Predation on Southwestern Mule Deer

Mountain lions kill mule deer, but the effects of predation on deer populations depend on many interrelated factors.

Throughout history many carnivores, such as mountain lions, black bears, bobcats, mountain lions, coyotes, golden eagles, wolves, and transient jaguars preyed on mule deer in the Southwest. The population-level effects of these predators are variable and depend on how productive the deer population is, the ratio of predators to prey, what other predators are present, and what other prey species are also feeding the local predators. These predator-prey relationships are complicated because deer population fluctuations are not simply a matter of one factor determining deer abundance. Today’s deer managers must understand the current assemblage of predators and consider their potential impact as part of a holistic management program.

Predation is a complex and contentious subject that can cause friction between researchers, wildlife managers, and the public. Often it seems that biologists keep pointing to weather and habitat while some members of the public believe agencies are simply reluctant to do anything about the predator problem. There’s good reason for confusion and the apparent split between the “habitat” and “predator” camps. The effects of predators on southwestern mule deer herds are so complicated that you can find scientific studies to reinforce any view you have on predators, if you’re selective. Unfortunately, the truth behind how and when predators affect mule deer populations is mired in details. The habitat-predator connection

In reality, “habitat” and “predators” are simply two parts of a complex equation that describes what makes deer populations fluctuate. One basic ecological principle called “carrying capacity” is at the heart of understanding the relationship between habitat quality and predators. Skipping the technical jargon, let’s just say when the habitat cannot support any more deer, then fawns saved from the jaws of coyotes will die from something else. For example, consider the mule deer killed by mountain lions in an over-populated herd right before a bad drought or harsh winter. Since a lot of deer will die of starvation or malnutrition during those harsh conditions, those killed earlier by lions did not have any effect on the number of deer

the next year. When the deer population is well below the maximum carrying capacity of the habitat, then there’s enough food, water and cover to support those deer saved from preda-

Paying attention while in the field can help determine what predators are in the area and represent sources of deer mortality. hoto p eson V ar h ouis l

tors. Since drought conditions and carrying capacity fluctuate dramatically from year to year, it’s difficult to predict whether the habitat will be able to support more deer six months from now. This makes it difficult to forecast how effective a predator management program will be. The interaction between predation and habitat carrying capacity is illustrated well by a New Mexico lion study, where removing more than half the lion population was followed by a severe drought. In this case, they saw no benefit to mule deer fawn recruitment or adult survival because the habitat couldn’t support the deer saved from predation. If mountain lion population reduction had been done for the purposes of increasing deer abundance, it would have been a waste of time and money. Predator impacts When deer populations decline, predators become a focus of concern because they can kill considerable numbers of deer. Mountain lions are the most important predators of adult mule deer, but coyotes can kill a large number of young fawns. These

two predator species garner most of the attention when it comes to predation on mule deer in the Southwest. Elsewhere in the West, grizzlies, wolves, black bears, bobcats, and golden eagles kill mule deer at varying degrees. Of these, only black bears at locally high densities and wolves on islands have been shown to substantially influence mule deer abundance.

Predators certainly can and do affect deer populations, but just because they’re affecting deer populations doesn’t mean they’re the primary factor regulating how many deer we have. Predators have been shown to accelerate deer declines caused by poor habitat conditions—drought or harsh winter—and delay the recovery of deer populations. However, coyotes and mountain lions don’t suddenly cause declines of mule deer populations living in good habitat.

Knowing when to manage predators

One cannot make any wide-sweeping generalizations about how predators affect mule deer populations because of the variability of their environment. We can say with confidence predators eat deer, and beyond that, we need to get into the details. Predators certainly are an important part of the ecosystem as a whole, but that doesn’t free them from the need to be managed. Wildlife biologists spend a lot of time managing prey populations, so it only makes sense we also manage predator populations when needed.

Experiments that exclude coyotes from an area are often followed by an increase in the deer population, indicating coyotes depressed deer abundance. If you could duplicate that and remove 100% of the predators from an area, and the habitat can support more deer, the deer population will increase. However, what can be done in a small research enclosure is impossible to replicate cost effectively on a statewide level or even a few counties.

The fact is, there’s a place for predator management, but managers should implement predator reduction plans only when there is a high probability for success in accomplishing a clearly defined goal. Killing predators in a vague attempt to blindly “help the deer” is not likely to accomplish anything that would be noticed on annual deer surveys. No predator management should be done without some long-term monitoring of deer populations and an understanding of what actually affects them. Documenting the conditions when predator management does or does not work will help achieve wildlife management goals without unnecessary waste of time and money, and will help avoid conflict with the media and those who don’t ever want to see a predator killed. It would be nice if issues like predation were clear-cut, and we could all decide the best course of action for helping mule deer populations. The truth is predator-prey relationships are complicated and must be evaluated on a case-by-case basis. Broad-brushed general statements about the impacts of predation and predator management are not helpful without specifics.

Coyotes kill a lot of fawns every year and our effectiveness in reducing that predation loss varies with many other factors.

Embrace the complexity

Deer population dynamics aren’t simply a matter of one factor determining deer abundance, but instead governed by extremely complex interrelationships among many environmental factors. This doesn’t mean we throw up our hands and give up. It means we have to make informed decisions about predator management. We must learn from the past predator management actions and look realistically at biological and logistical facts, and consider for each case whether predator management is the right tool to manage that deer population at that time.