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On the Prevalence and Relative Importance of Interspecific Competition: Evidence from Field Experiments

In a strictly defined sample of competition studies using controlled field experiments, covering 215 species and 527 experiments, competition was found in most of the studies, in somewhat more than half of the species, and in about two-fifths of the experiments. In most of these experiments interspecific competition was not distinguished from intraspecific competition. In the few studies in which the two were separated, interspecific competition was the stronger form in about onesixth of all experiments done. When competition was demonstrated, intraspecific competition was as strong or stronger than interspecific in three-quarters of the experiments. Some evidence from this literature survey suggests that negative results may be underrepresented, so that the absolute values of these figures may be too high. Since this bias should apply also to studies of all taxa, habitats, or other interactions it should not greatly affect estimates of the relative prevalence of competition. Since these estimates come from field experiments open to other influences such as predators, grazers, weather, disturbances, etc., they should provide a fair approximation of the relative prevalence of interspecific and intraspecific competition in natural ecological communities. The prevalence of competition in these studies varied. Marine organisms showed consistently higher frequencies of competition than terrestrial ones as did large-sized organisms as compared to smaller ones. Plants, herbivores, and carnivores showed similar frequencies of competition in all habitats compared. The incidence of competition varied considerably from year to year and place to place. In some categories, evidence concerning competition is sparse. More studies are needed of all freshwater species, marine vertebrates, parasites, effects on resource partitioning, and particularly the relative strengths of interspecific versus intraspecific competition. When both members of a pair were studied and some competition found, only one member was affected in well over half the experiments. Such strong asymmetrical competition is not always consistent in direction; reversals in the rank order of competitive superiority have been demonstrated by field experiments and direct observations. Some positive interactions were found. These may have been a consequence of actual positive influences or of negative ones acting indirectly through other species. The latter may also apply to some of the negative interactions interpreted as competition in these studies. If only the input and output of an experiment are known, it is difficult to decide what mechanism produced the observed effect. While many of the experiments probably have been correctly interpreted, the present survey illustrates how difficult it is to produce a clear and unambiguous demonstration of interspecific competition.