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 of these animals when they are born. These flies are numerous and their increase must be checked, probably by birds. So if certain insectivorous birds – whose numbers are probably regulated by hawks and other predators – were to increase in Paraguay, the flies would dwindle, the cattle and horses would become feral, the vegetation would change (as I have observed in some parts of South America), the insects and thus the insectivorous birds (as in Staffordshire) would be affected, and so on, in ever greater circles of complexity. This series begins and ends with insectivorous birds – not that natural relationships are ever so simple. Battle within battle must be perpetually recurring, with varying success, yet in the long run, forces are balanced and the face of Nature remains uniform over long periods of time, although the slightest change would give victory to one organism over another. Our ignorance is so profound and our presumptions are so high that we marvel at the extinction of a species; failing to see the cause, we invoke cataclysms to desolate the world and invent laws to govern the duration of forms of life!
I will give one more example showing how plants and animals widely separated on the scale of nature are bound together by a web of complex relationships.1 The exotic and peculiarly structured Lobelia fulgens is never visited by insects in this part of England and therefore never sets seed. (Many orchids absolutely require moths for pollination. Comparably, bumblebees are indispensable for the pollination of heartsease, because other bees do not visit this flower.) My experiments show that if bees are not essential to the pollination of clovers, they at least help significantly. But the common red clover is visited only by bumblebees, because no other bees can reach the nectar. So I have little doubt that if the bumblebee became very rare or extinct in England, the heartsease and red clover would follow. The number of bumblebees in a region depends greatly on the number of field mice, which destroy their combs and nests. Mr. H. Newman has long studied bumblebees and believes that “more than two-thirds of them are thus destroyed all over England.” Now, everyone knows that the number of mice depends mostly on the number of cats: Mr. Newman says, “Near villages and small towns I have found the nests of bumblebees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.” Therefore, the presence of many cats in a region may determine – through mice and then through bees – the frequency of certain flowers!
Every species faces multiple checks acting at different periods of life and different seasons or years. One or a few of these are generally the most potent, but all conspire in determining the average number or even existence of a species. In some cases widely disparate checks act on the same species in different regions. Looking at the plants and bushes on an entangled bank, we are tempted to attribute the kinds of plants and their proportional numbers to “chance,” but that would be wrong! When an American forest is cut down, a very different vegetation springs up, but the trees now growing on ancient Indian mounds in the southern United States are as diverse as in the surrounding virgin forests. A momentous struggle must have ensued for long centuries between various types of trees, each annually scattering seeds by the thousands. What war must have gone on between insect and insect, between insects, snails, and other animals with birds and beasts of prey, all striving to increase and all feeding on one another, trees, seeds, seedlings, or the other plants that first covered the ground and checked the trees’ growth! Throw up a handful of feathers and they all fall to the ground according to definite laws, and yet that is a very simple problem when compared to the interactions that determined, over centuries, the kinds and proportional numbers of trees now growing on the old Indian ruins!
Organisms that depend on each other, as a parasite depends on its prey, are generally far apart on the scale of nature. This is particularly common with those directly engaged with each other in a struggle for existence, such as locusts and grass-feeding ruminants. But the struggle is usually the most severe between members of the same species, because they inhabit the same regions, require the same food, and are exposed to the same dangers. The struggle is usually just as severe between varieties of the same species, and sometimes the contest ends quickly. If several varieties of wheat are sown together and the resulting seed is mixed and resown, the most fertile varieties or those best suited to the soil or climate will grow better, yield more seed, and in a few years supplant the others. To keep up a mixed stock of