Inefficient killers

Most people believe that predators are efficient and all-powerful killing machines that keep the numbers of their prey well below levels that would be possible without their depredations. But most people are wrong! As is often the case in nature, what seems superficially to be so is not when we look more carefully. Predators are generally inefficient.

The food of carnivores, unlike that of herbivores, is a concentrated source of protein. It exists in discrete 'packages', the bodies of other animals. If there are enough of these available to be caught there is enough good food to support reproduction and growth. Unfortunately, nearly all these packages are very mobile, and thinly and patchily spread in a variable environment. They may not be all that easy to find. Furthermore, even when they are quite plentiful and visible, prey animals are equipped with a range of sophisticated behaviours for avoiding becoming the victim of a predator. So, what seem superficially to be lots of easily caught animals are actually mostly difficult or impossible to catch.

All this means that predators are confronted with an environment every bit as harsh and inadequate as are herbivores. They are chronically short of food. As is the case with herbivores, however, this shortage does not normally impinge on adult animals. Most carnivores in their prime can catch enough prey to sustain themselves. The problem arises with females trying to reproduce, and with their young when they are undergoing their rapid early growth. At these times a lack of protein becomes critical. So much so that females commonly fail to breed, and most young that are produced soon starve. Precious few of their kind ever reach their prime.

You don't believe me? Let me give you some examples.

I have watched tiny, freshly-hatched New Zealand katipo spiders trying to eat aphids that I fed to them, being repelled and distressed by the aphids' caustic secretions sticking to their mouthparts and legs. I have seen how frequently an insect hitting an orbweb spider's web breaks free before the spider can get to it. On the other hand I have seen spider-hunting wasps caught by the very spiders they attacked. I have watched small parasitic wasps, which must sting and stun their prey before laying an egg in it, flung away by the potential victim's wild thrashing, or repelled by its secretions or vomit. Large caterpillars and adult aphids are virtually immune from attack because of these tactics as these wasps can successfully do battle with only the very small early stages of their prey. But even then they are far from efficient in their behaviour. When a female wasp encounters potential prey she will frequently go into a frenzy, thrusting repeatedly but aimlessly in all directions with her sting, more often missing than hitting her intended victims. Observing larger parasitic wasps that seek out newly pupated caterpillars in leaf litter on the forest floor, I found them to be trebly handicapped. First they search randomly with their feelers among the litter, and frequently pass within a hair's breadth of a potential victim without detecting it. Then, when they do find one, they are commonly flung away by its thrashings and never locate it again. Finally, even when they do manage to stay with one long enough to try to sting it, their sting cannot penetrate the thick tough integument of the chrysalis. To be successful, a wasp must find a chrysalis that has just transformed from the caterpillar and when its skin is still soft enough for the sting to penetrate.

These examples, however, are only some of my own casual and unquanti-fied observations - 'mere anecdotal evidence' my critics would call them. So I will now relate evidence that I have taken from properly conducted and published studies.

Was this article helpful?

0 0

Post a comment