Views of Communities
Community is an assemblage of species living close enough together for potential interaction. Community structure emerges from an interaction of many environmental variables that permits species to exist in certain places. There are two views to discuss why certain species are found together in communities.
Properties of Communities & Coevolution
Species diversity or richness is the number of species that make up a community which include relative abundance. Some species are quite rare in a community where as other species are plentiful. Dominant species are those which are abundant and have major impact on the community as a whole. Grouping of communities according to similarities in overall form without regard to the actual species is the basis for the biomes. Various feeding relationships of a community determine the flow of energy and cycling of nutrients from plants to herbivores and then to carnivores. Community stability is the ability of the community to bounce back to its original composition in the wake of some disturbance such as a fire or a disease that kills most individuals of a dominant species. Coevolution is an interspecific phenomenon which is of great importance in community ecology.
Interspecific competition for limited resources determines species diversity in some communities. Closely related species can coexist if there are one or more significant differences in their niches. Predation has important roles in the evolution of defensive adaptations in the prey species. Symbiosis has different impacts on a community. Parasitism resembles the predator-prey relationship but does not kill the host. It shows coevolution. Dynamic multiple interactions of organisms with both biotic and abiotic aspects of their environment results in a complex community property, the composition of species.
Succession involves changes in species composition of a community over ecological time. Primary succession occurs where no organisms previously existed whereas secondary succession occurs after disturbance of an existing community. Facilitation, inhibition and tolerance are the causes of succession.
Geographic Aspects of Diversity
Biogeography is the study of the past and the present distribution of species, deals with species diversity and composition in realms that have boundaries, ultimately associated with the patterns of continental drift. Islands are instructive in studying the role of dispersal in determining the species composition of communities.
Ecosystem is the level of ecological study that includes all the organisms in a given area along with the abiotic factors with which they interact. Most ecosystems are driven by energy from sunlight. Energy flow and chemical cycling are two inter related processes that occur by transfer of substances through the feeding levels of ecosystems. Trophic levels begin with producers, autotrophic organisms that support all other components of the community. The main producers in photosystems are photosynthetic autotrophs. Primary productivity is the rate at which light energy is converted to the chemical energy of organic compounds by autotrophs in an ecosystem. Biogeochemical cycles are the various nutrient circuits which involve both biotic and abiotic components of ecosystems. Three important chemical cycles are carbon, nitrogen and phosphorus cycles. The biosphere is a kind of superorganism with a self regulated metabolism that helps counter fluctuation in the physical environment. Processes occurring at one location can have far reaching effects and consequences.