The Effect of Scale on Congener Coexistence: Can Molluscs and Polychaetes Reconcile Beetles to Ciliates?

The traditional view based on Darwin and Gause states that taxonomically (and therefore ecologically) closely related species have to exclude each other more often than distant ones. Since Jaccard, this fact is regarded as evidence for species-to-genus ratios (S/G) being lower in insular or local biotas than in corresponding entire pools. But these arguments are not convincing, and the opposite point of view (coexisting principle) also exists. I treat the co-occurrence of congeneric species as a statistical phenomenon to be studied on the basis of meta-analysis methodology in terms of null-hypothesis testing. According to this hypothesis, congeners co-occur randomly and independently in samples from a total species pool. This is verified for various groups (marine ciliates and polychaetes, fresh-water molluscs and carabid beetles) using both S/G-ratios and similarity indices. Species lists of different ecological range (from regional faunas to single samples) are compared with a neutral model of random occurrence. The results depend significantly on the focus scale. The congeners usually tend to co-occur more frequently than expected, when habitat types or regions are treated as sampling units. However, the opposite tendency towards a separation is revealed on finer ecological scales (local communities or samples). Coexistence of closely related species should be treated as a complex scale-dependent phenomenon of niche separation (on the local level) within the pool of species similar in their environmental requirements and history (on the regional or biotopical level). The main processes affecting species diversity and distribution (competition, adaptation to the environment and speciation) manifest simultaneously but on different scales, and could mutually compensate each other. The scale factor should be taken into account to resolve the above contradiction (exclusion or coexistence).