The Necessity of predators: Part 3

The Necessity of predators: Part 3

By: Jonathon Sprague Woolf  Date: 06/30/1998 Category: | Wildlife Journal |

The previous sections have shown that predators are necessary both to a prey species and to the ecosystem in which the prey species lives. This is not "rocket science," as the saying goes. It isn't even a very new idea. The first great wave of American conservationists, men like Aldo Leopold and John Muir, understood the necessity of predators 50 and 100 years ago. They didn't have advanced mathematical models, or computer analysis, or the benefit of a century of experience with what happens after predators are exterminated. But they knew. They understood that predators are essential to an ecosystem, especially large predators that kill and eat large prey animals. A century of experience has only served to confirm what they tried to tell us. Predators are necessary to an ecosystem. If we don't have natural animal predators, then we have to simulate the effects of predation.

Now, how can we do that? How can we simulate the effects of predators on prey animals? Well, predators control prey numbers. How can we do that? We can try to control their reproduction somehow, or we can kill the surplus. There really aren't any other options.

In the Great Game of Nature the object is to perpetuate your genes. Simply surviving is not enough; the ultimate goal is to survive and reproduce. If an individual has great reproductive success, its genes will eventually be spread far and wide through the species gene pool. This means that any genetic variant that leads to greater reproductive success will be favored. In insects, this means that resistance to insecticides can rapidly spread through a population. In bacteria, it means that resistance to antibiotics can spread rapidly. In large mammals like deer and elk . . . what does it mean in them? Very simply, it means that any method of controlling population that depends on controlling how the animals reproduce is doomed to inevitable failure.

Animal rights activists claim that when food is scarce, female whitetail deer alter their reproductive cycle in a way that reduces their reproductive success, aborting or resorbing embryos and producing single fawns instead of twins. A few studies exist that tend to support this idea: when a deer population is over-stressed by lack of food, the number of pregnant does and the average number of fawns per pregnant doe both drop sharply. Interpretation of these studies is difficult, since some essential data is missing from the sources that I have. Nothing is said, for example, about which does are getting pregnant and which ones aren't.

If some specific does continue to produce twins every year while others produce single fawns or no fawns at all, then the difference between the ones that reproduce and the ones that don't is a genetic, heritable change. Such a change becomes a variant in the population which is subject to natural selection. If only the does with certain genes survive and reproduce, then after a few generations, most of the does in the population will carry the high-reproductive-rate gene; the reproductive success rate will go back up; and the herd will begin moving again toward a ruined habitat and a disastrous population crash. Population biologists have studied such situations many times, and it's startling how quickly such a successful gene can spread through a population. All other factors being equal, in this scenario the does with the lower reproductive rate are at an evolutionary disadvantage, and will disappear rapidly from the population. It doesn't matter that in the long run, the lowered reproductive rate might better ensure the species's survival. Evolution acts to maximize short-term reproductive success, not long-term survival.

On the other hand, if the reproductive success and frequency of twins drop all across the population, we have a different situation on our hands. When reproductive success drops below the replacement rate, it results in an over-all aging of the population, as fewer young animals come into the herd. Aging does keep eating their share of the available food, but have lower reproductive success rates. It's easy to see how this can result in a population with too few reproducing does, in which case a population crash of a different sort occurs as the reproductive rate drops drastically. It could even drop below the "viability level," the minimal population required to maintain a healthy, diverse gene pool. A population with a small gene pool becomes inbred, a ripe target for disease.

Both models, in other words, lead to disaster. Both eventually run afoul of the inevitable laws of evolution and ecology. This proposed "solution" by animal rights activists will not work, will never work, because it runs counter to the basic rules of Nature.

Some animal rights activists say we could put deer on birth control: somehow provide them with chemical contraceptives so that fewer deer reproduce each year. There are a great many logistical and practical problems with this idea, any one of which is enough to wreck it. But even if all those problems could be solved, it still wouldn't work.

Suppose you find a way to dose a herd of deer with contraceptives. There are three possible outcomes for any such program:

  1. It will achieve 100 percent coverage with 100 percent effectiveness. This is clearly undesirable by any standard, because it will result in extinction of the herd in a few years.
  2. It will achieve 100 percent coverage, but will not be 100 percent effective because some deer are genetically resistant. So, some deer will reproduce even though they got the contraceptive. These deer will pass along their resistance to their offspring, and those to theirs. That resistance then becomes an evolutionary advantage. If only resistant deer reproduce, then very quickly all the deer in the herd will be resistant.
  3. It will not achieve 100 percent coverage of the herd. That is, some deer will not get the contraceptive because for some reason they don't encounter it. Maybe they avoid the areas where the contraceptive is introduced into the herd - salt licks, perhaps. If it's applied by dart, then deer that are wary and difficult to shoot will escape being dosed. In any case, only those deer that avoid the contraceptive will reproduce, and eventually the entire herd will tend to avoid whatever method was developed for applying the contraceptive. If the contraceptive is not 100 percent effective in dosed deer, then the resistant ones will also pass on their resistance as in outcome number two. Eventually, most or all of the herd will have one or both defense mechanisms against the contraceptive.


How do we know this is what will happen? Because basically, a chemical contraceptive applies the same sort of pressure on a population that a poison like an insecticide does. Insects use a variety of methods, including avoidance and resistance, to defeat insecticides. The same evolutionary forces are at work in deer and other large animals faced with chemical attack, and the same types of countermeasures will be effective for them. We can't defeat bacteria with antibiotics, or insects with insecticides. Why should we think we can control deer with contraceptives, when all the evidence says we can't? Chemical contraceptives won't work, can't work, because the evolutionary process will act to nullify them as fast as we can introduce them.

Now, why can we expect predation in the form of hunting to work, if these other methods don't? Because there is a difference between how animals counter predation and how they counter the types of biological attacks that animal rights activists favor. Biological methods are countered by genetic patterns and/or learned behavior. Predators are countered by genes, behavior, and a sizable dose of luck. In any population controlled by predators, such as African antelope, the average prey animal is just as strong/fast/agile as it needs to be to stand an even chance of escaping a predator's attack, and the average predator is just strong/fast/ agile enough to stand an even chance of catching an average prey animal. No chase has a certain outcome - the prey animal needs luck to escape, and the predator needs luck to make a kill. Genes can be inherited, and behavior can be learned, but luck is entirely random. A single mis-step can cost a gazelle its life when a cheetah is hot on its trail. A zebra foal that stops beneath the wrong tree at the wrong time can find its life ended very suddenly as a leopard drops from above to kill it. A deer running from a pair of wolves may take a wrong turn and find itself faced with a dead end. These are all cases of pure, random luck. Evolution cannot adapt animals to a random phenomenon. So the animals will always be vulnerable to predation. And predation is the only consistently reliable method for controlling their numbers.



Nature herself tells us that the best way to control a population of animals is the way that she developed 550 million years ago: predators. If we need any additional proof, we have it in the boom-and-bust population cycles of whitetail deer, elk, and mule deer. These animals need predators. They evolved side by side with predators; they're adapted to the threat of predators. We've wiped out the animal predators over most of North America, so we have take it on ourselves to do the job they've been doing from time immemorial. Human predation - hunting - is necessary for the health both of the species and of the ecosystem in which the species lives. No other solution is likely to work, because the evolutionary process will nullify such solutions as fast as we try them. Hunting - systematic, carefully controlled, scientifically managed hunting, in the same ways that predators would hunt - is the only evolutionarily sound long-term method for controlling the populations of whitetail deer, mule deer, elk, and other large herbivorous animals.


References and further reading:

  • Beak of the Finch, The -- Jonathan Weiner, © 1994, Vintage Books
  • Nature's Keepers -- Budiansky, Stephen. © 1996, Vintage Books
  • On Methuselah's Trail -- Ward, Peter Douglas, © 1992, W. H. Freeman & Company
  • "The Big Ones Are Back!" National Wildlife, Oct/Nov 1978
  • Playing God in Yellowstone - Alston Chase. © 1986, 1987 Harcourt Brace & Company .


About The Author

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Jonathon Sprague Woolf -

Member/Volunteer/Partner/Article Writer of the National Animal Interest Alliance.

All Authors Of This Article: | Jonathon Sprague Woolf |
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