Coastal ecosystems are often driven by a mixture of strong periodical forcing in abiotic conditions and intense stochastic events. Repeated disturbance of assembled communities by storm surges precludes equilibrium-based mechanisms of coexistence. Stochastic biomass removal will lead to successional shifts in species composition, and the dominance of species will not only be based on niche traits, but also on dispersal traits, depending on patch connectivity. Niche and dispersal processes jointly determine priority effects in the colonization sequence.
To disentangle and predict the effects of dispersal and niche requirements on the assembly of salt marsh communities we set up a field experiment in the Wadden Sea of Germany. Twelve islands were constructed on the tidal flats approx. 500 m offshore from the salt marsh. Each island consists of three heights, corresponding to the lowest, intermediate and upper level of the adjacent salt marsh. Six islands feature bare sediment. The other six were planted with sods from lower salt marsh, where only the intermediate elevation of the islands corresponds to the realized niche of this lower salt marsh community. Planted islands are expected to show lower successional speed due to competition by unpaid extinction debts as compared to islands with bare soil. The set up was mirrored in the existing salt marsh where dispersal limitation is absent.
Results/Conclusions
We report the initial phase of plant community assembly and the feasibility of this novel experimental approach. Experimental plots within the salt marsh and on the islands were comparable in terms of abiotic conditions although temperatures were slightly more extreme on the islands. Dispersal limitation had a strong effect on the colonization of the bare plots, with significantly less individuals colonizing the islands than the bare plots within the salt marsh. These species colonized plots at all elevations although they were absent in the natural salt marsh community at the lowest and highest elevation. The initial colonization sequence was characterized more strongly by dispersal traits than growth traits. Species were sorted according to their tolerances to salinity and inundation after the first season indicating that abiotic filtering leads to rapid payment of extinction debts. The island experiment forms a technically challenging but novel tool to address the dynamics of community and functional assembly in coastal meta-ecosystems. It allows to test whether the assembly of species composition is related to an assembly of functions.