The Maintenance of Taxon Diversity in an Asexual Assemblage: An Experimental Analysis

Clonal complexes can be viewed as model systems that permit an assessment of factors organizing a guild whose members show extreme ecological similarity. The present study couples a description of distributional and abundance patterns of unpigmented clones of Daphnia pulex at a low—arctic site with manipulation experiments that test the environmental and biological variables responsible for generating these patterns. The descriptive studies, carried out in 1985 and 1989, revealed 36 clones in 150 habitats with individual ponds containing an average of 1.7 and a maximum of 5 clones. Distributional patterns and relative abundances of the clones were stable over the 4—yr interval. Three clones dominated the assemblage, comprising >75% of the animals. One of these clones dominated saline habitats while the other two displayed a checkerboard distribution in low—salinity ponds. The latter distributional pattern appeared linked to the presence of a predatory copepod, Hesperodiaptomus arcticus, in some ponds. Manipulation experiments showed that abiotic variables (e.g., salinity) played a role in excluding clones from some habitats, but that many habitats were invasible by a broader spectrum of clones than was ever detected in them. Experiments showed that clones were ordinarily superior in their home environments, supporting the role of competition in limiting local richness. The experimental results were not totally congruent with the clones' natural distributions, as one clone became dominant in all of the low—salinity manipulation ponds. This outcome was reversed by introducing H. arcticus into ponds. The results confirm that clonal diversity is maintained by physical and biological variation in the system, and demonstrate the substantial taxon diversity can be sustained in the absence of marked ecological divergence.