Why Predators Need to be Managed

By Pete Cimellaro, Chairman – Conserve and Protect Arizona

The role of predators in affecting wildlife populations is one of the most controversial issues associated with modern day wildlife management. It is often suggested that left alone, the balance of nature will eventually dictate the relationship between predator and prey, and that predation will be a function of density for some species. As an example, it is thought by some that when deer numbers increase, so will predation but not to the level that deer numbers will be driven much below carrying capacity of the habitat. Conversely, when environmental conditions drive decreases in deer numbers and prey availability is lower, predator populations will decline as well. Again, this is the balance of nature theory kicking in. It’s nice in theory but natural systems don’t always follow this pattern and many factors actually affect the dynamic relationships that do occur in nature.

One of the major factors that affect the function of natural systems is the presence of the human footprint on the landscape. In 1900, there were about 123,000 people in Arizona; a century later there were more than 5.1 million residents of this state. More than a startling fact, these people live, work and recreate on the same landscape with an amazing impact to not only natural habitat but also on wildlife themselves. Fawning areas have been converted to houses. Water sources such as the Salt and Gila rivers run underground, which is now being pumped to supply urban development. Simply, the ability for “hands off” wildlife management has been lost and the need for professional wildlife management has never been more paramount.

To develop an understanding of the relationship between top predators such as mountain lions and their prey, examining some scientific studies that have monitored the impact of predation on several different wildlife species is logical in forming an informed opinion on this issue.  

There are a number of studies that support the position that mountain lions can be responsible for elk population declines. A 25-year study in Oregon found that long-term trends in elk recruitment were most influenced by mountain lion densities above all other factors. A separate 17-year study in western Washington which included manipulation of mountain lion densities to test the role of predation on elk, indicated mountain lion predation reduced elk abundance and that when lions were experimentally removed, elk populations rebounded. Importantly, based on examination of carcasses, the authors of this study found that 75% of the elk killed by mountain lions were an additive factor or that these were prime animals that likely would not have died from natural causes.  

In studies that examined the impact of multiple predators, most pointed to mountain lions as having the greatest impact to both deer and elk populations. During the recolonization of wolves to Glacier National Park researchers documented that mountain lions had a greater impact on elk and deer than wolves. In this study, predation from both wolves and mountain lions was the primary factor limiting growth of deer and elk populations in that system. In this study, which was published in the Journal of Wildlife Management, the authors examined body condition using a health indicator of femur marrow fat and concluded that mountain lions readily killed prime-aged animals and that the femur fat index suggested that these animals would have survived the winter. So much for the concept that predators select the old and sick and actually improve herd structure.

In addition to impacting elk populations, the literature shows that predation can also play a role in affecting mule deer populations.  A decade-long study in New Mexico showed that when habitat conditions were poor (during a multi-year drought), mountain lion predation was the major cause of mule deer mortality contributing to the population decline.  With low fawn recruitment in bad years, predation shifts more to adult animals and that has a greater effect on the deer population particularly when adult females are killed.  This New Mexico study of predation found that losses from mountain lions could accelerate the rate of a deer population decline and also delay its recovery.

Similarly when the Round Valley mule deer population in California declined 83% (1984-1990) because of deteriorating habitat conditions, the number of mountain lions did not decline proportionately.  As a result, when habitat conditions improved the deer population did not respond as expected because of the heavy mountain lion predation that substantially delayed the increase in deer numbers.  This study showed the effects of predation may not have triggered the decline, but clearly showed the predation limited population growth under more favorable habitat conditions.

In researching wildlife populations it is difficult to account for all of the variables that can influence the population of wildlife being studied.  But a study conducted on the Three Bar Wildlife Area in Arizona, was able to take advantage of an enclosure that contained about 700 acres of predator-free habitat surrounded by similar habitat but with a variety of predators present. The research found that the mule deer population within the enclosed area grew to more than 100 animals while the density outside the enclosure remained about 3% of that inside. More telling, the researchers looked at a wide variety of habitat factors such as forage availability and quality and there was no difference between the two areas. The conclusion of the study was that predation was the cause of the difference in population density between the enclosure and the surrounding areas.

Mountain lion predation is also an important management concern for bighorn sheep.  In reviewing literature, it was stated that based on field research, that mountain lions are the only species capable of substantial predation on bighorn sheep. Case studies demonstrate that this is a reality in wild settings and many wild sheep populations have been adversely impacted by mountain lion predation. In a study of peninsular bighorn in California, a subspecies that was listed as endangered, researchers monitored the fate of radio collared bighorn. Over a nearly six year study, lions killed 26 of the 42 monitored bighorn. One of the conclusions of this study was that mountain lion predation at the rate observed could impede recovery.

In another instance in Arizona, an introduced population of sheep had increased post release to a high and sustained level for several years. Hunters reported finding a large number of mountain lion depredations on bighorn sheep, which were reported to the state wildlife agency. With follow up surveys, the agency determined that the population had declined markedly and as a result implemented a research project to determine the cause of the decline. What resulted was an encompassing study that looked at habitat conditions, health status of the bighorn, food resources, and the agency reduced mountain lion densities to test the role of predation in the decline. After years of study, it was determined that the bighorn herd was healthy and that the habitat was capable of supporting the bighorn and that food was not a limiting factor. When a limited number of mountain lions were removed, however, the bighorn population rebounded. Another important finding, which has been reported in other research findings is that all mountain lions do not prey on bighorns equally and that individual mountain lions become more adept at preying on bighorn and the removal of select predators can result in a bighorn population recovery.

This brings us back to a point made earlier, it is important to recognize that wildlife management is a science that has developed and been refined for decades. Wildlife should be managed by the educated professionals that use science to make informed decisions on the most effective way to ensure continued presence of our wildlife. Predator-prey relationships are dynamic and defy generalizations across ecosystems and predator species. All natural systems are complex and depending on a wide variety of factors such as weather and rainfall, can function under a combination of circumstances where prey populations can be either predator limited or food limited. These influences may change in the same population through time with changing environmental conditions or animal numbers. The effect of mountain lions on deer, bighorn sheep, and elk populations is determined by the number of prey in relation to habitat carrying capacity, weather conditions, other sources of mortality, and prey recruitment rate; but make no mistake, under the right conditions, mountain lions can and do reduce many of the larger prey species.  

With all of the above in mind, it has been demonstrated in the case studies above and in many more that all predators, but more so mountain lions, have the ability to cause the decline in wildlife numbers; and managing mountain lion populations can lead to improved populations in numerous species. With the effect of humans on the natural world being felt more each day, professional wildlife management is more important than ever before and without doubt, this also dictates effective management of all predators.