Little evidence for limiting similarity in a long-term study of a roadside plant community

1. Understanding the assembly of ecological communities is central to managing such communities for conservation, restoration and invasion resistance. A central plank of modern theory is limiting similarity, i.e. a finite limit to the similarity of coexisting species. 2. Here, we test the theory of limiting similarity in a roadside plant community that has been monitored for almost 50 years. We measure differences between observed trait distributions ('functional diversity, FD') and those expected on the basis of three different null models, several trait combinations and four different methods of measuring FD. 3. The general pattern was of lower observed than expected FD, or no difference between observed and expected FD, independent of FD measure and trait set. Some rare species had 'unusual' traits; thus if the null model assumed all species had equal chance of occurring, observed FD was generally lower than expected by chance, but if the null model assumed the probability of occurrence of rare species was proportional to the observed frequency of occurrence or excluded rare species, observed FD was not significantly different from expected. 4. Synthesis. The results do not support the idea that niche differences are central to the assembly of this plant community. However, they do suggest that rare species may be functionally unusual, and that the patterns revealed by this kind of analysis can be strongly influenced by the choice of null model, and much less so by the choice of FD measure.