A Meta-Analysis of Competition in Field Experiments

A meta-analysis was conducted on field-competition experiments published in six journals over a 10-yr period. We analyzed the effects of competition on the biomass of organisms belonging to 93 species in a wide variety of habitats. Competition had a large effect overall, with a great deal of heterogeneity in that effect among organisms. There were large differences among trophic levels in competitive effects, but the relative magnitude of competition at different trophic levels was contrary to the predictions of ecological theory. Primary producers and carnivores displayed small to medium effects. In these two groups, interspecific effects did not differ from intraspecific effects, nor did effects differ in terrestrial versus aquatic habitats. The effects of competition on herbivores ranged from large effects on anurans and lotic arthropods, to medium effects on marine mollusks and echinoderms, to effects that were not statistically distinguishable from zero for terrestrial arthropods. Interspecific competitive effects among these herbivore groups were generally less than intraspecific effects. Among primary producers, the effects of competition were not different in high- and low-productivity habitats. Across all taxa, large organisms did not experience greater competitive effects than small organisms, and competitive effects did not depend on the size attained in the absence of competitors. The effects of competition were weakly density-dependent in cases in which it was possible to examine the effects of the density of neighbors. Experiments conducted on caged organisms resulted in greater competitive effects than those with free-roaming and unenclosed organisms. Experiments with small sample sizes, short durations, and poor experimental design were more variable than experiments that were larger, longer, and better planned.