Community assembly of terrestrial testate amoebae: how is the very first beginning characterized?

Testate amoebae play an important role at the very first beginning of succession on land. We used litterbags buried into four different soils to study the early colonization (which occurred within less than 55 days) and establishment of testate amoebae. The litterbag cellulose exposed at the youngest mining site poor in nitrogen and phosphorus was colonized firstly in high abundances, whereas the substrate introduced into the reference sites of undisturbed soil was colonized slowly and in low densities. Besides the (expected) small-sized r-strategists (e.g., Euglypha rotunda, Tracheleuglypha dentata, and Trinema lineare), large-sized K-strategists (e.g., Centropyxis spp., Phryganella acropodia) occurred in remarkably high densities on all sites. Species that colonized the cellulose in high densities (e.g., P. acropodia and T. dentata) were found extremely rarely in the adjacent source substrate and vice versa, stressing the importance of the target substrate quality. In the course of the experiment, the influencing environmental factors became more complex, as shown by redundancy analysis (RDA). Concerning the amoebal community, there was a change from variability to stability, as visualized by cluster analysis. Adjacent litterbags within an investigation site revealed amoebal species and abundances with an increasing similarity during exposition time, whereas the litterbags between the four investigation sites were colonized differently. These observations point to a stochastic (variable) beginning of community assembly, changing to a more deterministic (stable) course. No species replacement has been observed, which is an essential part of most successional theories. Thus, the more flexible concept of "community assembly" should be considered instead of "succession" for protozoa. The stochastic beginning of community assembly and the lack of species replacement are explained by a neutral community model.