Predator Control: Who is Wile E. Coyote and who is the Roadrunner?

Conventional wisdom dictates that if there are fewer things that eat something else, then the result will be more of the something else.  But Mother Nature doesn’t always share our thinking.  And no matter how much we study it and get results to the contrary, human nature can’t seem to shake this belief.  Are we the Wile E. Coyote to the eastern coyote Roadrunner?

Case in point: coyotes and deer.  Deer and coyote research in the southeastern U.S. has documented high rates of fawn mortality due to coyotes and a peer-reviewed commentary in The Journal of Wildlife Management suggested that coyotes may play a major role in reducing deer densities in the Southeast. This research in South Carolina and Georgia  has documented high mortality rates of white-tailed deer fawns due to coyotes, that deer are an important food item for coyotes, and that indices of deer recruitment have shown declines (number fawns per adult female in the harvest has declined).

Pennsylvania uses the same index to monitor deer recruitment and has found no change over the past decade. And my colleagues and I published our own peer-reviewed commentary in The Journal of Wildlife Management suggesting that available evidence indicates the simpler solution of reducing harvest of antlerless deer can compensate for increased coyote predation in most situations.

But regardless of the data, conventional wisdom won’t let go.  So let’s look at the data a little more closely.  Dr. John Kilgo and colleagues in South Carolina actually put the effects of coyotes to the test by removing an average of 4.2 coyotes per square mile per year (Jan-Apr) for 3 years (a total of 474 coyotes on 37 sq. miles) and monitoring the survival of 216 fawns.

Deer-Forest Study

Before the culling of coyotes, fawn survival was 22.8%. During the first year of coyote removal, fawn survival more than doubled to 51.3% but after 2 years of coyote removals fawn survival was lowest – only 20.2%! In the third year, fawn survival was 43.1%.

They concluded that “coyote control may not be a viable tool for most land managers in the eastern United States hoping to improve recruitment or increase deer population density.”  (Mother Nature thumbing her nose at our conventional wisdom.)

Even though they documented an increase in fawn survival, “The level of coyote removal necessary to increase deer recruitment to desired levels is unknown but appears difficult to achieve through ordinary trapping.” And coyote control may be even more difficult on smaller tracts of land because coyotes travel over large areas and they can have high immigration rates into territory vacancies. (Again, Mother Nature having a good laugh.)

The authors also noted that the “marginal gains in recruitment … may not justify such an investment.” The investment they were referring to was $123 per coyote removed (approximately $58,000 on 37 sq. miles over 3 years – assuming each coyote pelt could be sold for $76 to recover some costs). In Pennsylvania we have about 3,300 sq. miles of state forest land and about 2,300 sq. miles of State Game Lands.

At over $500 per square mile per year, society will have to decide if it’s economical to reduce coyote populations to effect a deer population increase, or just reduce the antlerless deer harvest.

While all this talk about coyotes and deer has spawned debate and even research here in Pennsylvania, there is more research going on across the pond on a sibling species that may lend more insight to this discussion.  But what do red foxes in France have to do with deer, coyotes, and conventional wisdom in the U.S?

Deer-Forest StudyNicolas Lieury and his colleagues’ question was more theoretical in nature but probably more important to understand. Without going into the gory details, they wanted to know how culling of foxes affected their population growth rate. That is, they wanted to know how the fox population responded to increased mortality – was it through increased immigration rates or increased reproductive rates? 

The experiment was replicated on 5 areas, of which each area was about 100 square miles and consisted of having local hunters and trappers increase their culling rate of red fox (no bounties were offered). They killed on average 3.8 foxes per sq. mile per year.

Culling could not reduce late winter densities more than 25-32% below carrying capacity which was about 4 fox/sq.mile. They had to cull almost half (45%) of the pre-breeding population annually to maintain a population density of 2.6 fox/sq. mile!

The only time of year that culling was effective was in winter. If culling occurred any other time of year, culling was totally compensated by immigration!

So is there a take-home message? Well, red fox are not coyotes (although in the same family with similar social behavior) but I think it’s clear that if we are to ever have any hope of devising an effective coyote control program (i.e. conventional wisdom actually holding a chance against Mother Nature), we need to understand the relative importance of coyote reproduction and immigration in relation to culling.

Only then will we know if we an afford it. Or if it’s worthwhile. Or if humans will forever be Wile E. chasing the Roadrunner.

-Duane Diefenbach

Photo credits: red fox by Rob Lee; fawn by PGC

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