Assembly Rules Demonstrated in a Saltmarsh Community

Abstract

1 Assembly rules are generalised restrictions on species presence or abundance that are based on the presence or abundance of one or several other species, or types of species. Such rules were sought in a saltmarsh community, as a step towards determining how common assembly rules are in natural plant communities. 2 Variance in quadrat richness was only two-thirds of that expected under a null model. The effect was stronger than in plant communities previously examined. It is in the direction that would be expected if there were niche limitation, although other explanations, especially limitations to the packing of individual plant modules, are possible. 3 Highly significant guild proportionality (i.e. low variance of guild proportions between quadrats) was observed for two a priori guild classifications: narrow vs. broad leaves and monocots vs. dicots. This suggests another assembly rule, that avoids effects of plant-module packing. 4 The Wilson-Roxburgh iterative optimization method was used to find the intrinsic guilds, i.e. those groups of species that tend to be mutually exclusive in the real community. Three such searches produced very similar guild classifications, which converged to becodhe identical after further whole-dataset optimizations. This intrinsic guild structure placed all the monocots in one guild, together with other narrowleaved species. It is concluded that real guilds occur in this saltmarsh. Guild membership could subsequently be correlated with leaf morphology, as in a lawn previously examined by similar methods. This suggests that canopy interactions may be important in controlling species coexistence. 5 It is concluded that there are regularities in community structure in the saltmarsh examined that are strong evidence of assembly rules.