THE EFFECT OF SCALE ON CONGENER COEXISTENCE - CAN MOLLUSKS AND POLYCHAETES RECONCILE BEETLES TO CILIATES

The traditional view based on Darwin and Gause states that taxonomical ly (and therefore ecologically) closely related species have to exclud e 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 (''coexi sting principle'') also exists. I treat the co-occurrence of congeneri c species as a statistical phenomenon to be studied on the basis of me ta-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 be etles) using both S/G-ratios and similarity indices. Species lists of different ecological range (from regional faunas to single samples) ar e compared with a neutral model of ''random occurrence''. The results depend significantly on the focus scale. The congeners usually lend 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 communitie s or samples). Coexistence of closely related species should be treate d 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 m ain processes affecting species diversity and distribution (competitio n, adaptation to the environment and speciation) manifest simultaneous ly but on different scales, and could mutually compensate each other. The scale factor should be taken into account to resolve the above con tradiction (''exclusion or coexistence'').