Summary 1We sought evidence among the plant species of a New Zealand sand dune community that limiting similarity controls the ability of species to coexist. Sampling was at four spatial scales, from a single point up to a scale of 50 m 2 . Twenty-three functional characters were measured on each of the species, covering the morphology of the shoot and root systems and nutrient status, and intended to represent modes of resource acquisition. 2 Patterns of association between plant species at the four scales were examined for any tendency for plants with similar functional characters to coexist less often than expected at random (e.g. if a point has three species, do they have notably different characters?) The observed results were compared with the patterns expected under a null model using a range of test statistics. 3 A test over all characters found that the mean dissimilarity between nearest-neighbour species in functional space, and the minimum dissimilarity, were greater than expected under the null model at the 0.5 × 0.5 m scale. This supports the MacArthur & Levins model, although the actual community did not show an even spread of species over functional space. 4 Limiting similarity effects were seen even more consistently in separate characters when within-species variation was taken into account to calculate measures of overlap. The characters involved were mainly those related to rooting patterns and leaf water control, and thus perhaps reflecting the acquisition of nutrients and/or water. 5 Our results seem to be amongst the most convincing support for the theory of limiting similarity, and the only example involving vegetative processes in plant communities. The characters involved suggest that species can more readily coexist if they differ in their water-use pattern, reducing competition between them.

The concept of niche overlap appears in studies of the mechanisms of the maintenance of species diversity, in searches for assembly rules, and in estimation of within-community species redundancy. For plant traits measured on a continuous scale, existing indices are inadequate because they split the scale into a number of categories thus losing information. An index is easy to construct if we assume a normal distribution for each trait within a species, but this assumption is rarely true. We extend and apply an index, NO(K), which is based on kernel density functions, and can therefore work with distributions of any shape without prior assumptions. For cases where the ecologist wishes to downweight traits that are inter-correlated, we offer a variant that does this: NO(Kw). From either of these indices, an index of the mean niche overlap in a community can be calculated: NO(K,community) and NO(Kw,community). For all these indices, the variance can be calculated and formulae for this are given. To give examples of the new indices in use, we apply them to a coastal fish dataset and a sand-dune plant dataset. The former exhibits considerable non-normality, emphasising the need for kernel-based indices. Accordingly, there was a considerable difference in index values, with those for an index based on a normal distribution being significantly higher than those from an index which, being based on kernel fitting, is not biased by an assumption for the distribution. The NO(K) values were ecologically consistent for the fish species concerned, varying from 0.02 to 0.53. The sand-dune plant data also showed a wide range of overlap values. Interestingly, the least overlap was between two graminoids, which would have been placed in the same functional group in the coarse classification often used in functional-type/ecosystem-function work.

Summary1. We sought evidence for limiting similarity, a basic aspect of community structure, in three zones of a saltmarsh. Sampling was conducted at three spatial scales (grains), from a single point up to the scale of several square metres. Twenty-three functional traits, related to the structure of the shoot and root systems and to nutrient status, were measured on each species present, separately in each community. 2. Patterns of association between plant species were compared with those expected under a null model, to assess whether plants with similar functional traits tended to coexist or to separate, i.e. whether there was environmental filtering or limiting similarity. A patch null model was used, a type that tends to be conservative but that avoids spurious evidence of limiting similarity caused by environmental pseudoreplication. One overall and four univariate test statistics were calculated, to capture possible patterns in trait space whilst minimising the problem of multiple testing. 3. In the Shrub community, overall evidence for even spacing of co-occurring species in functionaltrait space, the pattern expected from the theory of limiting similarity, was seen at the area scale. In univariate tests in that community, there was evidence for even spacing in leaf lobation and leaf succulence, especially at small scales. 4. In the Rush community, there was significant evidence for limiting similarity in several traits, especially those related to canopy interactions, but also in some root characteristics. However, clustering in other traits, presumably owing to microenvironmental filtering, reduced overall tests for limiting similarity to 'marginal significance' (0.1 > P > 0.05). 5. In the species-poor and salt-stressed Salt turf, significant departures from the null model were sporadic and not consistent, although chlorophyll characters and leaf nitrogen concentration tended to be clustered. 6. Synthesis: There was evidence for niche limitation in two of the communities -Rush and Shrubapparently based on canopy interactions in both cases and perhaps also root interactions in the latter community. Limiting similarity can be an important force in community assembly. However, in situations when it cannot be demonstrated, we do not know whether trait-based competition is absent or whether its signal is overwhelmed by other processes.