The competition-colonization trade-off has long been considered an important mechanism explaining species coexistence in spatially structured environments, yet data supporting it remain ambiguous. Most competition-colonization research examines plants and the dispersal-linked traits of their seeds. However, colonization is more than just dispersal because rapid population growth is also an important component of colonization. We tested for the presence of competition-colonization trade-offs with a commonly used artificial assemblage consisting of protozoan and rotifer species, where colonization was the ability of a species to establish populations in patches. By ranking species according to their colonization abilities and their pairwise competitive interactions, we show that these species exhibit competition-colonization trade-offs. These results reveal that the competition-colonization trade-off exists within nonplant assemblages and that even in a laboratory setting, species are constrained to be either good competitors or colonizers but not both.

The predatory protozoan Stentor coeruleus Ehrenberg, 1830 is known to show photosensitivity and photodispersion, avoiding regions of high light intensity as an antipredation strategy. This physiological and behavioral response to light likely has demographic consequences. We manipulated light intensity to determine population responses of S. coeruleus and the resulting effects on its prey Colpidium striatum Stokes, 1886. We show that S. coeruleus maintained the highest population density under ambient light levels and low densities under both high and no light treatments. The results from the no light treatment were surprising because little work has been done on possible important behavioral and physiological processes cued by light. These results add power to the use of S. coeruleus as a model predator system to test ecological dynamics and processes associated with predation.