Text C. Ecosystems: Mechanisms of Population Balance

1. There are some specific mechanisms that provide population balance in nature. It is necessary to focus on one mechanism at a time, but keep in mind that in natural ecosystems all of the mechanisms are working in concert to create the overall balance. Knowledge of these mechanisms will make us aware of how ecosystems may be upset and the consequences that may result.

2. A classic example of population balance is that between the lynx, a member of the cat family, and hares, a member of the rabbit family, as observed in Canada from 1850 to 1930. When the hare population is low, each hare can find abundant food and plenty of places, to hide and raise offspring. In other words, the hares' environmental resistance is relatively low, and their population increases despite the presence of the lynx predator. As the hare population increases, however, each hare has relatively less food and fewer hiding places. More hares provide easier hunting for the lynx so that, with plenty of hares to feed lynx young, the lynx population begins to fall. As the hare population falls, the food and shelter available to each hare again increase. Also, surviving hares are those that are healthiest and best able to escape from the lynx. Hunting becomes harder for the lynx; many of them starve, and their population begins to fall. These factors sum up to lower environmental resistance for the hares, and their population increases again, repeating the cycle. These events explain the fluctuating but continuing balance found between the hare and lynx populations.

3. Much more abundant and ecologically important in population control are a huge diversity of parasitic organisms. These organisms range from tapeworms. which may be a foot or more in length to microscopic disease-causing protozoans, fungi, bacteria, and viruses. All species of plants, animals, and even microbes may be infected by parasites. In terms of population balance, parasitic organisms act in the same way as large predators. As the population density of the host organism increases, parasites and their vectors (agents that carry the parasites from one host to another), such as disease-carrying insects, have little trouble finding new hosts, and infection rates increase, causing dieback. Conversely, when the population density of the host is low, transfer of infection is impeded, and there is a great reduction in levels of infection, a condition that allows the population to recover.

4. Parasites may not kill their host, but they generally weaken it and make it more vulnerable to adverse conditions and to attack by larger predators. It is commonly observed that the animals killed by large predators are infected with parasites, whereas animals killed by hunters are generally healthy. In a food web, a population of any given organism is affected by a number of predators and parasites simultaneously. Consequently, the balance can be thought as a balance between the population of an organism and its natural enemies. The wide swings in populations noted in the hare-lynx case are generally typical of very simple ecosystems involving relatively few species. Balances between an organism and several natural enemies are generally more stable and less prone to wide fluctuations because different natural enemies come into play at different population densities. Also, when the preferred prey is at a low density, the population of the natural enemy may be supported by its feeding on something else. Thus, the lag time between increase of the prey population and that of the natural enemy is diminished. These factors have a great damping effect on the rise and fall of the prey population.

5. In all such balances, however, whether simple or complex, it is extremely important to recognize that a high degree of adaptation is involved on the part of both the prey or host and the natural enemy. This adaptation is such that a given natural enemy is incapable of completely eliminating its prey or host but yet is capable of limiting the prey or host population to a certain density. Putting any predator and prey or host and parasite together does not lead to an automatic balance. Such lack of balance is shown all too clearly by what may occur when a species from one region is introduced into another. Such introductions may lead to what are commonly called ecological disasters as balances fail.

6. In discussing predator-prey balances, it was said that in lean times the excess carnivore population – the lynx, for instance, – simply starved. Actually, another factor is often involved in the control of carnivore and some herbivore populations: territoriality, which refers to individuals or groups claiming a territory and defending it against others of the same species. For example, the males of many species of songbirds stake out a territory at the time of nesting. Their song has the function of warning other males to keep away. Male wolves and other carnivores, including dogs, stake out a territory by spotting it with urine, the smell of which warnes other males to stay away. The territory defended is large enough to assure the "owners" of being able to gather enough food to successfully rear a brood. The size of the territory defended varies with resources available. In lean times territories are larger; in good times they are smaller.

7. The obvious advantage of territoriality is that individuals that are able to successfully claim and defend a territory will have enough resources to rear a well-fed, healthy next generation. Those individuals unable to claim a territory generally meet an unhappy end. Continually chased out of one territory after another, they fall victim to any of the factors of environmental resistance, or at the very least they are unable to breed and raise young. Territoriality does not change the basic principle of population being a dynamic balance between biotic potential and environmental resistance. In the face of limited resources, however, territoriality creates a mechanism of selecting the strongest and fittest to survive and breed, while eliminating the genes of the weaker individuals.

8. Territoriality is an instinctive behavioral trait in many species. By keeping populations in check, territoriality helps to maintain the balance of the ecosystem and thus to ensure survival of the species. Many people have observed that humans are also a territorial animal, even to an extent that greatly exceeds any other species. Almost all of us aspire to owning a piece of land that we can put a fence around and call ours, and the bigger the piece of land, the better. Throughout history, virtually all wars between nations have involved territorial disputes, and nations continue to arm themselves to the teeth with the most sophisticated weaponry available – always, they say, to defend themselves against the threat of territorial encroachment by their neighbors.