Abstract
Ecological drift can override or interact with the effects of deterministic niche selection on small populations to drive the assembly of small communities. We tested the hypothesis that small communities are more dissimilar among each other because of ecological drift than large communities, which are mainly structured by niche selection. We used a unique dataset on insect communities sampled identically in a total of 200 streams in climatically different regions (Brazil and Finland) that differ in community size by fivefold. Null models allowed us to estimate the magnitude to which beta diversity deviates from the null expectation under a random assembly process while taking differences in species richness and relative abundance into account. Beta diversity of small communities was consistently closer to null expectations than beta diversity of large communities. However, although beta diversity and community size were strongly related in both regions, the type of relationship varied according to the type of dissimilarity coefficient. While incidence-based beta diversity was lower than expected and negatively related to community size only in Brazil, abundance-based beta diversity was higher than expected and positively related to community size in both regions. We suggest that ecological drift plays an important role in small communities by increasing the chances of species with low competitive ability to occur within the metacommunity. Also, while weak niche selection and high dispersal rates likely reduced variation in community composition among large tropical streams, niche selection was likely sufficient to cause non-random variations in genera relative abundances among large communities in both regions. Habitat destruction, overexploitation, pollution, and reductions in connectivity have been reducing the size of biological communities; these environmental pressures will make smaller communities more vulnerable to novel conditions and community dynamics more unpredictable, as random demographic processes should prevail under these conditions. Incorporation of community size into ecological models should provide conceptual, empirical and applied insights into a better understanding of the processes driving changes in biodiversity.