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 is more common to meet with stunted forms due to the directly detrimental action of climate. In Arctic regions, snow-capped mountains, or absolute deserts, the struggle for life is almost exclusively with the elements.
Thus, the action of climate is mainly indirect by favoring other species. For example, there are prodigious numbers of garden plants that can perfectly well endure our climate but never become naturalized, because they cannot compete with native plants or evade destruction by native animals.
Epidemics tend to ensue when a species increases inordinately within a small area due to very favorable circumstances. (At least this seems to be generally true for game animals.) This is a limiting check independent of the struggle for life. But even some of these so-called epidemics appear to be caused by parasitic worms that have been disproportionately favored for some reason (possibly because they spread more easily among crowded animals); this produces a sort of struggle between the parasite and its host.
In many cases preservation of a species depends on the maintenance of a large number of individuals relative to its enemies. It is easy to raise corn, rapeseed, and other grains in a field, because the seeds far outnumber the birds that feed on them. The birds cannot increase proportionally to this superabundance of food in one season, because their numbers are checked by winter. But anyone who has tried knows the difficulty of getting seeds from a few wheat or other such plants in a garden; in my case I lost every single seed. The necessity for a large stock in some species explains some singular natural phenomena, such as the extreme abundance of otherwise rare plants in certain areas and the density of some “social” plants even at the extremes of their range. In such cases a plant will survive only where conditions are so favorable that many individuals can exist together and save one another from destruction. I’ll add, without elaboration for now, that the positive effects of frequent intercrossing and the negative effects of close inbreeding are probably relevant to some of these examples.
Many recorded cases show the unexpected and complex relationships among organisms that have to struggle together in one region. I will give a simple but interesting example. On the estate of a relative in Staffordshire, there is a large, barren, and untouched heath. Several hundred acres of it were enclosed and planted with Scotch fir twenty-five years earlier. In the planted part, the native vegetation changed remarkably, more so than is generally observed when passing from one soil to another. Not only did the proportional numbers of heath plants change, but twelve species (not counting grasses and carices) that were absent from the heath flourished on the plantation. The effect on insects must have been even greater, because the heath was frequented by two or three insectivorous bird species, but the plantation harbored six bird species not found on the heath. Here we see how introducing a single type of tree had potent effects; the only other interference was enclosure of the land to keep out cattle. Indeed, I observed the importance of enclosure near Farnham, in Surrey, where there are extensive heaths with a few clumps of Scotch fir on distant hilltops. In the last ten years large spaces have been enclosed, and self-sown firs are now springing up in multitudes, so densely that all cannot survive. When I ascertained that these young trees had not been sown or planted, I was surprised by their numbers; I went to several places where I could see hundreds of acres of unenclosed heath and saw literally no Scotch firs except for the old planted clumps. But on looking closely I found many seedlings and little trees that were perpetually browsed down by cattle. In one square yard, several hundred yards from one of the old clumps, I counted thirty-two little trees. Judging from growth rings, one of them had tried to raise its head above the shrubs of the heath for twenty-six years and failed. No wonder that as soon as the land was enclosed, it became thickly covered with firs. Yet the heath was so barren and extensive that no one would have suspected cattle of having searched it for food so effectively!
In this case cattle determined the existence of Scotch fir, but in some parts of the world insects determine the existence of cattle. Paraguay offers perhaps the most curious example. Here cattle, horses, and dogs have never run wild, and yet to the south and to the north they swarm in a feral state. Azara and Rengger have shown that this is caused by an abundance in Paraguay of a certain fly that lays its eggs in the navels
