SPECIES RICHNESS OF STREAM STONES - AN INVESTIGATION OF THE MECHANISMS GENERATING THE SPECIES-AREA RELATIONSHIP

In an upland fourth-order stream, the fauna dwelling on both natural and artificial stones was found to be highly correlated with stone surface area. The power function model provided a good description of the species-area relationship of the stones. For both types of stones, passive sampling was rejected as an explanation for the species-area relationship. The compilation of cumulative species richness, proceeding in order from the smallest area to the largest area and vice versa revealed a negative fragmentation effect. Groups of small stones harboured an impoverished fauna compared with large stones of the same surface area. Small stones did not harbour a distinctive fauna, but simply a sub-set of the species found on larger stones. To ascertain the significance of habitat diversity in generating the species-area relationship, grooves were cut into artificial stones and the fauna on grooved and ungrooved stones compared. Increased grooving on stones of identical surface area, increased both abundance and species richness significantly. Passive sampling, tested by rarefaction, did not satisfactorily explain this increase in species richness. Thus, for stream stones, habitat diversity appears to be a strong contributing factor for the increase in species richness with surface area.