Studies of the ecology of macroorganisms have produced a body of theory about the nature of biological interactions and their effects on species in the field. This body of theory describes the ways that species affect each other and also predicts the outcome of specific interactions such as competition or predation. It is reasonable to ask whether the same body of theory applies to species of microorganisms. We performed a simple field experiment to test the hypothesis that biological interactions influence species of cellular slime molds living in forest soils. Members of the guild of cellular slime mold species co—occur on both microhabitat and geographic scales and in laboratory cultures, the species all consume the same food resources. We experimentally elevated the density of one cellular slime mold species and detected significant population responses in the remaining species in the cellular slime molds: (1) we observed resource limitation in the species that we added, and (2) we observed that the densities of cellular slime mold species are interrelated.

Populations of the cellular slime mold Polysphondylium pallidum contain clones that differ in their abilities to use bacterial resources. We found abundant diversity among isolates obtained from plots of several square metres and from single soil cores 1.1 cm in diameter. The variation we were measuring was not due to genetic changes induced by our manipulation. Previously, isolation of different species of cellular slime molds from sample areas, metres to hectares in extent, has prompted studies of mechanisms of species coexistence in the face of assumed competition for bacterial resources. We conclude that competition has not only failed to eliminate different species from the community but has failed to eliminate clonal diversity from within species. Previous explanations of species coexistence need reevaluation.

A guild of cellular slime molds (CSM) consisting of two isolates from each of five species, representing two genera, and obtained from the same square meter of forest soil exhibited extensive growth rate variation when tested on a suite of 18 bacteria isolated from the same soil. Significant growth rate differences were found at each taxonomic level examined: among species of different genera, between genera, among species within genera, and between isolates (=clones) within species. The type of bacteria used as prey determined the relative rank of the growth rates in different CSM isolates, as well as the taxonomic level at which significant differences were found. We suggest a possible reconciliation between a previous hypothesis, based on competition, and contradictory experimental work on resource partitioning in this guild of bacterial predators. Our results raise a question about the efficacy of using single genotypes to represent a species when ecological ideas are developed through laboratory investigations.

The presence of Ca2+ (concentration, ca. 0.4 mM) in the growth medium causes cells of Desulfovibrio vulgaris (Hildenborough) to aggregate, leading to a decrease in plating efficiency. When the Ca2+ concentration in the medium was reduced 20-fold, cell aggregation did not occur and the plating efficiency increased from an initial value of 34% to a final value of 56%.