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 a nd artificial stones was found to be highly correlated with stone surf ace area. The power function model provided a good description of the species-area relationship of the stones. For both types of stones, pas sive sampling was rejected as an explanation for the species-area rela tionship. The compilation of cumulative species richness, proceeding i n order from the smallest area to the largest area and vice versa reve aled 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 s ub-set of the species found on larger stones. To ascertain the signifi cance 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 significan tly. Passive sampling, tested by rarefaction, did not satisfactorily e xplain this increase in species richness. Thus, for stream stones, hab itat diversity appears to be a strong contributing factor for the incr ease in species richness with surface area.