Null model approaches to evaluating the relative role of different assembly processes in shaping ecological communities
Various local processes simultaneously shape ecological assemblages. β-diversity is a useful metric for inferring the underlying mechanisms of community assembly. However, β-diversity is not independent of γ-diversity, which may mask the local mechanisms that govern community processes across regions. Recent approaches that rely on an abundance-based null model could solve this sampling issue. However, if abundance varies widely across a region, the relative roles of deterministic and stochastic processes may be substantially misestimated. Furthermore, there is additional uncertainty as to whether null models used to correct γ-dependence in β-diversity should be independent of the observed patterns of species abundance distributions or whether the models should reflect these patterns. Here, we aim to test what null models with various constraints imply about the underlying processes shaping β-diversity. First, we found that an abundance-driven sampling effect could substantially influence the calculation of γ-corrected β-diversity. Second, we found that the null models that preserve the species abundance patterns could better reflect empirical patterns of spatial organization of individuals. The different implications generated from different applications of the null model approach therefore suggest that there are still frontiers regarding how local processes that shape species assemblages should be quantified. Carefully exploring each facet within different assembly processes is important.
KeywordsAbundance β-diversity Community assembly Biogeographical gradient Species abundance distributions
The authors acknowledge those who contributed to the collection of data for the data sets used here. We thank T. Shiono for his critical comments on the earlier draft. Our original data were collected through studies under the budgetary support of the Sumitomo Foundation, the Japan Securities Scholarship Foundation, the Global COE Program (E03) from the Japan Society for the Promotion of Science (JSPS), and the Graduate School of Environment and Information Science, Yokohama National University (Category B). The study was partly supported by a JSPS Grant-in-Aid for Young Scientists (Category A). We appreciate further constructive comments from D. Laughlin and the two reviewers. The experiments comply with the current laws of the country (Japan) in which the experiments were performed.
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