Competitive Effect and Response: Hierarchies and Correlated Traits in the Early Stages of Competition

(1) Competitive ability can be compared between species in two ways: effect of different neighbour species on performance of a single target species or response of different arget species to a single neighbour species. In a 5-week glasshouse experiment, an additive design was used for all combinations of seven species as both target and neighbour species to determine if there were consistent hierarchies in competitive effect and/or response, what traits of individuals determined position in these hierarchies, and whether or not effect and response competitive ability were related during the early stages of competition. (2) Five weeks after sowing, significant non-linear regressions of target biomass on neighbour density were found for 59% of the forty-nine species combinations and significant linear regressions on neighbour biomass were found for 51% of the species combinations. The slopes of these regressions represent per-plant and per-gram competition coefficients, respectively. (3) Neighbour species differed strongly in competitive ffect per plant. Differences in effect per gram, response per plant, and response per gram were much weaker. Nevertheless, consistent competitive hierarchies were found for both effect and response on both a per-plant and per-gram basis. (4) Different traits determined position in the effect and response hierarchies. Neighbour species with larger seed mass and larger maximum potential mass had stronger per-plant competitive ffects, whilst neighbour species with higher maximum relative growth rates had stronger per-gram competitive effects. The reverse of this latter pattern was seen for competitive response: target species with lower maximum relative growth rates were better response competitors. Mean effect and response competitive ability of the seven species were uncorrelated with each other. (5) These differences in traits associated with strong effect and strong response competitive ability emphasize the importance of distinguishing between them in experimental studies, at least during the early stages of competition.