THE PREDATOR AND ITS PREY

New Zealand Listener, Volume 35, Issue 901, 9 November 1956, Page 30

THE PREDATOR AND ITS PREY

By

L. R.

RICHARDSON

Professor of Zoolagy, Victoria University College — -- Se

SKED whether, if he were an animal, he would prefer to be a predator or belong to a species preyed on by others, the average person would elect to be a predator and would pick a hard and hungry life. The picture most of us have of the predator enjoying a comfortable life feeding at choice on the animals on which it preys is one which we are learning is far from the rule. It may apply to some few predators, but not to all. There is still as much opportunity for fresh research into this relationship between various animals, as in any other major aspect of zoology; and the promise of results which can profoundly change our understanding of some of the very old but persistent etroneous ideas of life under natural conditions. — ; For instance, there is the commonly held belief in a "balance of nature,"

that the number of the individual animals in a species is controlled ordinarily to a certain level around which the numbers fluctuate only slightly. We are accustomed to think that animals continue to reproduce abundantly and that predators eat the surplus, or if there are no predators, that the number will increase until food-shortage develops and starvation kills out the surplus to restore the "balance." We need to distinguish between major predators which feed on large prey, and minor predators which feed, for example, on insects, worms and other tiny

but abundant animals, Any predatorprey principle should apply equally to predators of ‘both gyoups. If food-short-tage is important in keeping a balance in natute, then we would reasonably expect to find that where major predators are absert the health of the individuals of a minor-preglator species will be poorer than if the numbers were held down by major predators to a level where the food-supply was fully adequate. There are many places where major predators are lacking, or insignificant in numbers. Islands provide us with excellent examples of this, and when we @amine such islands we fail to find any picture of overall illhealth, stunting in size, or other indicatiofis of consequences from food shortage in the species of larger size. We are led, then, to a peculiar conclusion. On an island, like many around our coast, where the larger species such as birds or tuatara represent the endanimals of a food-chain and are not | themselves subject to control by a major predator, we fail to find evidence of illhealth from malnutrition. We must conclude that the control over number is established befere malnutrition takes place. In other words, in the absence of : major predator, the "balance of nature" is brought about before a sig"nificant strain is placed on the food‘supply. If this were not thé case, then there would be violent cyclical flunctuations in numbers, far more so than is ‘found in such circumstances, _ The excellent work being done by K. -R. Allen and his associates in New Zea- | land on the food of trout can be taken as an example of a study of a minor predator-prey relationship and correlates well with much work in the United ‘States, especially that of the pioneer -and later studies on the quail, which demonstrate among other matters a case of a major predator-prey relationship. The two examples tegether give a littleknown picture of this relationship. Mr. Allen has given ample evidence that the abundant small aquatic animal life of a trout-stream is available as | food for a trout only under very definite conditions, The trout is carnivorous, It feeds to a small extent on insects which Ea by chance into the water; but the of the food is insects and other ‘mail animals living in the water. The trout can eat only the prey which it can see and capture. However, many of these small animals are, from the trout’s viewpoint, secretive in habit, living under stones or otherwise concealed. Of these, the trout can eat only the: relatively few individuals which for one reason or another leave their cover and can be seen and caught. In fact, just as McAtee found when he studied the insects eaten by birds in North America, the different kinds of insects are not eaten according to their relative abundance, but according to their availability. The most numerous do not necessarily form the greatest part of the food of a bird, or of a trout. The same thing was beautifully demonstrated with bob-white quail by Errington and his associates at Iowa where, through much careful field study, it was found that the number of quail in a given area was stable and related to the amount of suitable cover, in this case shrubs, thickets, brush and the

like, where the quail found adequate food, shelter and the satisfaction of all its needs, just as do the aquatic insects under stones. The quail seldom leave their cover. Under ordinary conditions predators such as fox, cats, hawks, and so on, capture only the exceptional individual quail which, like the insect, becomes accidentally "exposed." This and other evidence shows that in order to keep eating, some major predators may have to patrol ten to twenty miles to get food for a day, and even then often sleep on an empty stomach. Many other examples can be given to show, as do the above, that major and minor predators must lead a daily life of steady expenditure of energy if they are to feed. The habits of the prey give the highest measure of protection against a predator, and the bulk of a prey species leads a life free from the danger of predation. In the light of this knowledge, it is most difficult to believe that predation under normal circumstances has any significant control over the balance of nature, that is the balance of numbers under natural conditions. Elton in England by analysis of the fur returns of the Hudson’s Bay Company has shown cyclical fluctuations in the numbers taken of pelts of different fur species and that, for example, as the return of rabbit skins risés over a period of years, this is paralleled by a rise in the number of fox and other predator pelts. Under the old idea of predator-prey relationship, this was interpreted as a simple cause and effect-more rabbits to feed more foxes; but it is becoming

clear that the increase in the numbers of the various species is not based on a simple foodchain effect. It is not a sequence of events but a single event happening equally with each species brought about by a common and still unknown factor operating at the same time on all the species concerned. A new attitude to, and understanding of, the preda-tor-prey relationship is valuable to us in New Zealand. The disappearance of many native animals from some areas, and the reduction in numbers in others, is often blamed on introduced predators; but if predation is to continue to that extent, then the predator must be able to invade and destroy the prey inside the protection of its cover and its habits. This would be a far less likely cause of local or general extinction than if at the same time or independently human activities destroy the cover and force the prey to either change its habits, or die. Few species, in North America the woodchuck is a famous example, have changed their habits to the extent that they can survive deprived of their original natural cover, On the other hand, if the contest is to the death, the pre-

dator will come to starvation icng before the prey is -wiped out, a situation which is the same whether the predator preys on one species or many, for. as the number of prey diminish, the predator must expend an increasing amount of energy to catch its daily food until the expenditure of energy exceeds the. value of the food taken. The predator starves to death, but there will still be,some of the prey left to breed, and the prey will survive. We commonly regard many of our pest species as reaching pest proportions because here they are freed from predator and other forms of biological control. Few stop to think that the possum and wallaby in Australia were essentially without’ a primary predator control. Arboreal in habit, the possum was free from major control by the dingo, or from the eagle and other predatory birds which, in many cases, are really carrion eaters. It is more than hard to pick any Australian animal which could have exerted control in the old sense over the possum. The wallaby was, like the bob-white quail, protected by its habits and its cover, in this case a country sufficiently open that the wallaby could detect a predator and escape before being seized. Now that we can devalue the status of the predator and of the food supply as major forms of biological control under ordinary conditions, we must look to other factors for the answer to our pest problems. From field experience we can be confident that disease is not an ordinary primary controlling factor. We can be equally confident that there is a primary controlling factor which can be determined only when we bring to bear on our problems a new enquiry unfettered by traditional thought.