Ecology

Compared with the potential biological "demand," the environment has a limited ability to "supply" the requirements of life. As a result, the rates of critical ecological processes, such as productivity, are constrained by so-called limiting factors, which are present in the least supply relative to the biological demand. A limiting environmental factor can be physical or chemical in nature, and the factors act singly, but sequentially. For example, if a typical unproductive lake is fertilized with nitrate, there would be no ecological response. However, if that same lake was fertilized with phosphate, there would be a great increase in the productivity of single-celled algae. If the lake was then fertilized with nitrate, there would be a further increase of productivity, because the ecological requirement for phosphate, the primary limiting factor, had previously been satiated.

This example illustrates the strong influence that the environment has on rates of processes such as productivity, and on overall ecological development. The most complex, productive, and highly developed ecosystems occur in relatively benign environments, where climate and the supplies of nutrients and water are least limiting to organisms and their processes. Tropical forests and coral reefs are the best examples of well-developed, natural ecosystems of this sort. In contrast, environmentally stressed ecosystems are severely constrained by one or more of these factors. For example, deserts are limited by the availability of water, and tundra by a cold climate.

In a theoretically benign environment, with an unlimited availability of the requirements of life, organisms can maximize the growth of their individual biomass and of their populations. Conditions of unlimited resources might occur (at least temporarily), perhaps, in situations that are sunny and well supplied with water and nutrients. Population growth in an unlimited environment is exponential, meaning that the number of individuals doubles during a fixed time interval. For example, if a species was biologically capable of doubling the size of its population in one week under unlimited environmental conditions, then after one week of growth an initial population of N individuals would grow to 2N, after two weeks 4N, after three weeks 8N, after four weeks 16N, and after eight weeks it would be 256N. A financial analogy will help to put this tremendous rate of population increase into perspective: an initial investment of $100 growing at that rate would be worth $25,600 after only 8 weeks.

Clearly, this is an enormous rate of growth, and it would never be sustainable under real-world ecological (or economic) conditions. Before long, environmental conditions would become limiting, and organisms would begin to interfere with each other through an ecological process known as competition. In general, the more similar the ecological requirements of individuals or species, the more intense the competition they experience. Therefore, competition among similar-sized individuals of the same species can be very intense, while individuals of different sized species (such as trees and moss) will compete hardly at all.

Competition is an important ecological process, because it limits the growth rates of individuals and populations, and influences the kinds of species that can occur together in ecological communities. These ecological traits are also profoundly influenced by other interactions among organisms, such as herbivory, predation, and disease.