SPECIES RICHNESS, SPECIES COMPOSITION AND POPULATION-DYNAMICS OF PROTISTS IN EXPERIMENTAL MICROCOSMS

1. Laboratory experiments using communities assembled from bacteria plus one, two or four protist predator-prey pairs tested whether species richness affects the persistence, abundance and temporal variability of the protist species. Comparisons among six different food webs of four protist species tested whether the effects of species composition rival the effects of species richness. 2. Most populations in species-rich food webs were less abundant than populations of the same species in food webs with fewer species. Increased species richness increased the variability of one species. Combining predator-prey pairs increased the number of extinctions. 3. The species composition of food webs also affected the persistence and mean abundances of many species, and changed the variability of two species. Strong effects of species composition may limit the ability of ecologists to explain population dynamics by examining general features of community structure.