Every species has its niche, its place in the grand scheme of things. Consider a wolf-spider as it hunts through the litter of leaves on the woodland floor. It must be a splendid hunter; that goes without saying for otherwise, its line would long since have died out. But it must be proficient at other things too. Even as it hunts, it must keep some of its eight eyes on the look-out for the things that hunt it; and when it sees an enemy it must do the right thing to save itself. It must know what to do when it rains. It must have a lifestyle that enables it to survive the winter.
It must rest safely when the time is not apt for hunting. And there comes a season of the year when the spiders, as it were, feel the sap rising in their eight legs. The male must respond by going to look for a female spider, and when he finds her, he must convince her that he is not merely something to eat—yet. And she, in the fullness of time, must carry an egg-sack as she goes about her hunting, and later must let the babies ride on her back. They, in turn, must learn the various forms of fending for themselves as they go through the different molt of the spider’s life until they, too, are swift- running, pouncing hunters of the woodland floor. Wolf-spidering is a complex job, not something to be undertaken by an amateur. We might say that there is a profession of wolf-spidering.
It is necessary to be good at all its manifold tasks to survive at it. What is more, the profession is possible only in very restricted circumstances. A woodland floor is necessary, for instance, and the right climate with a winter roughly like that your ancestors were used to; and enough of the right sorts of things to hunt; and the right shelter when you need it; and the numbers of natural enemies must be kept within reasonable bounds. For success, individual spiders must be superlatively good at their jobs and the right circumstances must prevail. Unless both the skills of spidering and the opportunity are present, there will not be any wolf-spiders. The “niche” of wolf-spidering will not be filled. “Niche” is a word ecologists have borrowed from church architecture. In a church, of course, “niche” means a recess in the wall in which a figurine may be placed; it is an address, a location, a physical place. But the ecologist’s “niche” is more than just a physical place: it is a place in the grand scheme of things. The niche is an animal's (or a plant's) profession.
The niche of the wolf-spider is everything it does to get its food and raise its babies. To be able to do these things it must relate properly to the place where it lives and to the other inhabitants of that place. Everything the species does to survive and stay “fit” in the Darwinian sense is its niche. The physical living place in an ecologist's jargon is called the habitat. The habitat is the “address” or “location” in which individuals of a species live. The woodland floor hunted by the wolf-spiders is the habitat, but wolf-spidering is their niche. It is the niche of wolf-spidering that has been fashioned by natural selection. The idea of niche gets at the numbers problem without any general questions that ecologists want to answer—the question of the constancy of numbers. The common stay common, and the rare stay rare, because the opportunities for each niche, or profession, are set by circumstance. Wolf-spidering needs the right sort of neighbors living in the right sort of wood, and the number of times that this combination comes up in any country is limited.
So, the number of wolf-spiders is limited also; the number was fixed when the niche was adopted. This number is likely to stay constant until something drastic happens to change the face of the country. Likening an animal’s niche to a human profession makes this idea of limits to number very clear. Let us take the profession of professing. There can only be as many professors in any city as there are teaching and scholarship jobs for professors to do. If the local university turns out more research scholars than there are professing jobs, then some of these hopeful young people will not be able to accept the scholar's tenure, however, cum laude their degrees. They will have to emigrate or take to honest work to make a living. Likewise, there cannot be more wolf-spiders than there are wolf-spider jobs, antelopes than there are antelope jobs, crabgrass than there are crabgrass jobs. Every species has its niche. And once its niche is fixed by natural selection, so also are its numbers fixed.
This idea of niche gets at the numbers problem without any discussion of breeding effort. Indeed, it shows that the way an animal breeds has very little to do with how many of it there are. This is a very strange idea to someone new to it, and it needs to be thought about carefully. The reproductive effort makes no differ ence to the eventual size of the population. Numerous eggs may increase numbers in the short term, following some disaster, but only for a while.
The numbers that may live are set by the number of niche-spaces (jobs) in the environment, and these are quite independent of how fast a species makes babies. But all the same, everyone must try to breed as fast as it can. It is in a race with its neighbors of the same kind, a race that will decide whose babies will fill the niche-space jobs of the next generation. The actual number of those who will be able to live in that next generation has been fixed by the environment; we may say that the population will be a function of the carrying capacity of the land for animals of this kind in that time and place. But the issue of whose babies will take up those limited places is open. It is here that natural selection operates. A “fit” individual is, by definition, one that successfully takes up one of the niche-spaces from the limited pool, and the fitness of a parent is measured by how many futures niche-spaces her or his offspring take up. “Survival of the fittest** means survival of those who leave the most living descendants.
A massive breeding effort makes no difference to the future population, but it is vital for the hereditary future of one’s own line. Therefore, everything that lives has the capacity for large families. Yet these are degrees of largeness in wild families, and these degrees of largeness make sense when looked at with an ecologist’s eye. The intuitively obvious consequence of a law that says, “Have the largest possible family or face hereditary oblivion,’* is the family based on thousands of tiny eggs or seeds. This seems to be the commonest breeding strategy. Houseflies, mosquitoes, salmon, and dandelions all do it. I call it “the small-egg gambit.’’
It has obvious advantages, but there are also costs, which the clever ones with big babies avoid. For users of the small-egg gambit, natural selection starts doing the obvious sums. If an egg is made just a little bit smaller, the parent might be able to make an extra egg for the same amount of food eaten, and this will give it a slight edge in the evolutionary race. It is enough. Natural selection will, therefore, choose families that make more and more smaller and smaller eggs until a point of optimum smallness is reached. If the eggs are any smaller than this, the young may all die; if they are any larger, one’s neighbor will swamp one’s posterity with her mass-production. The largest number of the smallest possible eggs makes simple Darwinian sense. But the costs of the small-egg gambit are grim. An inevitable consequence is that babies are thrown out into the world naked and tiny. Most of them as a result die, and early death is a common lot of baby salmon, dandelions, and the rest.
In the days before Darwin, people used to say that the vast families of salmon, dandelions, and insects were compensations for the slaughter of the young. So terrible was the life of a baby fish that Providence provided a salmon with thousands of eggs to give it a chance that one or two might get through. It seems a natural assumption and one that still confuses even some biologists. But the argument is the wrong way around. A high death rate for the tiny, helpless young is a consequence of the thousands of tiny eggs, not a cause. A selfish race of neighbor against neighbor leads to those thousands of tiny eggs, and the early deaths of the babies are the cost of this selfishness.
There is this to be said for the small-egg gambit, though; once you have been forced into it, there are the gambler’s compensations. Many young scattered far and wide mean an intensive search for opportunity, and this may pay off when the opportunity is thinly scattered in space. Weed and plague species win this advantage, as when the parachute seed of a dandelion is wafted between the trunks of the trees of a forest to alight on the fresh-turned earth of a rabbit burrow. The small-egg gambits of weeds may be likened to the tactics of a gambler at a casino who covers every number with a low-value chip. If he has enough chips, he is bound to win, particularly if big payoffs are possible. He does have to have very many chips to waste, though.
Therefore economists do not approve of gamblers. To the person with an economic turn of mind, the small-egg gambit, for all its crazy logic, does not seem a proper way to manage affairs. The adherents of this gambit spend all their lives at their professions, winning as many resources as possible from their living places, and then they invest these resources in tiny babies, most of whom are going to die. What a ridiculously low return on capital. What economic folly. Any economist could tell these animals and plants that the real way to win in the hereditary stakes is to put all your capital into a lesser number of big strong babies, all of which are going to survive. Several animals in fact do this.
I call it “the large-young gambit.'* In the large-young gambit one either makes a few huge eggs out of the food available, or the babies grow inside their mother, where they are safe. Either way, each baby has a very good chance of living to grow up. It is big to start with and it is fed or defended by parents until it can look after itself. Most of the food the parents collect goes into babies who live. There is little waste. Natural selection approves of this as much as do economists. Big babies who have a very good chance of long life mean more surviving offspring for food-investment in the end. This prudent outlay of resources is arranged by birds, viviparous snakes, great white sharks, goats, tigers, and people. Having a few, largely young, and then nursing them until they are big and strong, is the surest existing method of populating the future. Yet the success of this gambit assumes one essential condition.
You must start with just the right number of young. If you lay too many monster hen’s eggs or drop too many bawling brats, you may not be able to supply them with enough food, and some or all will die. You have then committed the economic wastefulness of those of the tiny eggs. So, you must not be too ambitious in your breeding. But the abstemious will also lose out, because its neighbor may raise one more baby, may populate the future just that little bit better, and start your line on a one-way ride to hereditary oblivion. You must get it just right; not too many young, and not too few. Natural selection will preserve those family strains which are programmed to “choose” the best or optimum size of the family.
Many ecologists have studied birds with these ideas in mind, and they have found that there is often a very good correlation between the number of eggs in a clutch and the food supply. In a year when food is plentiful, a bird may lay, on the average, one or two eggs more than in a lean season. The trend may be slight but sometimes is obvious. Snowy owls, which are big white birds of the arctic tundra, build vast nests on the ground. They feed their chicks on lemmings, the small brown arctic mice. When lemmings are scarce, there may be only one or two eggs in each owl’s nest, but when the tundra is crawling with lemmings, the nests may well have ten eggs each.
The owls are evidently clever at assessing how many chicks they can afford each year. But people are cleverer than snowy owls and have brought the large-young gambit to its perfection. They can read the environment, guess the future, and plan their families according to what their intelligence tells them they can afford. Even the infanticide practiced by various peoples at various times serves the cause of Darwinian fitness, rather than acting as a curb on population. There is no point in keeping alive babies who could not be supported for long.
Killing babies who could not be safely reared gives a better chance of survival to those who are left, and infanticide in hard times can mean that more children grow up in the end. Thus, every species has its niche, its place in the grand scheme of things; and every species has a breeding strategy refined by natural selection to leave the largest possible number of surviving offspring. The requirement for a definite niche implies a limit to the size of the population because of the numbers of the animal or plant are set by the opportunities for carrying on life in that niche.
The kind of breeding strategy, on the other hand, has no effect on the size of the usual population, and the drive to breed is a struggle to decide which family strains have the privilege of taking up the limited number of opportunities for life. Every family tries to outbreed every other, though the total numbers of their kind remain the same. These are the principles on which an ecologist can base his efforts to answer the major questions of his discipline.
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