Document: ZAC-3-52-47

Priority effects determine the outcome of competition between species with similar competitive abilities.

LONG, Z.*

Rutgers University, New Brunswick, New Jersey, USA ¹

Abstract:
Various theories use differences between species resource consumption rates and resource requirements to predict outcomes of competition. Some evidence, however, demonstrates that outcomes of competition may not be deterministic, but depends on initial abundances of competing species. Evidence for such priority effects comes mainly from experiments that are short in duration relative to the generation times of the organisms studied. It remains unclear if differences caused by initial conditions can persist over time or if competition always leads to convergence on a deterministic outcome. I tested if certain species always outcompete others regardless of initial conditions or if differences in initial conditions can persist despite competition using aquatic microcosm experiments. I grew four bacterivores, Paramecium caudatum, Paramecium tetraurelia, Tetrahymena thermophila, and Colpidium striatum, in monoculture and measured their ability to reduce levels of their resource, bacteria, to determine relative competitive abilities. P. caudatum, P. tetraurelia, and C. striatum did not differ significantly in the level to which they reduced bacteria, but they all reduced bacteria to significantly lower levels than T. thermophila. I allowed each species to reach high abundance in monoculture, and then subjected each species to invasion by low numbers of the remaining other three bacterivores, and invasion by low numbers of the other three bacterivores and a generalist predator, Actinosphaerium. All species that were initially dominant maintained dominance after approximately six generations, except T. thermophila which declined significantly in abundance when subjected to invasion by the other three bacterivores. The initially high abundance of T. thermophila did not prevent invasion by the better competitors. These results identify a clear priority effect: When species are similar in competitive ability initial conditions determine the outcome of competition.

Keywords: Priority effects, competition, aquatic microcosms

This abstract is being presented at: 3:15 PM in session:
Oral Session #11: Trophic Cascades.