Limitations to species coexistence: evidence for competition from field observations, using a patch model

Plant community structure in four dune slacks is examined for evidence of competition, expressed as local constraint on species biomass. Such constraint would result in a low variance in total quadrat biomass, compared to a null model. A method of analysis is introduced, using a patch-based null model, which is intended to discount most of the effects of environmental heterogeneity. The method is applied to data in which previous methods had failed to find such structure. There was significant biomass constraint (i.e. low variance in biomass) examining total plant-community biomass in Site 1, and there was a trend in this direction also in Site 2. When guilds defined on morphology were examined, significant biomass constraint was found for the Rhizomatous guild in Site 2. There was a trend to low variance for this guild also in Site 4, but it was significant only in a one-tailed test. After a consideration of other mechanisms, it is concluded that the most likely explanation for this biomass constraint is interspecies competition. Site 2, which showed the strongest evidence of biomass constraint in the present analyses, was also the site that demonstrated a significant deficit of variance in richness in earlier work, and was the most species-rich of the four sites and the least disturbed. The advantages of the new method, for obtaining evidence on competition, are discussed in comparison to pot experiments and field-perturbation experiments. We conclude that community structure can be seen in the pattern of species biomasses provided that a suitable method of analysis is used, i.e. examining small-scale effects and excluding the effects of environmental heterogeneity. In some cases such effects can be related to guild membership. We hypothesise that constraint on biomass is likely to be stronger in more species-rich and less disturbed communities.