Do Coyotes Cause Deer Declines?

Coyotes kill deer.  This fact is undisputed.  The real question is:

Do coyotes cause deer populations to decline?

This question has led to years of research.  While there are still strong advocates on both sides, enough science has been done that there is an evidence based answer to this question.

Coyotes will scavenge deer carcasses any time of the year (1) and usually have deer hair in their scat, but they primarily hunt deer during the spring and the winter in areas with snow.  There is some evidence of coyotes hunting adult deer in areas without snow, but this is not well documented.  Studies from northern areas show during mild winters coyotes kill less deer and switch to other prey like snowshoe hare(2), and it is hard for coyotes to kill adult deer without snow to slow and exhaust deer.  It is mainly the alpha male and female of territorial breeding pairs that kill deer and livestock, and younger and transient animals stick to smaller prey and scavenging (3).  In fact the presence of the alpha pair is just as important for deer hunting success as the coyote pack size(4).

In the spring coyotes focus on hunting deer fawns, and in areas without wolves or bears coyotes are usually the largest source of fawn mortality.  In various studies coyotes have caused fawn mortality as high as 50% and as low as 9% (5, 6).

Killing lots of fawns means that coyotes are bad for deer right?  Unfortunately it is not that simple.  There are two main kinds of predation, compensatory and additive predation.  Compensatory predation describes a situation where a predator kills prey that would have died from other causes later in the year (i.e. starved to death) while additive predation are predator kills that add to the total mortality rate (i.e.  predators kill healthy deer that would have survived the winter).  The real question is whether coyote deer kills are additive or compensatory.  If predation is additive than coyotes are lowering the deer population, but if most predation is compensatory than coyotes are only killing deer that would die from something else.

One way to figure out which kind of predation is happening is to shoot a bunch of coyotes in one place and not shoot them in another place and look at the difference in fawn survival (some researchers build a fence and then shoot the coyotes).  Many early coyote removal studies removed coyotes in the spring and then counted the ratio of fawns to does in the fall (7).   These studies showed that coyote removals increased the number of fawns that survive to the fall, BUT they did not follow fawn survival through the winter and we don’t know if those fawns survived to become adult deer.

Researchers saw the problem with these studies and conducted some very large and intensivecoyote deer research throughout the United States.  A 9 year study in Idaho removed both mountain lions and coyotes(8) and several large studies in Wyoming, Utah, and Colorado removed coyotes from areas ranging from 40 to 300 square miles(9–11).  None of these studies found any evidence that coyote removal caused an increase in the deer population.  The study in Idaho actually found that fawn predation was more affected by the abundance of other prey than coyote removal.   The two best coyote removal studies with white-tailed deer populations also show that coyote removal does not affect overall population growth.  The first study in southeast Texas showed that coyote removal increased fawn survival but did not change the population density(12).  Another long term study of a deer population in a predator free fenced area in Texas found that deer density increased for a few years and then sharply declined to match the populations outside the predator exclosure (13) and the deer in the predator free area had a much poorer diet (14).  The researchers concluded that coyotes actually had a stabilizing effect on the deer herd.

The fact that coyotes are not causing deer populations to decline can also be seen in the effect white-tailed deer are having on forest ecosystems throughout the eastern United States.  Overabundant white-tailed deer have been shown to decrease tree regeneration (15), changed the composition of the entire plant community within reach of a feeding deer (16), and changed shrubs enough to decrease survival and abundance of nesting songbirds (17).

Do coyotes cause deer declines?

The evidence says no.  There is no evidence that coyotes are the factor that keeps deer populations from growing and actually evidence that some predation may keep the deer herd from overshooting the food supply.  Agree or disagree?  Leave a comment below, but please be civil.

Literature Cited

1. M. Bekoff, Canis latrans, Mammalian species , 1–9 (1977).

2. B. R. Patterson, L. K. Benjamin, F. Messier, Prey switching and feeding habits of eastern coyotes in relation to snowshoe hare and white-tailed deer densities, Can. J. Zool.76, 1885–1897 (1998).

3. B. N. Sacks, J. C. C. Neale, FORAGING STRATEGY OF A GENERALIST PREDATOR TOWARD A SPECIAL PREY: COYOTE PREDATION ON SHEEP, Ecological Applications12, 299–306 (2002).

4. M. Elbroch, K. Rinehart, Behavior of North American Mammals (Houghton Mifflin Harcourt Trade & Reference Publishers, 2011).

5. W. B. Ballard, D. Lutz, T. W. Keegan, L. H. Carpenter, J. C. deVos, Deer-Predator Relationships: A Review of Recent North American Studies with Emphasis on Mule and Black-Tailed Deer, Wildlife Society Bulletin29, 99–115 (2001).

6. W. B. Ballard, H. A. Whitlaw, S. J. Young, R. A. Jenkins, G. J. Forbes, Predation and Survival of White-Tailed Deer Fawns in Northcentral New Brunswick, The Journal of Wildlife Management63, 574–579 (1999).

7. G. G. Stout, Effects of Coyote Reduction on White-Tailed Deer Productivity on Fort Sill, Oklahoma, Wildlife Society Bulletin10, 329–332 (1982).

8. M. A. Hurley et al., Demographic response of mule deer to experimental reduction of coyotes and mountain lions in southeastern Idaho, Wildlife Monographs178, 1–33 (2011).

9. J. L. Harrington, M. R. Conover, Does Removing Coyotes for Livestock Protection Benefit Free-Ranging Ungulates?, Journal of Wildlife Management71, 1555–1560 (2007).

10. D. E. Brown, thesis, Utah State University.

11. R. M. Bartmann, G. C. White, L. H. Carpenter, Compensatory Mortality in a Colorado Mule Deer Population, Wildlife Monographs , 3–39 (1992).

12. S. L. Beasom, in Transactions of the North American Wildlife and Natural Resources Conference, (1974), vol. 39, pp. 230–240.

13. J. G. Teer et al., in Transactions of the North American Wildlife and Natural Resources Conference, (1991), vol. 56, pp. 550–560.

14. J. G. Kie, D. L. Drawe, G. Scott, Changes in Diet and Nutrition with Increased Herd Size in Texas White-Tailed Deer, Journal of Range Management33, 28–34 (1980).

15. D. M. Waller, W. S. Alverson, The White-Tailed Deer: A Keystone Herbivore, Wildlife Society Bulletin25, 217–226 (1997).

16. T. P. Rooney, D. M. Waller, Direct and indirect effects of white-tailed deer in forest ecosystems, Forest Ecology and Management181, 165–176 (2003).

17. D. S. deCalesta, Effect of White-Tailed Deer on Songbirds within Managed Forests in Pennsylvania, The Journal of Wildlife Management58, 711–718 (1994).