Evidence for assembly rules: limiting similarity within a saltmarsh


Correspondence author. E-mail: bastow@bastow.ac.nz


1. 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 functional-trait 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 > > 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 Shrub – apparently 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.