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Abstract. Species richness can affect both the size and stability of populations. Many simple theories predict that complex speciose communities should be less stable than simple ones. In contrast, stability of collective community attributes, such as functional groups, may be greater in more complex communities. We used aquatic microcosms containing microbial food webs to generate an experimental gradient of species richness. Biweekly counts of population samples permitted estimation and comparison of density, persistence time, and temporal variation in density for populations and trophic functional groups embedded in communities of varying richness. Densities of nearly half of the species declined as species richness increased, demonstrating broad community-wide density compensation that resulted from more abundant competitors and predators in species-rich communities. Persistence time, a measure of population stability, also declined with increasing species richness. Another measure of population stability, temporal variation in density, indicated that abundances of most species embedded in diverse communities varied no more than in less diverse communities. In contrast to results for individual species, temporal variation of entire functional groups composed of multiple species decreased as species richness increased. The stability of individual populations was not predicted by that of functional groups. Interspecific interactions and statistical averaging may both contribute to the reduced temporal variability of functional groups created by aggregating multiple species.
Key words: complexity; density compensation; diversity; functional groups; microbes; persistence; protists; stability.
INTRODUCTION
Despite a long history of theoretical interest (MacArthur 1955, Elton 1958, May 1972, Pimm 1982), there are few empirical studies showing that population dynamics depend on community complexity (Pimm 1991, Tilman 1996). This fundamental uncertainty underscores the need for experiments linking species richness and population dynamics. One reason for the dearth of studies is the difficulty of measuring population dynamics in complex natural communities. The difficulty stems from the long duration of studies needed to monitor dynamics for several generations, and the uncertainty in assessing total species richness. Studies of communities of microorganisms assembled in microcosms can yield insights about dynamics that would be difficult or impossible to tease from natural systems (Gause 1934, Hairston et al. 1968, Vandermeer 1969, Luckinbill 1979, Lawler 1993a, b, McGradySteed et al. 1997). Microcosms of different complexity can be readily assembled, and generation times of microorganisms are sufficiently short so...