DOMINANCE AND STRUCTURAL BIMODALITY IN MEDITERRANEAN-GRASSLAND GRADIENTS

The ordering of communities located in oligotrophic zones of central-w estern Spain points to a broad dispersion of extremes of the trophic g radient, represented by slopes. One such extreme is characterized by t he 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 v ery humid communities), since situations of strong dominance ale produ ced. This, together with evidence of dominance toward the center of th e gradient, means that the general response of the vegetation fits a b imodal model, with peaks al 3.48 and 3.69 bits, in opposition to the t heory that the maximum diversity expected in such situations would be unimodal and would occur in moderately resource-poor habitats. On cons idering each of the gradient extremes separately, the above mentioned dominance is due to different species, depending on the community in q uestion. This gives rise to Conditions of manifest heterogeneity in th e configuration of the landscape, reaching 0.56 bits in shrub communit ies and 0.75 bits in more eutrophic communities. Compared with the rem aining communities, distribution can be considered to be ''coarse grai n.'' In the latter communities, high diversity and low heterogeneity a re linked; species are repeated from one site to another; and distribu tion is ''fine grain'' (heterogeneity does not surpass 0.35 bits in an y case). Accordingly, the results are also in disagreement with the th eory that the poorest communities (situated on the upper parts of the slopes) would show a fine-grain distribution, whereas the richest ones (low down on the slopes) would have a coarse-grain distribution. Here , both the poorest and richest communities show a coarse-grain distrib ution. This paints to the dangers of advancing theor-ies that have not been tested under all possible circumstances, although the case studi ed here can, to a certain extent, be considered as particular among th e grasslands predominating in the zone.