All biologists worth their salt know that each and every form of life has the capacity to multiply and increase at a truly astonishing, indeed a frightening rate. It is easy to do calculations demonstrating the truth of this. For example, assuming (in all cases) that all descendants survive, one bacterium dividing every 20 minutes would produce approximately 300 grams of bacteria in 24 hours; 150 million tonnes in a month. A female housefly, laying a minimum of 600 eggs in her lifetime, would, at the end of a summer of some eight to 10 generations, have 1.9 x 1020 descendants - or roughly 200 million cubic metres of fly. A female vole reaches sexual maturity in 28 days, has a gestation period of 21 days and produces six to eight young in each litter. In a year she would have a million descendants. By way of contrast, female elephants do not mature sexually until they are 30 years old, have a gestation period of 21 months, and produce an average of only six young in their lifetime. Yet in 750 years one female would have 19 million descendants.
Clearly none of these things happens or the world would be swamped by any one of these creatures. However, sometimes such rates of increase are achieved for brief periods. Then the explosive growth of numbers in a very short time is truly spectacular. Think of plagues of locusts.
When I was an undergraduate I was taught that animals did not increase like this because every species has natural enemies which quickly kill most of the 'surplus' individuals. This is something that seems intuitively obvious -we can easily observe this predation happening all around us in nature. So, on those rare occasions when some animal does reach plague proportions, the assumption is that this must be because something has prevented its natural enemies from regulating its numbers. From this it follows that the way to control populations of pests introduced from another country - be they plant or animal - is to import their natural enemies which had not come with them. When I was a young Forest Entomologist my job centred around these beliefs. It was not until I was confronted with an outbreak of native New Zealand caterpillars defoliating introduced North American pine trees, that I started to doubt this received wisdom. Here was an animal, usually in such low numbers that it is hard to find, either on its natural or adopted hosts, and with a full suite of natural enemies attacking it, suddenly becoming so abundant on these introduced plants that it was destroying them. But it was not doing so on all of them, even when they were in quite close proximity to each other: nor on any of its native food plants. There must be some other explanation. And so there is. However, it took me many years of study and research before I understood what it is.
Perhaps, not surprisingly, it is a very simple explanation. But it is one that is not at all apparent, even to the quite careful observer. The real reason animals do not increase and swamp their environment is because they cannot obtain enough of the sort of food they must have to reproduce and grow. Without this, females can produce few young, and most that are born quickly starve. It is only when, briefly, and for a variety of reasons, there is an increase in the availability of such food that more animals survive. Then, if this increase of their food is large and sustained, we observe plagues and outbreaks.
This book explains how all this comes about in nature and describes some of the many 'ingenious' ways in which animals have evolved to cope with this usually chronic shortage of an essential resource.
If you are like many people - and especially if you watch 'tooth and claw' natural history documentaries on television - you will doubt me. So, too, do many professional biologists. But not, interestingly, those scientists whose work is connected with the nutrition and growth of laboratory and farm animals and birds. Nor do farmers who raise animals for a living. Frequently the response of such people is 'So, what's new?' But I hope that if you are a doubter, you will read what I have to say rather than dismiss it without considering the evidence. Then, perhaps, you may be sufficiently motivated to start looking more closely for yourself at what really goes on in this wonderful, if harsh and pitiless world of ours.
Above and beyond all this, however, I think you will be - as I have always been - fascinated and captivated by the many marvellous ways in which animals have evolved to survive in this inadequate world.
T.C.R. White School of Agriculture and Wine Waite Agricultural Research Institute The University of Adelaide March 2005
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