population hits some limitation, at which point you've reached carrying capacity and the population ought to stabilize.

The Allee effect, named after ecologist and textbook author Warder Allee, occurs when birth rates rise (or death rates drop) as population size increases. This can happen if finding mates becomes easier with higher population density, or if individuals hunt or forage more efficiently in larger populations.

I don't know whether anyone has looked for Allee effects in turkeys, but if anyone out there wants to fill me in, I'm all ears.

The other option, after all, is that the population growth rate is constant. Exponential growth has an explosive appearance, since doubling from 100 to 200 turkeys would be quite different than doubling from 10,000 to 20,000. The turkey population may simply not have reached any limit on its growth rate yet.