Patterns of plant community assembly in invaded and non-invaded communities along a natural environmental gradient

Questions: Is the stress gradient hypothesis (SGH) effective in predicting patterns of community assembly in coastal dune plant communities along the sea-inland environmental gradient? Does the introduction of invasive plant species disrupt these patterns, leading to a collapse in community structure? Location: Sandy coastal dunes of the Lazio region (Central Italy). Methods: We randomly sampled coastal dune plant species in 2 m 9 2 mplots (4 m 2). Multivariate techniques were used to classify these plots and allowed identification of four plant communities along the sea-inland environmental gradient, three of which were invaded by Carpobrotus aff. acinaciformis (iceplant). For each community, we computed two different indices of co-occurrence that take into account species abundance, and then used Monte Carlo permutations alongside appropriate null models to determine whether overlap in the use of space was greater (aggregated community structure) or smaller (segregated community structure) than what would be expected by chance alone. Results: For the four non-invaded communities, the analysis highlighted how community assemblage patterns were strongly tied to the sea-inland environmental gradient. In the two foredune communities, overlap in the use of space was greater than expected by chance, indicating an aggregated assemblage. In contrast, progressing along the zonation, assemblage patterns first shifted to random in the transition dune community and then became segregated in the fixed-dune community. As for the three communities that were also found in an invaded state, the presence of iceplant was associated with a random structure in community assemblage. Conclusions: Taken together, our results are consistent with the SGH, which predicts that along an environmental gradient, facilitation drives the assembly of plant communities where conditions are harsh, whereas competition is predominant where conditions are less severe. The comparison of the three invaded communities with their non-invaded counterparts revealed how iceplant invasion has led to a shift to randomness in community structure, as has been documented forwildfires and other disturbances.