On Predator-Prey Relations
We have recently posted two engaging “popular” articles by Dr. Charles E. Kay concerning predator-prey relations (or relationships or interactions).
Dr. Kay (of Utah State University) is one of our premier wildlife ecologists and is the author/editor of Wilderness and Political Ecology: Aboriginal Influences and the Original State of Nature [here], author of Are Lightning Fires Unnatural? A Comparison of Aboriginal and Lightning Ignition Rates in the United States [here], co-author of Native American influences on the development of forest ecosystems [here], and numerous other scientific papers.
In Wolf Predation: More Bad News [here], Dr Kay discusses apparent or predator meditated competition, using wolves, moose, caribou, and deer as examples.
Predator meditated competition is a tricky concept. Most people are aware that predators can reduce a prey population, and that the predator population can then fall due to a lack of prey. As the predators decline, the prey population rebounds. Then the predator population rebounds, and the cycle begins anew.
But this model of predator-prey relations is overly simplified. In the real world, predators often have alternative choices besides one type of prey. If the alternative prey is sufficiently numerous, the predator populations do not always decline so much. The primary prey is thus still subject to predation, and it can be driven to extinction.
In effect, the various prey populations are in competition with each other, not for food but for predator avoidance.
In general, prey populations are never food-limited if there are predators around. For instance, caribou feed on tundra lichen, of which there is an abundance in Canada. But there is not an abundance of caribou, because wolves keep their population low, at something like 10% or less of the “carrying capacity” of the tundra. In turn, wolf populations have been held in check by the limited number of caribou (which is the fault of the wolves) or by human culling of wolves (not the wolves fault, but historically true).
But when other prey, such as moose and whitetail deer are present, wolf populations can persist despite limited numbers of caribou. In effect, moose and caribou (and deer) compete to avoid wolves, i.e. predator meditated competition.
The victors in that battle, in most cases, are the moose. Wolves selectively decimate caribou populations much more easily than moose populations. When both prey types are present, wolves will extirpate the caribou because the moose provide an alternative prey that maintain the wolves even when caribou are in very low numbers.
Historically and prehistorically moose were absent from most of western North America and eastern Canada, as well. Even in Alaska, moose were historically limited to a few, very remote areas. Since European settlement, however, moose numbers have exploded, as has the area occupied by those animals. There are more moose in North America today than at anytime in the last 12,000 years, except for the 1950’s-60’s when predator control was widespread and effective. Historically, caribou numbers were low and those animals so widely spaced that they could support only a few or no wolves. The addition of alternative prey, though, has allowed wolves to increase and the wolves then drive the more vulnerable caribou ever downward. That is to say, the addition of moose did not buffer, or reduce, predation pressure on caribou but instead increased predation on caribou, the exact opposite of what most people would predict.
But the situation is even more complicated than that, because bears and cougars are also often present, as are deer and rabid arctic foxes, and the predator-prey relations can get very complex. Throw into the mix human beings (for the last 12,500 years or more), and all kinds of extinctions and irruptions can take place (especially when we give our fellow predators special status).
When prey populations are held in check at 10% of the carrying capacity, and a new predator is introduced, the prey can disappear. Such was the case when human beings first arrived in the Western Hemisphere and over 40 species and 30 genera of large mammals rapidly went extinct (see Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America by Dr. Paul S. Martin [here]).
Not only did prey species go extinct, so did many predator species. Many carnivores could not survive when their prey disappeared. Human beings, however, are omnivores and we can survive on a variety of animal and plant foods. Human beings did not kill off saber-toothed tigers and dire wolves so much as we starved them out by slaughtering the mammoths, which were barely holding on due to chronic predation keeping their populations at a minimum.
One bottom line is that “habitat” is rarely a prime factor in animal population dynamics.
So we have islands that are poor caribou habitat, but which have no predators versus a nearby national park that is excellent caribou habitat but which contains wolves. Now according to what many biologists and pro-wolf advocates would have you believe, habitat is the all important factor in maintaining healthy ungulate populations, while predation can largely be ignored. Well, nothing could be further from the truth. Habitat it turns out, is irrelevant…
Despite the supposedly “poor” habitat in the Slate Islands, Bergerud and his research team recorded the highest densities of caribou ever found anywhere in North America. Moreover, those high densities have persisted since at least 1949 when the herd was first censused. More importantly, the density of caribou in the “poor” habitat, but predator-free, Slate Islands was 100 times that in Pukaskwa National Park where predators hold sway. 100 times or 10,000% more caribou per unit area. A significant difference by any objective standard.
Another bottom line is that specially protected predators (like wolves) can irrupt (rapidly expanding population numbers) and drive prey species to oblivion. If wolves have alternative prey, and even if they don’t, elk can be effectively eliminated. Which is what is happening today in Idaho, Montana, and other states where wolves have been re-introduced.
The implications for wildlife management are profound. Setting aside “habitat” for predators and prey is doomed to failure. Population management is the only means whereby species (predator and prey) can be saved from extinction. Please read the article for a more in-depth analysis of this truism.
The second essay (that we recently posted) by Dr. Kay is The Kaibab Deer Incident: Myths, Lies, and Scientific Fraud [here]. By the way, both of these articles were first published in Muley Crazy Magazine [here], the absolute best hunting publication in the West!! (by their own admission and we agree with them).
In The Kaibab Deer Incident: Myths, Lies, and Scientific Fraud, Dr. Kay relates the story of irruptions and near extirpations of the Kaibab deer herd. It’s a tale that includes Aldo Leopold, famously the author of Sand County Almanac, a classic of nature writing that is worshiped by many. Dr Kay writes:
The Kaibab was established as a Forest Reserve in 1893 and in 1906 was designated as the Grand Canyon National Game Preserve by President Theodore Roosevelt. Today, the southern end of the plateau is in Grand Canyon National Park, while the rest of the area is managed by the U.S. Forest Service. When the Kaibab was declared a game preserve in 1906, hunting was prohibited and the federal government began an extensive predator control program. Between 1907 and 1923, an average of 40 mountain lions, 176 coyotes, 7 bobcats, and 1 wolf were killed each year. In all, only 30 wolves were ever killed by government agents on the Kaibab. Instead, the main predators were mountain lions and coyotes. The Forest Service also reduced the number of livestock permitted to graze the plateau.
In response to those measures, the mule deer herd irrupted from around 4,000 animals in 1906 to an estimated 100,000 head in 1924. As might be expected, the growing deer population severely overgrazed both the summer and winter ranges. This lead to a number of studies and reports, as well as a dispute between the federal government and the state of Arizona. In short, the Forest Service said that the deer herd needed to be reduced to prevent further range damage but the state refused to open the area to hunting. In response, the federal government claimed that it could kill deer on the Kaibab to protect habitat without a state permit. Needless to say, Arizona objected and the ensuing legal battle made it all the way to the U.S. Supreme Court.
The Supreme Court agreed that the Kaibab deer herd had exceeded the range’s carrying capacity and that overgrazing by mule deer had denuded public lands. The Court also sided with the federal government in ruling that the Forest Service could authorize hunting on the Kaibab without state approval. This legal precedent still stands and means that when push come to shove, the federal government can control wildlife populations on public lands. Arizona had no alternative but to capitulate, but it was too late because the plateau’s mule deer had experienced a major die-off and by 1931 fewer than 20,000 animals were left.
This case is often cited as an example of “carrying capacity” overload, and why predators are important components of “natural balance”.
But there is no such thing as “natural balance” — real world factors are so much more complex than that simplistic model, and indeed real world factors have always impinged on the Kaibab Plateau. In particular, as Dr. Kay points out, anthropogenic predation has been the prime factor for millennia.
Humanity on the Kaibab during the Holocene included not only the Southern Paiutes but also Pueblo I through V cultures, Basket Maker I, II, and III cultures, and the Archaics (~8,000 BC) [here].
People have been the keystone predators worldwide for a very long time. The residents of the Kaibab Plateau were not only hunters, they were farmers who check-dammed the arroyo washes to grow corn. If a deer was stupid enough to venture anywhere near the adobe/brush huts, it was dinner. And the humans burned like crazy. The poor deer, what few there were, were herded by fire into kill zones.
It appears that occasionally the people even ate each other, although nobody likes to talk about it. Predators will be predators! Meat larger than lizards was probably a rare delicacy on the Kaibab.
Whatever wildlife extirpations and irruptions have occurred in the last century or so, they are the direct result of the elimination of the keystone predators, human beings.
As I noted above, predator-prey relations are complex. Simple models do not suffice to explain real world population dynamics.
Dr. Kay explains all this better than I do. Please read his articles (and books), which are cutting edge and paradigm-challenging, and which demonstrate the complexities of the wildlife management issues that we face today.
