Dominance and structural bimodality in Mediterranean‐grassland gradients

Abstract

The ordering of communities located in oligotrophic zones of central‐western Spain points to a broad dispersion of extremes of the trophic gradient, represented by slopes. One such extreme is characterized by the presence of shrub and the other by a relatively high degree of soil humidity and an abundance of nutrients. In both cases, the result is low alpha diversity (1.98 bits in shrub communities and 1.67 bits in very humid communities), since situations of strong dominance are produced. This, together with evidence of dominance toward the center of the gradient, means that the general response of the vegetation fits a bimodal model, with peaks at 3.48 and 3.69 bits, in opposition to the theory that the maximum diversity expected in such situations would be unimodal and would occur in moderately resource‐poor habitats. On considering each of the gradient extremes separately, the above‐mentioned dominance is due to different species, depending on the community in question. This gives rise to conditions o...