Chapter Summary

The principle of limiting factors states that for every physical factor in the environment, there are both maximum and minimum tolerable limits beyond which a given species cannot survive. Often the factor in shortest supply or closest to the tolerance limit for a particular species at a particular time is the critical factor that will determine the abundance and distribution of that species in that ecosystem. Random genetic variations create diversity in a population that gives some individuals advantages in a given set of circumstances. The best-suited organisms will survive and reproduce more successfully than the ill-suited ones. Eventually, the genes for these successful characteristics predominate in the population and the species becomes adapted to its environment and to a particular role. This process leads to evolution of a species either through gradual replacement of the original parental type or a splitting of a population into two species.

Habitat describes the place in which an organism lives; niche describes either the role an organism plays or the total set of conditions that control its distribution. Natural selection often leads to niche specialization and resource partitioning that reduce competition between species. Organisms interact within communities in many ways. Predation—feeding on another organism—involves pathogens, parasites, and herbivores as well as carnivorous predators. Competition is another kind of antagonistic relationship in which organisms vie for space, food, or other resources. Symbiosis is the intimate living together of two species. Mutualism means that both species benefit; commensalism means that one species benefits while the other is indifferent.

Some fundamental properties of biological communities are productivity, diversity, complexity, resilience, stability, and structure. Productivity is a measure of the rate at which photosynthesis produces biomass made of energy-rich compounds. Tropical rainforests are generally the most productive of all terrestrial communities; coral reefs and estuaries are generally the most productive aquatic communities. Diversity is a measure of the number of different species in a community, while abundance is the total number of individuals. Often the most productive and stable communities are highly diverse and profusely populated, but sometimes a high degree of specialization makes an ecosystem more, rather than less, susceptible to disturbance. Ecological complexity refers to the number of species at each trophic level as well as the total number of trophic levels in a community. Structure concerns the patterns of organization, both spatial and functional, in a community. Often a keystone species, or group of species, plays an unusually important role in determining community structure, composition, or function. All of these characteristics are affected both by physical and chemical factors as well as biological interactions between the organisms that make up the community. Edge effects at the boundaries between different habitat types can be important where landscapes are fragmented into isolated patches.

Ecological succession and development are processes by which organisms alter the environment in ways that allow some species to replace others. Primary succession starts with a previously unoccupied site. Secondary succession occurs on a site that has been disturbed by external forces. Often succession proceeds until a mature, diverse, climax community is established. These mature communities may have self-perpetuating processes that make them resistant to change and resilient to disturbance. Whether diversity always leads to stability, however, is controversial. Communities that are disrupted regularly by fires or other natural disasters sometimes establish dynamic equilibrium or disclimax communities dependent on constant renewal. Introduction of new species by natural processes, such as opening of a land bridge, or through human intervention can upset the natural relationships in a community and cause catastrophic changes for indigenous species.