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.