The Role of Predator Management: A Wildlife Manager’s Perspective
Article by Dr. John M. Tomeček, Assistant Professor and Extension Wildlife Specialist, Department of Wildlife and Fisheries Sciences, Texas A&M University
Photos by: Butch Ramirez, Russell Graves, Wyman Meinzer, and Larry Ditto
Predation as an Ecological Force
Just as one would not manage game animals without first understanding their influence on forage resources, so too must we not manage predators before knowing the influence on prey.
To understand how predators can affect prey, and what we, as managers, can or cannot change, we must first examine the predator/prey functional relationships. We must also always remember that every action has consequences, both intended and unintended: there is no such thing as a free lunch.
The first functional relationship to discuss is a low-density equilibrium. In this situation, the abundance of prey animals is maintained at a fairly stable, but low level over time. The prey exist far below the landscape’s carrying capacity based on forage production. Predator control in this system will generally result in more prey because fewer animals are consumed and more young reach maturity. This population never reaches carrying capacity, and eventually predator numbers increase to an extent that the system returns to the low, stable density of prey.
The second relationship is the concept of multiple equilibria. Here, there is at least more than one level of prey abundance or density regulated by both predation and landscape carrying capacity. Essentially, prey animals increase in numbers above some lower level. When they do, predator numbers rise to meet them and hold both predator and prey populations steady at some higher equilibrium. Minor fluctuations occur, but the relationship is fairly stable. This is density-dependent predation.
When predator numbers are reduced by lethal management, however, prey are released from the stability of their earlier relationship, and increase in abundance to a higher level. Ultimately, the carrying capacity of the landscape—the vegetation that makes up wildlife habitat—may suffer as it is consumed to the maximum limit at which prey can survive. Under this equilibrium, both habitat and prey exist in a less-than-ideal condition, with prey numbers checked due to starvation.
A third functional relationship considers “stable limit cycles.” In this case, weather drives increases in prey abundance, as relatively better conditions improve forage availability. Predation is density-independent, meaning that the number of prey consumed does not increase with their increasing density. Ultimately, however, prey numbers grow to an upper limit where they reach or exceed vegetative carrying capacity, typically as weather conditions change to limit forage resources, such as under drought or harsh winters. Predation now intensifies as prey abundance declines due to lack of food, and prey are drawn back down to a low limit at which predation declines as relatively few prey make them inefficient for predators to search out. Eventually vegetative resources will improve, and the cycle beings again between these limits.
Finally, we often see what we call “recurrent fluctuations.” In this functional relationship, prey abundance is generally some low-density equilibrium, but punctuated by an infrequent event that causes increases in prey abundance. These increases are typically rapid and increase numbers up to the level of carrying capacity. As food competition continues, declines in prey are the result of food competition and intensified predation as an unusually high density of prey have incentivized predators to forage in that area. Ultimately, prey numbers will decline back to a stable level, but the new equilibrium may not be the same as before—higher, lower or similar to the same—it is difficult to tell, but it can have serious ramifications for long-term prey population declines.
In all of these relationships, there is a common theme: prey abundance is bounded by the food availability and the predation rate. In any system, fluctuations occur that cause numbers of predators and prey to vary widely, but these systems were “built” with these species in mind. Predators play a vital role in maintaining the balance of an ecosystem when prey numbers grow so large that they threaten the stability of the ecosystem’s vegetation.
The Effects of Predation
Recently, I had a conversation with a rancher, discussing the effects of predation on his sheep flock. I am fairly sure he thought I was insane for discussing the different effects predation can have on prey animals.
“Son,” he instructed me, “there’s one and only one effect of predation—dead animals. And that’s that.”
Although I could not disagree with this land steward and passionate animal husbandry practitioner, that predators do kill prey, I submit that the story goes much further than that. The killing of an animal can have both positive and negative consequences, and our judgement of those depends entirely on our goals and needs. A wise biologist once told me that anyone’s position on a matter depends largely upon where they sit: both in predator politics, as well as life. Before we discuss the costs and benefits of predation, we must first examine the effects of predators on prey.
Predation’s primary effect, if we must choose a primary effect, is that an animal life is lost. As the rancher told me, that’s the story, chapter and verse. For a livestock raiser, that may be true: primary effects from predation regard the total number killed.
For the wildlife manager, the story may be similar, but the implications are very different. When we consider the loss of livestock, we have lost food (or fiber) for human consumption, as well as income for the producer. In the context of wildlife, however, we may have gained some sorely needed population management, or we could have experienced a backward slide in our goals. All things hang on the objectives in our wildlife management plan.
Examining primary predation is single-species focused. We consider what the loss of a single animal means to the population. In the event of rare or endangered species, a single death can have strong influence over the species’ future. For less rare species, however, predation at some stable rate is natural and healthy when one considers the ecosystem. An unnatural assemblage of predators, however, may skew this effect in such a way as does not benefit habitat or ecosystem health in the long run. As we examined in our functional relationships, this may well mean fluctuations that result in long-term declines in mean prey animal abundance.
Is the limit of predation an effect on individual animals? Just as with our story of livestock predation, we must dig deeper still to uncover what predation means in terms of the system—plants and animals—that determine the health of the land of which we are stewards.
We can watch primary predation happen: see the kill occur, and watch the consumption of a prey animal. The act of primary predation, however, sends ripples through the prey community. These are the secondary effects, much more nebulous than the primary effect, but just as important—if not more important—in the end result of the influence of predators on prey.
Over time, science became more aware that prey animals learn, adapt and attempt to avoid predation—which can have a key role in the way that predation positively or negatively influences an ecosystem.
The chief example cited by conservationists is the unintended influence of the grey wolf reintroduction of Yellowstone National Park. Before wolves returned to Yellowstone, the elk population of the park was large, and in many ways, regulated only by landscape carrying capacity—in short, the vegetation suffered. Aspens were a thing of memory in many areas of the park.
After wolves were reintroduced, however, the oldest members of the herd were removed—primary predation in action—and the rest of the herd began to take notice of areas where they were more easily killed by these large predators. As a result, aspens in these areas began to recover, partially as a result of fewer mouths to feed in the park, but also a result of those mouths choosing to use habitat less favorable to wolf predation.
Similar stories have been told the world over, lending credibility to a theory of the “culture of fear,” that wildlife learn what areas to avoid in order to minimize the risk of death by predator. Although this may benefit ecosystems, the lingering question remains whether animals experiencing critical declines or during reintroduction, can afford for critical habitat, perhaps some of the last remaining habitat, excluded them by the presence of a more generalist, predatory species. This leads to the next question any wildlife manager should consider: how does one manage predation?
Habitat Management or Predator Management?
At first glance, I suspect the wildlife manager to cock an eyebrow at the section’s heading. Who would ask me to manage habitat instead of predators? Predator management evokes visions of trap lines, snares, and death. Habitat management brings to mind the gentle, nurturing of a gardener, even if that sometimes means weeding, thinning and burning to encourage life. For many, we have been taught that these are opposing concepts. I submit to you that they are complementary and should be treated as having different goals.
When considering that a population of prey animals might be declining, the first thought a wildlife manager ought to have is: “Do prey have adequate resources on my property, and can they access them?” When we imagine a piece of overused Texas Hill Country—complete with bare rocks and browse line—most managers would agree that a person needs to fix their habitat problem before thinking about losses to predators. In terms of our functional relationships, we are allowing carrying capacity to regulate prey numbers. This produces low-quality game due to chronic malnourishment, and is not the work of a good land steward. If habitat is well-managed for your target species, or better yet for a healthy ecosystem, and it is accessible, then we can discuss further predator management. Thus, predator management exists within the context of present habitat.
As every land manager knows, land isn’t land: there are inherent differences in productivity and present condition. The most veteran wildlife manager knows that good quality habitat is a process rather than a goal. There is always room for improvement, and as such, we must constantly ask ourselves, “How can I make the land under my care better?”
Predation management goals may change as habitat changes over time, for better or worse. If food is abundant and accessible, but predation seems to be a problem, we can consider two sets of factors that influence predation rates: those created by the habitat and those created by predators themselves.
In the first case, factors that increase predator access, such as perching sites for avian predators, or linear habitat, such as beaches, which allow fast, efficient searching by terrestrial predators, increase rates of predation. Lack of escape cover as well as highly concentrated, essential resources, such as limited water, increase the ability of a predator to efficiently capture prey with limited effort. Thus, we ask ourselves if our habitat management practices have generated a landscape more to the benefit of predators or prey.
In the realm of animals, both predators and prey have biological factors that influence rates of predation. For the predators, we ask basic question such as the number of predator species, the abundance of each and whether they maintain exclusive territories. In some cases, territories limit predation access, given that a particular individual or perhaps a pair actively defends their hunting grounds. The loss of a territory holder, however, may increase rates of predation.
Additionally, invasive predators, particularly exotic, invasive predators, such as wild pigs, tend to have a disproportionately high influence on rates of predation, as they are not part of the system as it evolved. Finally, if predators “bridge” to alternative prey during times of food scarcity, rates of predation may be higher still.
If one chooses still to engage in lethal removal of predators, given that the habitat under present conditions can support more prey animals without degrading habitat, then management must be undertaken to address specific concerns. Perhaps the manager is concerned about fawn survival in white-tailed deer. If relatively few fawns are noted after well-designed, properly conducted surveys, predators may be the cause of lower fawn survival. On the other hand, a manager should never engage in broad scale control without identifying a problem, the cause and navigating habitat-based solutions first.
The Way Forward: What Every Wildlife Manager Needs to Know about Managing Predators
Tradition taught wildlife managers that “the only good predator is a dead predator.” The reality that we have learned over time is that a predator is just one more creature in an ecosystem, and generally speaking, the force of predation is positive in terms of ecosystem maintenance. Under the influence of humans, however, and given human desire for consumptive use of native wildlife, reaching goals sometimes necessitates managing predation.
The manager’s first consideration is determining a “threshold of tolerance” for predation rates. As we know, left to their own devices, predators and prey naturally fluctuate in both abundance and density. Within the context of management goals, however, one may have limits to what can be tolerated. Naïve game managers may say that they wish no predation would occur, but the reality is that their hunters, then, would have to hunt at such times and intensities to mimic predation’s effects, not only on adults, but also on juveniles, the old and infirm, and other individuals. When necessary, most find this distasteful.
Still, for those whose goal is restoring a critically under-populated species, be it rare or endangered across its range or merely locally, the loss of even one individual may not be tolerable. This is what we call a restoration cycle.
Just like pronghorn in the Trans-Pecos region or Desert Bighorn Sheep, or in the early 20th century era of white-tailed deer restoration, there are times when a reintroduced population is not yet capable of sustaining natural rates of predation. This is a time when aggressive control of predation may be warranted, but no management regime is eternal: it must be progressively reevaluated to determine if the population is ready for natural predation. After all, it isn’t just numbers killed that predation brings us, but also the secondary effects that help shape ecosystems.
In order for a manager to do all these things, one cannot just “shoot from the hip.” As romantic as it may seem to make decisions with the heart alone, the art of wildlife management also requires the science of wildlife ecology. Many facets of predator ecology and predator-prey dynamics are given to us by science, so managers must also be scientists.
Meticulous record keeping including surveys, plant and animal, as well as weather data, grazing regimes and many other items, plays into the formation of wildlife management plans and protocols. Without this data in, we cannot know if our management is headed in the right direction. Knowledge of how the system functions should inform what managers see at their local scale, and help them to know when to enact management to curb predation, and when to let the process take its course.
When predation does occur, and we are fortunate enough to find the carcass, the identification of predation is essential. Particularly if the loss of any of the killed species is unacceptable. Management without data is just guessing, and so too is lethal removal of a predator without knowing the source of one’s losses.
Well-established forensic guidelines to determine the cause of a predation mortality exist, with the national standard published by Texas A&M AgriLife Extension Service for many years. Beyond trying to assess “whodunit,” many managers find that they have problems related to toxic plants, poor mothers or other, non-predation mortality sources. These, too, are data sources for savvy wildlife managers.
In today’s world, it isn’t enough to say anymore that predators need to be lethally managed. Aside from taking prey that are game species, many predators are valued as game animals in the U.S. and abroad. The difficulties of sustainably taking “trophy” predators such as mountain lions, grey wolves, brown bears and other such animals is that their numbers are usually small, but their impacts on ecosystems great. Today’s managers must consider how to sustainably harvest both sides of the equation to keep the ecosystems of the properties they steward in balance.