A struggle for existence inevitably follows from the high rate at which all living things tend to multiply. Every reproducing organism must suffer destruction at some point; otherwise its numbers would swell geometrically to such inordinate proportions that no region could support them. Because more individuals are produced than can possibly survive, there must be a struggle for existence – either one individual with another of the same species, or with individuals of other species, or with the physical environment. This is the doctrine of Malthus applied to the whole organic world; for here there can be no artificial abundances of food or prudent restraint from mating. Although some species may now be increasing in numbers, they cannot all increase, because the world would not hold them.
Every species, without exception, increases at such a high rate that without checks the earth would become covered by the progeny of a single pair. Even slow-breeding humans have doubled in twenty-five years. At this rate, in a few thousand years there would literally be no standing room. Linnaeus calculated that if an annual plant produced only two seeds – and no plant is as unproductive as this – and their seedlings produced two seeds each, and so on, then in twenty years there would be a million plants. The elephant is thought to be the slowest breeder of all known animals, and I have tried to estimate its minimum rate of natural increase. Conservatively estimating that elephants breed from age thirty through age ninety, producing three pairs of young during the interval, then after five hundred years there would be fifteen million living elephants descended from the first pair.
But there is better evidence than just theoretical calculations: the numerous cases of animals in the wild increasing at astonishing rates when conditions were favorable for several consecutive seasons. Even more striking are cases of domestic animals that have run wild. Had they not been confirmed, rumors of slow-breeding cattle and horses in South America and Australia increasing at great rates would have been incredible. Similarly, there are examples of invasive plants becoming common across whole islands in less than ten years. Several of the plants covering entire square leagues of the plains of La Plata, almost to the exclusion of all other plants, were introduced from Europe. I hear from Dr. Falconer that several American plants introduced to India now range from Cape Comorin to the Himalaya – a change that has happened in less than four hundred years. The fertility of these plants and animals had not been suddenly and temporarily increased. The obvious explanation is that the environments were very favorable, so fewer old and young were destroyed and many of the young could reproduce. The geometric ratio of increase, which always produces surprising results, explains the extraordinarily rapid growth and spread of invasive organisms.
In the wild almost every plant produces seed and almost every animal pairs annually, which means that the potential for geometric growth is widespread – but held in check. Familiarity with large domestic animals tends, I think, to mislead us. We do not see them succumb to great destruction, and we forget that thousands are slaughtered each year for food. In the wild an equal number would somehow have to be destroyed.
The only difference between organisms that produce eggs or seed by the thousand and those that produce extremely few is that the less productive would require a few more years of favorable conditions to populate an entire region. The condor lays a couple of eggs and yet may be more numerous than the ostrich, which lays twenty. The Fulmar petrel lays only one egg but is believed to be the most numerous bird in the world. One fly deposits hundreds of eggs and another (like the louse fly) deposits only one, but this does not determine how many individuals of each are supported in a region. Producing many eggs is important to species that are dependent on fluctuating quantities of food, because it allows for quick increases in number; however, the real importance of laying many eggs is to make up for the destruction that happens at some, usually early, period of life. If an animal can protect its eggs or young, then average numbers can be maintained even if only a small number of offspring are produced. But if many eggs or young are destroyed, many have to be produced, otherwise the species would become extinct. If a tree lived, on average, for a thousand years, then a single seed every thousand years would sufficiently maintain its numbers – assuming that every single seed were to survive and germinate in an appropriate place. So the average number of any animal or plant depends only indirectly on the number of its eggs or seeds.
These considerations are essential when thinking about nature: every organism strives to increase its numbers, each individual struggles at some period of its life, and destruction inevitably falls on the young or the old during each generation or at recurrent intervals. Lighten any check, mitigate the destruction ever so little, and the number of individuals will quickly increase. The face of Nature is like a yielding surface with ten thousand sharp wedges packed close together and driven inward by incessant blows; sometimes one wedge is struck, and sometimes another with even greater force.
What moderates the natural tendency to increase is obscure. Consider the most vigorous species; by as much as it swarms in numbers, so much greater will be its tendency to increase. There isn’t a single case for which we know exactly what the checks are. This should not surprise anyone who recognizes how little we know about this topic even with respect to humans, so much better known than any other animal. Several authors have treated this subject, and in my future work I will discuss some of the checks at length, especially those influencing the feral animals of South America. Here I mention only the main points. Eggs or very young animals seem to suffer the most, but this is not always the case. Plants face a vast destruction of seeds, but based on some observations I have made, the seedlings suffer the most from germinating in ground already thickly covered by other plants. Seedlings are also destroyed in vast numbers by various enemies. I dug and cleared a three-by-two-foot piece of ground where there could be no choking by other plants and marked all the seedlings of native weeds as they came up. Out of 357 seedlings, 295 were destroyed, mostly by slugs and insects. If a piece of turf is mowed or browsed by ruminants over a long period of time and then allowed to grow uninhibited, the more vigorous plants gradually kill the less vigorous (although fully mature) plants. In this way, on a little three-by-four-foot plot of turf, nine species out of twenty perished because all were allowed to grow freely.
Of course, the amount of available food defines the upper limit to a species’ rate of increase, but frequently it is predation that determines its average number. The stocks of partridge, grouse, and hare on large estates depend chiefly on the destruction of vermin. If in England not a single game animal were shot for the next twenty years, and if at the same time no vermin were destroyed, there would probably be less game than now (even though hundreds of thousands of game animals are killed annually). However, in some cases there is no destruction by predators – as with the elephant and rhinoceros. Even the tiger in India rarely dares to attack a young elephant protected by its dam.
Climate plays an important part in determining the average number of a species; periodic extremes of cold or drought are the most effective checks of all. I estimate that the winter of 1854–1855 destroyed four-fifths of the birds on my own grounds. This is tremendous – a human epidemic that kills 10 percent is considered extraordinarily severe. At first sight the action of climate seems independent of the struggle for existence, but insofar as climate reduces food supplies, it can precipitate the most intense competition among individuals – be they of the same or different species – that subsist on the same kind of food. Even when climate acts directly – for example, through extreme cold – it is the least vigorous or those with the least food that suffer most. When we travel from south to north or from a humid region to a dry region, we invariably see some species gradually disappear. It may be tempting to attribute the whole effect to the conspicuously changing climate and its direct action, but this would be incorrect. Even where a species is most plentiful, it constantly suffers enormous destruction at some period of its life from predators or from competitors for space and food. If these enemies or competitors are favored by any slight changes in climate, they will increase. And because each area is already fully stocked by inhabitants, the other species will decrease. So when we travel southward and observe fewer and fewer individuals of some species, the cause involves both the species in question being hurt and others being favored. When we travel north, the effect is similar but less pronounced, because all kinds of species, and therefore competitors as well, decrease northward. When going north or ascending a mountain, it