Predation is another factor that may limit the size of populations. In this sense, pre-dation includes everything from one kind of animal capturing and eating another to parasitism (the condition of an organism living in or on another organism, at whose expense the parasite is maintained). Predation and parasitism are two ends of a biological spectrum, between which there is no clearly marked distinction. They are governed by similar principles.
When experimental populations are set up under simple conditions in the laboratory, the predator often exterminates its prey and then becomes extinct itself, having nothing to eat. If refuges are provided for the prey, however, its population will be driven to low levels but can recover; one would then expect the populations of predators and prey to follow a cyclical pattern. The low population levels of the prey species provide scant food for the predators; then the predators in turn become scarce. As this occurs, the prey recover and again become abundant. Population cycles are characteristic of some species of small mammals, such as lemmings, and may be stimulated, at least in some situations, by their predators. An example of such fluctuating populations is expressed diagrammatically.
Relationships of this sort are of the greatest importance for biological control. If the prey becomes so rare that it is an infrequent event for the predator species to encounter it, the predator itself may become extinct. Ideally, in the case of deliberate biological control, the prey will survive in small numbers, thus making it possible for small populations of the predator to survive also and for the prey species to be controlled indefinitely.
In Australia, for example, prickly pear cactus (Opunt-ia), introduced from America, once overran the ranges and became so abundant that vast areas were effectively. closed to cattle grazing. The situation was changed dramatically with the introduction of the moth Cactoblastis. The larvae of the moth feed on the pads of the cactus and rapidly destroy the plants. Within relatively few years, the moth had reduced the cactus to the status of a rare species in many regions where it was formerly abundant. It is now exceptional to find an individual of Cactoblastis, but the moth is still present and evidently keeps the cactus in check.
The future is more problematic for the American chestnut (Castanea americanu a species of tree that has virtually been driven to extinction by the accidental intro duction of an Asiatic fungus, Cryptonectria (Endothia) parasitica. The chestnut used ii be a dominant or codominant tree throughout much of the forests of eastern and cen tral temperate North America. Chestnut blight, the disease caused by the fungus, wa first seen in North America in New York State in 1904 and killed most of the dies' nut trees in North America during the following 30 years. Today the American chesi nut survives largely as sprouts that grow every year from the trunks of trees that weie killed decades ago, A few populations also seem to be isolated from the chestnut blight in certain remote areas.
Many additional examples of the ways in which predator-prey relationships operate are familiar. Organisms that cause diseases that completely kill their host specie are not "successful," because once they have done so they have also eliminated theil own source of food. Thus those strains of the disease-causing organism that are less virulent will be favored by natural selection and will survive.
The history of the viral disease myxomatosis, which is caused by a virus introduced into Australia and New Zealand to control rabbits, provides an instructive example of this principle. The rabbits were brought to these countries as a convenient source of meat, but they soon ran wild, with devastating effects on the countryside. When the virus causing myxomatosis was introduced, most of the rabbits soon died. The most virulent strains of the virus disappeared with the dead rabbits, and less lethal strains became apparent in the remaining rabbit populations. At the same time, strains of rabbits that were resistant to the disease began to appear. Now the populations of both kinds of organisms have achieved the kind of equilibrium relationship in which they can coexist indefinitely.
An organism that causes a disease that always kills its host will die with it;
one that produces sublethal effects will have the opportunity to spread to another host.
The relationships between large carnivores and grazing mammals are a subject of great interest and importance in many parts of the world. Appearances are sometimes deceiving. On Isle Royale in Lake Superior, for example, moose reached the island and multiplied freely there in isolation. When wolves later reached the island by crossing over the ice in winter as the moose had done earlier, it was widely assumed at first that they were playing the determining role in controlling the moose populations. More careful studies, however, have demonstrated that this is not the case. The moose that the wolves eat are old and diseased, for the most part, and would not survive long anyway. In general the moose are controlled by the amount of food available to them, their diseases, and many factors other than the wolves.
The intricate interactions between predators and prey are an essential factor in the maintenance of groups of organisms occurring together that are rich and diverse in species. By controlling the levels of some species, the predators make possible the continued existence of others in that same community. In other words, by keeping ihe numbers of individuals of some of the competing species low, the predators prevent or greatly reduce competitive exclusion. Such patterns are particularly characteristic of biological communities in intertidal, marine habitats. For example, in preying selectively on bivalves, sea stars prevent them from monopolizing all the space in such habitats, opening them up to many other kinds of organisms.
A given predator may very often feed on two, three, or more kinds of plants oranimals in a given community in a way that partly depends on their relative abundance. In other words, a predator may feed on species A when it is abundant and then switch to species B when A is rare. Similarly, a given prey species may be a primary source of food for increasing numbers of species as it becomes more abundant, which tends to limit the size of its population automatically. Such feedback systems are key factors in determining the structure of many natural communities.