I specifically discussed the inefficiency of predators in Chapter 6, but the rest of the book illustrates that herbivores are just as inefficient at 'controlling' their 'prey'. Predators are inefficient because their prey have evolved ways to mostly stop being caught - to become inaccessible. If this had not happened then the end point would have been extinction of both prey and predator. Similarly plants have evolved, not so much to be inaccessible, but to be nutritionally inadequate for their predators - the herbivores - thus reducing them to similar levels of inefficiency, but once more avoiding mutual extinction.

We sometimes see speeded-up versions of this co-evolution in action as a result of human interference with the natural world. An especially good example happened here in Australia. When the myxomatosis virus was introduced to attack the devastating hoards of feral rabbits it swept through their populations, via its mosquito vector, reducing their numbers by over 90 per cent. The two organisms had never before encountered each other in nature, so rabbits were a new and untapped food resource for the virus; the virus a new predator against which the rabbit had no defence. But in relatively few years a double phenomenon emerged. The few rabbits that survived did so because they had some resistance to the virus. They quickly bred to considerable numbers, only to be once more decimated by the disease - but leaving a slightly larger and even more resistant residual population. And this happened many times, on each occasion with fewer and fewer rabbits dying. Simultaneously, however, the virus attacking each generation consisted of a less virulent strain than before, one less likely to kill a rabbit before a mosquito could transmit it to a new host. The end point is that rabbits are little affected by myxomatosis any more, and we have had to introduce another virus - calicivirus - to start the process all over again.

Throughout this narrative I have stressed that it is the food that is available to individual animals in a population which is the crucial element deciding how many of them will live. Much of the potential food in a habitat is not used because it is quite inaccessible. Whether it is because it can run too fast for you to catch, or it is too dilute in your diet of plant sap, is of little consequence. In either case you will die for lack of food in the midst of apparent plenty. And this has come about because of the constant co-evolution of eater and eaten.

Another thing that prevents animals gaining access to all the potential food in their environment is evolution's 'one way arrow': once launched down a particular evolutionary path there is no turning back. The senescence-feeding lerp insects I have discussed before provide a good example of how this works. They will feed only on mature gum leaves even when, within easy reach on the same twig, there are young expanding leaves containing much higher concentrations of amino acids than the mature leaves. Even when I confined them experimentally to new leaves they would not feed or lay their eggs on them, and newly hatched nymphs placed on new leaves wander until they die rather than feed. These insects are descended from ancestors that happened (for reasons we can never know) to feed on old leaves and survive. Now they are locked into that way of life by their inherited physiology and behaviour. They will respond only to specific cues from mature leaves, so new leaves might just as well be on another planet; they are not part of their world, and the food in them, no matter how good, is not available to them. In the same way cabbage white butterfly caterpillars will die without feeding if confined to a plant that is not a brassica, even though it might contain more food than the brassica.

So, to say a population is limited by its food does not infer that it eats all the food that is there. There is often lots left, but they cannot get at it. That there is this reservoir of unused food in many habitats is dramatically demonstrated from time to time when human activity results in an animal getting into a part of the world where it had never been before. There are many examples, especially in this part of the world - rabbits, wasps, millipedes, possums, pigs, goats, deer; to name but a few. Typically the numbers of such new introductions explode at the point of introduction and the population rapidly spreads. Conventional wisdom says this is because they no longer have their natural enemies to control their numbers. But I hope you have learnt enough from this book to realise that this is unlikely to be the answer. Far more probable is that the population has found a source of unexploited food that was not accessible to the native animals. A phenomenon repeatedly observed with such new introductions indicates this might be the case. It is the 'doughnut effect'. As a new population becomes established it increases to very high numbers before starting to spread. Then, as it spreads, its numbers at the point of origin drop markedly while at the 'wave front' moving out in all directions from the source, they remain large. They exhaust and overrun the hitherto unused supply of food as they spread. Another theory is that the invader actively out-competes and excludes whatever native animal was using the resource.

Mostly such conclusions are based on anecdote or untested assumptions. And in some cases they may well be true. However, the few studies that have looked carefully for this result have found it not to be the case. One such was done here in Adelaide. The recently arrived European wasp and the native Australian paper wasp both hunt insect prey which they feed to their growing young in their paper nests. It was feared the much more abundant and aggressive newcomer would quickly oust the native wasp by aggressively beating it for its food. However, it has not done so and monitoring the day-to-day activities of the two species demonstrated why. They hunt in different places within the same habitat, at different times and over a different range of ambient temperature. The differences in time and temperature relate to differences in the climate of their native habitats. But the difference in their hunting relates to their exploiting different food. The native wasps hunt exclusively in the foliage of plants for caterpillars. The invaders, on the other hand, are voracious omnivores (and scavengers - as anyone trying to have a garden barbecue with these wasps around will attest). They prey on a wide range of insects in many locations, but mostly catch adult flies. Caterpillars, however, make up less than 8 per cent of their diet, and it is probable that the ones they do take belong to different species from those the paper wasps catch.

A similar story, but with a much longer history, emerged when the European white butterfly was introduced into the eastern United States in the 19th century. It spread rapidly and reports from naturalists and casual observers indicated it was ousting a closely related native American white butterfly which was becoming increasingly rare and was thought would soon be exterminated all together. Yet 100 years later both species were abundant and living in the same locations. Close study of their respective life cycles revealed, however, that each species lives in a different world; their habitats are different. They eat different species of plants, and even when adult butterflies of both are flying and mating in the same field, they completely ignore each other. It seems the great success of the invader and the local extinctions of the native came about because major changes in land use with early European farming proliferated the preferred plants of the European butterfly while destroying much of those required by the native one.

Where then, at the end of this book, does this leave us? I would hope understanding why the world stays green; that it does so because all animals are pretty much struggling to survive. They live in

Figure A.1 Polistes wasp: Fears that these native Australian paper wasps would be supplanted when the introduced European wasp ate most of their prey proved unfounded. The two species have less than 8 per cent of their prey in common. Photo courtesy of Kym Perry.

a world where they are usually hard pressed to find enough food to persist, in spite of the myriad ways they have evolved to overcome this shortage. This is not because there isn't enough food in the world, but because much of it is unavailable, is spread too thinly, or is too hard to catch. From a consideration of all this it should not be difficult to appreciate that the idea of an efficient and 'balanced' nature is just a myth. Animals do not make optimal use of their habitat, but rather have evolved ways of maximising their access to what resources are present. Depending on the quirks of natural selection, this may or may not be the 'best' outcome as seen from our human standpoint. By and large nature is just coping - it gets by with what works at the time.

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