Soil Microarthropod Communities Along Salt Marsh Transects of the Wadden Sea Are Predominantly Structured by Niche...

The relative importance of deterministic versus stochastic processes in community assembly is widely discussed in ecology. Stochastic models explain community assembly by ignoring the niches of species, whereas deterministic models assume the niche concept to be key for understanding the assembly of species. Whether stochastic or deterministic processes dominate in shaping the community composition of soil animals is gaining increased attention. Here, we investigated the spatial heterogeneity of soil microarthropod communities (Collembola, Oribatida, Mesostigmata) along a salt marsh transect from the upper salt marsh (USM) to the lower salt marsh (LSM) to the pioneer zone (PZ) on three islands, that is, Norderney, Spiekeroog, and Wangerooge, in the Wadden Sea of Germany. We hypothesized that microarthropod communities are predominantly structured by niche-based processes and change in a deterministic way from the USM to the LSM to the PZ, whereas microarthropod communities differ little between the three islands of similar history and position in the Wadden Sea. Supporting our hypothesis, Oribatida and Mesostigmata communities differed more strongly between the three zones than between the islands indicating that environmental factors changing along the saltmarsh gradient select for specific mite communities in a deterministic way. Collembola communities also differed between the three zones but also between the islands indicating that they are structured by both environmental filtering as well as stochasticity. However, even the differences in mite and Collembola communities between the islands may-at least in part-be explained by environmental filtering, for example, by differences in nutrient input from the Ems river estuarine. Overall, the results indicate that stochasticity plays only a limited role in structuring microarthropod communities in the dynamic marine-terrestrial boundary of salt marshes pointing to the importance of niche differentiation and environmental filtering.