The Ecological Characteristics of Idiosyncratic and Nested Diatoms

Nestedness represents the degree to which species assemblages are proper subsets of larger assemblages. Highly nested assemblages are characterized by a high number of species conforming to nested subset pattern and a low number of species which depart from nested pattern, that is, idiosyncratic species. The main aims of this paper were to (i) examine whether local stream habitat factors and spatial variables govern the number of nested stream diatom species at sites, (ii) to examine whether nested and idiosyncratic species differ in ecological characteristics, and (iii) to study if these distinctions have implications for turnover in stream diatom communities. Stream diatom communities showed a highly significant nested pattern, with observed matrix temperature of 28.4 °C. However, number of idiosyncratic species was high, constituting 44% of total species number of the survey. The number of nested species at sites was not related to any of the measured habitat factors or geographical location of the sampling sites. Idiosyncratic species were significantly (ANOVA: P=0.002) more widely distributed than nested species. Partial mantel tests showed that idiosyncratic species exhibited faster turnover along geographical and environmental distance than nested species. These data showed that although the degree of nestedness was highly significant, stream diatom communities were nevertheless characterized by a number of idiosyncratic species departing from the nested subset pattern. It seems that the compositional turnover is faster in communities that are dominated by idiosyncratic species suggesting that turnover diversity in diatoms may be governed more by the distribution of idiosyncratic species.