Sunday, September 29, 2002		Nature

Eliminating predators can harm prey
Nutan Shukla

HOW does predation affect the population of prey? Naturally, too much predation tends towards the extermination of the prey, and no doubt this happens to some species when the balance of their relationship with their predator is seriously upset. Often, the introduction of a new predator into the set-up can endanger the survival of a species. But in natural and undisturbed circumstances, the activities of predators are necessary for the preservation, prosperity and the healthy growth of the prey species as a whole.

One reason for this is that the very act of predation tends to select the least hardy animals as prey. The old and week animals, those unable to run as fast as others, and those rejected by the herd etc, are likely to fall prey to the predators first. Even in cases where this type of mechanism can only operate less directly, predation still has a beneficial effect in maintaining prey population within acceptable overall limits. Where a predator is removed altogether, catastrophic effects can result.

Here is an example. The Americans decided to save the deer herd on the Kaibab Plateau in the state of Arizona. The deer bore great losses from predators, like pumas and wolves, and people began to destroy these predators. Before long, there was not a single puma or wolf left in the locality.

This seemed to be an excellent cause for rejoicing. Indeed, the deer herd began to grow quickly. But the joy proved to be premature; several years passed, and the deer began to fall ill and die en masse. Soon there were less of them left than there used to be before the wolves and the pumas were destroyed. In another few years, the deer on the plateau were threatened with complete extinction.

Later, it was concluded that the reason for this phenomenon was the absence of predators. Those who favoured killing predators to boost the deer population, perhaps, had no knowledge about the consequences of the drastic measures they were taking. Otherwise, they’d have realised that the increased numbers of the deer would devastate the plant life and, having destroyed it, would start to die of hunger.

Starvation, however, was not the only result of the total destruction of predators: the deer began to fall ill, and diseases spread fast among them. The people who had destroyed the predators did not know that they prevent epidemics. They not only eat dead animals, which are a source of infection, but they also destroy weak and sick animals, carriers of disease, and thus cut short its spread.

Coming back to introduction of predators, it often results in dangerous consequences. One species of ladybirds introduced into America to control cotton pests itself became a serious menace in the absence of its natural predator. Lack of the customary predators was probably one of the main reasons for the enormous and rapid spread of the rabbit after its introduction into Australia. The animal’s formidable breeding system was able to operate without hindrance and within a very few years the whole continent was covered with millions of rabbits, an almost uncontrollable pest.

Despite the benefits to the species and to the general balance of nature which predation provides, many animals go to great lengths to avoid falling victims to predators. Among insects many bizarre and often attractive forms have resulted from attempts at protective coloration. Camouflage, like that of the stick insects, provides one obvious type of protection, and hundreds of examples from among the insect community can be found. But less well-known are the insects which have developed warning colours to mimic some distasteful or noxious insect which a predator would be likely to avoid.

Many predators change their prey if they are forced by hunger. Foxes, which usually feed on small mammals, will eat berries and other plant foods if they are faced with starvation. In fact, most predators have fairly catholic tastes, because their food supply is less assured than that of their prey. An over-finnicky predator will not be able to survive long.