CAUSES AND CONSEQUENCES OF VARIATION IN COMPETITIVE ABILITY IN PLANT-COMMUNITIES

Several factors may define the cause and pattern of variation in competitive ability among individuals within a plant community, Variation may be a consequence of genetic or environmental variability. These two sources of variation may vary in their relative magnitudes. The relevant scale ot' genetic variation may occur at the individual genotype level or at the species level. The relevant scale of environmental variation may occur at the individual plant level or at the neighbourhood (or community) level. Relative competitive abilities may be effected by genotype-environment interaction or by genotype-genotype (or species-species) interaction. The complex relationship among these factors reveals the mechanistic basis for establishing a clear distinction among five specific hypotheses for species coexistence and diversity that are all variations of the general hypothesis that competitive abilities do not differ sufficiently among coexisting species to cause any competitive exclusion at the community level. These hypotheses are compared in terms of the degree to which they are restricted by assumptions and supported by existing data, and in the extent to which they involve evolutionary consequences of competition.