Competitive intransitivity and size-frequency distributions of interacting populations

The competitive ranking of three marine sessile organisms was determined from overgrowth observations and found to be intrasitive. Overgrowth results were dependent upon relative vertical relief which, in turn, was related to colony area. Discriminant function analysis demonstrates that competitive rankings can be largely predicted on the basis of knowledge of the size-frequency distribution of interacting populations. Because size increases with age for these species, size-dependent competition implies age-dependent competition. The role of age structure in determining the results of inter-specific interactions has been curiously neglected, particularly as it relates to competitive ability, yet the age dependence of competitive ability suggests (i) the occurrence of intransitivity in purely exploitative systems, (ii) a mechanism for frequency-dependent behavior, (iii) the production of limit cycles, and (iv) the possibility of size refugia.