Ryan Rauscher is the Conrad Area Wildlife Biologist for the Montana Department of Fish, Wildlife and Parks. Recently, he was asked to explain why predator management is important. He invited us to share his response with you:
The other day, I received a request to explain why predator management is a valuable tool to wildlife managers. The person was most interested in coyotes management to improve game numbers. There were no questions of what is the role of predation, impacts of predation, or predator and prey population dynamics. This person had already made up his mind regarding coyote management and was only looking for supporting evidence to strengthen his perceptions. As a biologist, I reviewed the scientific literature to ensure an accurate response to his question.
As mentioned before, one of the foundational principles of the North American Model of Wildlife Conservation is that science is the proper tool to manage wildlife. Science is unbiased decision-making using a rigorous method. Science is used to understand natural phenomena including the role of predation, predator management, and wildlife mortality.
Predator control as a management tool is a complex issue in part because different segments of society value predators differently and because previous research on effects of predator management can be ambiguous. Nonetheless, coyote management is often recommended by many sportsmen to increase wildlife populations for several reasons. First, encounter rates between coyotes and hunters that include seeing coyote tracks and scat, hearing coyote howls, and finding coyote kill sites are likely higher than most predators. This produces an intuitive reaction by sportsmen that coyote predation has a strong negative effect on deer populations. Secondly, investigations into the causes of mortality of deer specifically have consistently identified coyote predation as a common source of mortality. Stemming from this, the effect of coyote predation on mule deer has been more thoroughly investigated in recent times.
To understand predator effects on prey populations, a basic understanding between two types of mortality is required. Mortality from predation can either be compensatory and additive. Compensatory mortality occurs when one source of mortality offsets another source. An example of compensatory mortality would be a mule deer fawn in poor physical condition killed by a coyote during winter. The fawn likely would have died from malnourishment even if it had not been killed by a coyote. When compensatory mortality is occurring in a population, reducing one source of mortality will result in an increase in another source with no net decrease in total mortality. Compensatory mortality occurs most often when populations are near carrying capacity. Predator control when wildlife populations are at or near carrying capacity will not benefit prey populations. Conversely, additive mortality results in an increase in total mortality. An example of additive mortality would be a healthy adult mule deer doe killed in a vehicle collision during summer. This doe would have likely survived if not for the accident. The further a populations are below carrying capacity, the greater proportion of total mortality is additive. Applying predator management actions when predation is largely additive can increase prey numbers if predation has been proven to be a major contributor to total mortality.
Predator management can appear simple relative to compensatory or additive mortality. However predator management is complicated by a host of additional dynamic factors including forage and cover conditions, weather, alternate prey abundance, physical condition and vulnerability to predation. A single source of mortality can be compensatory under one set of conditions and additive under another. The challenge is to understand enough about the intricacies and interactions of all the factors to determine if and when predator management could be effective, and therefore potentially appropriate. Generally, intensive studies are required to understand the relative role of all potential limiting factors for a deer population. However, such studies are complex requiring a great deal of resources and difficult to complete. Thus previous relevant studies are often used in management decisions.
An intensive predator management study occurred in Colorado in 1992 where coyote density was reduced to evaluate the affect on deer population dynamics. Coyote control annually removed a large percentage of coyotes from a winter range study area and did reduce coyote predation on deer. However a simultaneous increase in malnourishment deaths occurred with no overall increase in survival rates. This switch between mortality causes, with no increase in survival, was viewed as strong evidence that coyote predation was compensatory.
Another more recent, research project in southeast Idaho in 2011 also tested the effectiveness of coyote removal from mule deer winter range as a management strategy to improve mule deer population survivorship. An average of 53 coyotes/1000 km2 per year was removed during a six-year period from a mule deer winter range. Coyote removal did result in increased fawn survival during years in which lagomorphs populations were low. However, the increase in survival was temporary. Subsequent December fawn to adult female ratios showed no population level increase resulting from coyote control efforts. Also, no effect of coyote removal was observed during years of normal lagomorph abundance.
In addition to large scale predator manipulation studies, other studies have quantified coyote predation on mule deer. In Colorado in 2009, coyote-caused mortality rates for two groups and age classes of deer were examined. One group received a nutritional supplement. The fawns in this group had a coyote-caused overwinter mortality rate of 4 fawns per 100. Adults receiving nutritional supplements had an annual coyote-caused mortality rate of 1 per 100. The other group under similar conditions did not receive a nutritional supplement. Fawns in this group experienced an overwinter coyote-caused mortality rate of 3 times higher than the first group. The adults in this group experienced an annual coyote-caused mortality rate of twice as high as the first group.
Other research assessed the secondary impact of coyote removal for livestock protection purposes on mule deer. Based on sex ratio data, researchers in 2007 found that coyote control efforts had no effect on deer population productivity.
As you can see from the above examples, the relative degree to which predation on deer affects population levels varies considerably through time and space, and there are no simple methods for determining if and when predator management is appropriate and can be used to increase deer populations. A cursory look at a deer herd or predator populations will not provide the information needed to determine if predation is limiting. Only after an in-depth evaluation of all the factors potentially affecting a deer population can a scientifically-sound predator management recommendation be made that will produce the desired effects.
The above begs the question, when is predator management activities appropriate? Again, speaking to deer and coyotes, several conditions must be met. First, the deer population must be below carrying capacity, a difficult thing to measure at times. Second, coyote predation must be identified as a major cause of mortality. Third, coyote management efforts can result in a significant decline in coyote numbers e.g. greater than 70%. Also, coyote management is timed just prior to coyote or deer reproductive periods. And finally, coyote management efforts are focused at an appropriate, local level.
Therefore, the assumption that predator control is a valuable tool for increasing wildlife populations, although it is standard dogma, is not as simple as it appears and doesn't hold true in many cases. It is often easy to confuse the fact predation exists with the effect of predation. Just because individual deer have been lost to coyote predation does not mean deer populations have been adversely affected.