Ecosystems: What Keeps Them The Same?

1. The most important point to recognize is that no forces or rigid structures exist that prevent ecosystems from changing. In fact, eco­systems can and do change, even drastically, as conditions are altered. The one thing that enables ecosystems to sustain a given composition of species over long periods of time is that all the relationships in the system are in a dynamic balance.

2. Each species in an ecosystem exists as a population - that is to say, an interbreeding, reproducing group. An ecosystem's remaining stable (sustaining itself) over a long period of time implies that the pop­ulation of each species in the ecosystem remains more or less constant in size and geographic distribution. Any continuing increase or de­crease in population would be observed as a change in the ecosystem. In turn, a population's remaining constant over a long time means that reproductive rate is equaled by death rate. Thus, the problem of eco­system balance boils down to a problem of how birth rate and death rate are balanced for each species in the ecosystem.

3. Maintaining or increasing a population depends on more than reproductive rate (number of live births, eggs laid, or seeds or spores set in plants) by itself. Recruitment, which is defined as making it through the early growth stages to become part of breeding, reproduc­ing population, is equally important. For example, many fish lay thou­sands, even millions of eggs, and plants typically set thousands of seed. Yet population increase may be nil because recruitment is so low; in other words, most of the young fish and plants perish in the early stag­es of growth. (Note that "low recruitment" is a polite way of saying high mortality of the young.) Conversely, even a relatively low repro­ductive rate may result in a substantial population increase when re­cruitment is high. Primates are an outstanding example of this latter strategy.

Ecosystems: What Keeps Them the Same?

4. Additional factors that influence population growth and geo­graphic distribution are the ability of animals to migrate or of seeds to disperse to similar habitats in other regions, the ability to adapt to and invade new habitats in addition to the one originally occupied, de­fense mechanisms, and resistance to adverse conditions and disease. All these factors taken together are referred to as the biotic potential of the species. Despite different strategies regarding biotic potential, there is one point in common: Every species has sufficient reproductive ca­pacity to rapidly increase its population if factors are favorable for a high recruitment. Indeed, each new generation will be multiplied by the number of female produced. For example, rabbits producing 20 offspring, 10 of which are female, may grow by a factor of 10 each generation: 10, 100, 1000, 10 000... Such a multiplying series is called an exponential increase. In populations it is commonly called a popu­lation explosion.

5. Populations in natural ecosystems do not explode because all con­ditions are seldom favorable for any extended period of time. One or more abiotic factors, such as unfavorable temperature, amount of available wa­ter, pH, or salinity, and/or one or more biotic factors, such as predators, parasites, disease organisms, or lack of sufficient food, become limiting. The combination of all these abiotic and biotic factors that may limit pop­ulation increase is referred to as environmental resistance.

6. One may already foresee the result of the interplay between bio­tic potential and environmental resistance. Sooner or later, any popu­lation increase will be curtailed by one or more factors of environmen­tal resistance. It is important to observe how this curtailment works, however. In general, the reproductive rate for a species remain fairly constant, because that rate is part of the genetic endowment of the species. What varies tremendously is recruitment. It is in the early stages of growth that individuals (plants and animals) are most vulnerable to precaution, disease, lack of food (or nutrients) or water, or other ad­verse conditions. Consequently, environmental resistance effectively reduces recruitment. Of course, some adults also perish, particularly the old and weak. If recruitment is at the replacement level, just enough to replace these adults, then the population size will remain constant. If recruitment is not sufficient to replace losses in the breeding popu­lation, of course, the population size will decline.

Unit Seven

Ecosystems: What Keeps Them the Same?

7. In certain situations, environmental resistance may effect re­production as well as causing mortality (death) directly. For example, loss of suitable habitat often prevents animals from breeding. Also, cer­tain pollutants adversally affect reproduction. However, we can still view these situations as environmental resistance either blocking a pop­ulation's growth or causing its decline.

8. In conclusion, whether a population grows, remains stable, or de­creases is the result of a dynamic balance between its biotic potential and environmental resistance. In general, biotic potential remains constant; it is shifts in environmental resistance that allow populations to increase or cause them to decrease. For example, a number of favorable years (low environmental resistance) will allow a population to increase; then a drought may cause it to die back, and the cycle may be repeated. It should be noted that balance is a relative phenomenon. Some balances fluctuate very little, others fluctuate widely, but as long as decreased populations restore their numbers, the system may be said to be balanced. Still, the questions remain: What maintains the balance within a certain range? What prevents a population from going into an explosion or into extinction? Indeed, in nature, neither possibility is ruled out.

9. In general, populations remain within a certain size range be­cause most factors of environmental resistance are density-dependent. That is, as population density (the number of individuals per unit area) increases, environmental resistance becomes more intense and causes an increase in mortality such that population growth ceases or declines. Conversely, as population density decreases, environmental resistance is generally mitigated, allowing the population to recover. This bal­ancing act will become clearer if we discuss specific mechanisms of population balance.

10. Human impacts, on the other hand, readily result in extinction
because they are not density-dependent. Impacts such as ecosystem
destruction, habitat alteration, pollution, and exploitation can be just
as intense at low population densities as at high. Furthermore, the bio­
tic potential of many species depends on a minimum population base
— a herd of deer, a pack of wolves, a flock of birds, or a school offish,
for example. If a population is pushed below a certain critical number
necessary to support a breeding population, biotic potential fails, and
extinction is virtually assured. Species whose populations are declin-

ing because of human impacts are defined as threatened. If the popu­lation is approaching or is at what scientists believe to be critical num­ber, the species may be defined as endangered.