The flawed logic behind Utah’s mountain lion culling study

When it comes to wildlife management, few topics generate more debate than predator control to support big game populations. The idea of killing predators to increase populations of their prey predates wildlife management as a formal discipline. The logic is straighforward: remove the predator that eats the animal people want more of, and that prey population should increase. On the surface, it seems simple. 

Nature, however, is rarely that simple. A wide range of scientific studies have examined the relationships among predators, prey, and management actions. If there is a common theme, it is that predator control does not produce universal or predictable results (1-7). In some cases, removing predators has led to increases in big game populations. In others, it has not. Outcomes depend heavily on ecological context, habitat conditions, weather, and the presence of other predators. Predator removal, in short, is not a guaranteed solution for increasing ungulate populations.

This debate is especially active in the western United States, where wildlife agencies face pressure to boost mule deer populations. Mule deer are valued by human hunters and are also a primary prey species for mountain lions, creating the perception of direct competition. Yet research suggests the relationship among mountain lion predation, mule deer population trends, and predator control is complex and not easily reduced to simple cause and effect (2, 8, 9). 

Utah’s proposed study

The state of Utah has launched a multi-year study to examine the effects of culling mountain lions on mule deer populations. The Utah Division of Wildlife Resources (DWR) plans to remove as many mountain lions as possible from six of the state’s 30 hunting units (10). Mule deer across Utah are already monitored using GPS collars that track survival and movement. The study aims to determine whether mule deer mortality and survival patterns change in areas where mountain lions are intensively removed. Funding for the project comes from two hunting organizations: the Utah Wild Sheep Foundation and the Sportsmen for Fish and Wildlife. 

At first glance, experimentally testing whether mountain lion removal affects mule deer populations appears consistent with science-based wildlife management. Research is essential for informed decision-making. However, closer examination of the study design raises several concerns.

The issue of experimental design

A central concern is the apparent lack of a control group. In scientific research, a control group is foundational. It represents an area or population where the treatment, in this case, intensive mountain lion removal, does not occur, allowing researchers to compare outcomes between treated and untreated areas (5).  Many people are familiar with this concept through medical studies that use placebos. Ecological research similarly requires comparison areas to isolate the effects of management actions (1, 2,5).

The Utah study appears to rely primarily on before-and-after comparisons within the six targeted units. While such comparisons can provide useful information (11), they may also be influenced by other factors occurring during the study period. For example, winter severity strongly affects mule deer survival (2, 8). A series of mild winters could increase deer survival regardless of predator removal, potentially complicating the interpretation of results.

Ideally, some hunting units would undergo intensive mountain lion removal while others would not, allowing direct comparisons over the same period (8). However, Utah already permits year-round cougar harvest with few limits. As a result, there may be no true “no-culling” areas available to serve as controls.  Although open harvest does not necessarily mean predator populations are substantially reduced, mountain lion harvest has increased in Utah over the past decade (12). Without clearly designated control areas, isolating the specific effects of the study’s intensified removals may prove difficult. 

Additional unanswered questions

Several additional questions remain unclear. Will data be collected on the mountain lions themselves? Are removals targeting specific age classes or sexes? Are lions being removed uniformly across each unit or concentrated in high-quality mule deer habitat? Does the state have reliable estimates of how many mountain lions inhabit each unit prior to removal? 

Baseline population estimates are important for determining how much reduction would be required to meaningfully influence mule deer populations. In addition, other predators, such as coyotes, can affect mule deer fawn survival (2, 12, 8). If mountain lion numbers decline but other predators continue to limit fawn survival, the overall effect on mule deer may be limited.

Broader management context

This study also unfolds within a broader political context. A 2020 state legislature requires predator populations to be “managed” when big game numbers fall below specified thresholds. These legislative directives shape the management landscape in which DWR operates.

At the same time, it is worth examining long-term mule deer trends. Mule deer populations have fluctuated for decades (2, 14, 10). Since 2010, state-reported data indicate that Utah’s mule deer population has hovered around 300,000 animals, with periodic fluctuations but no clear long-term decline (15). Measures such as adult and fawn survival, buck-to-doe ratios, and hunter success rates have remained relatively stable. At a statewide scale, the severity of the problem the study seeks to address is open to interpretation.

Conclusion

Utah’s mountain lion removal study is presented as an effort to evaluate predator control scientifically. However, questions about experimental design, the absence of clear control areas, and the broader management context may limit the strength of the conclusions that can be drawn. The outcome of the study will have implications not only for mountain lions and mule deer but also for public trust in wildlife management decisions. 

As debates over predator control continue, careful study design, transparency, and clear objectives will be essential to ensuring that wildlife policy is guided by credible and rigorous science. 

 

References

(1) Reynolds, J.C. and Tapper, S.C., 1996. Control of mammalian predators in game management and conservation. Mammal review, 26(2‐3): 127-155.

(2) Ballard, W.B., et al. 2001. Deer-predator relationships: a review of recent North American studies with emphasis on mule and black-tailed deer. Wildlife Society Bulletin, 99-115.

(3) Allen, B.L., et al. 2014. Sympatric prey responses to lethal top-predator control: predator manipulation experiments. Frontiers in Zoology, 11(1):56.

(4) Chitwood, M.C., et al. 2015. White‐tailed deer population dynamics and adult female survival in the presence of a novel predator. The Journal of Wildlife Management, 79: 211-219.

(5) Treves, A., et al., 2016. Predator control should not be a shot in the dark. Frontiers in Ecology and the Environment, 14(7): 380-388.

(6) Clark, T.J. and Hebblewhite, M., 2021. Predator control may not increase ungulate populations in the future: a formal meta‐analysis. Journal of Applied Ecology, 58(4): 812-824.

(7) Muthersbaugh, M.S., et al. 2026. Evaluating potential altered harvest and predator management strategies to increase white‐tailed deer population growth in the Southeastern United States. The Journal of Wildlife Management, 90: e70127.

(8) Hurley, M.A., et al. 2011. Demographic response of mule deer to experimental reduction of coyotes and mountain lions in southeastern Idaho. Wildlife Monographs, 178: 1-33.

(9) Elbroch, L.M. and Quigley, H., 2019. Age‐specific foraging strategies among pumas, and its implications for aiding ungulate populations through carnivore control. Conservation Science and Practice, 1: e23.

(10) Utah Division of Wildlife Resources. 2024. Utah Mule Deer Statewide Management Plan. https://wildlife.utah.gov/public_meetings/rac_minutes/2024-11-mule-deer-statewide-management-plan-2025-30.pdf

(11) Serrouya, R., et al. 2017. Experimental moose reduction lowers wolf density and stops decline of endangered caribou. PeerJ, 5: e3736.

(12) Bernales, H., and D. DeBloois. 2024. Utah Cougar Annual Report 2024. Annual Report 25–21. Utah Division of Wildlife Resources. https://wildlife.utah.gov/pdf/annual_reports/cougar/24_cougar_annual_report.pdf 

(13) Hamlin, K.L., et al. 1984. Relationships among mule deer fawn mortality, coyotes, and alternate prey species during summer. The Journal of Wildlife Management: 489-499.

(14) Mule Deer Working Group. 2019. Historical and current mule deer abundance. Western Association of Fish & Wildlife Agencies. https://wgfd.wyo.gov/media/31621/download?inline 

(15) Utah Division of Wildlife Resources. 2026. Supportuing Utah's Mule Deer. https://wildlife.utah.gov/deer.html