Intraspecific aggregation does not increase species richness in dune grasslands


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1. One of the possible mechanisms to maintain a high number of species in a plant community is that intraspecific aggregation due to limited dispersal reduces the importance of interspecific competition.

2. The hypothesis has been examined both theoretically and experimentally in the simplifying case of two species in a uniform abiotic environment, and there is a solid qualitative understanding that intraspecific aggregation reduces the importance of interspecific competition; however, there is considerable uncertainty as to the magnitude of this reduction in plant communities with many species in a heterogeneous environment and its effect on the number of coexisting species.

3. Here, the hypothesis was tested by comparing observed species richness at a site with a compound estimate of the intraspecific aggregation of all the species observed at the site, while taking the expected confounding effects of environmental spatial heterogeneity into account.

4. It is difficult to draw firm conclusions from the non-significant statistical results found in this study, as a lack of significance does not necessarily rule out the presence of an effect. However, based on the observation that species richness was not correlated with the degree of intraspecific aggregation, it was concluded that there was no additional support for the hypothesis that reduced interspecific competition due to intraspecific aggregation is important for maintaining the relatively species-rich flora in North-European dune grassland.

5.Synthesis. It is important to test whether it is possible to generalize the expected effects of intraspecific aggregation from manipulated experiments using only a few plant species to natural plant communities with many species, and the approach taken in this paper is a practically manageable approach.