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None of seven existing theories seeking to explain the coexistence of plant species, could singly explain the pattern of species richness found e.g. in calcicolous grasslands. A more general explanation of species coexistence utilizes an idealized concept, the null community, defined as an undisturbed community within stable zonal vegetation including the whole pool of potential species. The number of potential species will then be dependent mainly on evolutionary factors (speciation, species traits). The majority of 'real' communities have a smaller species pool for historical reasons - migration has not taken place. Within this local species pool, asymmetric interspecific competition is the main force reducing species richness. Competition takes place on an ecological level (within an ecological time scale), but the outcome of it is governed by evolutionarily determined traits of individuals. Ecological factors influence the number of coexisting species under two circumstances - when habitat conditions are changing and one species pool replaces another, or when some external factor (such as grazing or mowing) changes the traits of individuals, thus altering the outcome of competition. Present theories operate with only one factor - evolutionary, historical or ecological. In the framework of the generalized approach presented here each of these theories is useful to explain species coexistence on certain levels and under particular conditions.
Oikos © 1992 Nordic Society Oikos