Evidence for Competitive Exclusion of Introduced Natural Enemies in Biological Control: An Addendum

Abstract

The direct relationship between cigarette smoking and lung cancer is well known. However, this does not mean that all smokers will necessarily develop lung cancer or that nonsmokers will never contact lung cancer. The same kind of reasoning should be applied to the question of competitive exclusion of introduced natural enemies in biological control. In a previous paper, we established an inverse relationship between (1) the rate of (or percent) establishment of a large group of introduced predators and parasites and (2) the number of exotic incumbent species of natural enemies present and the number released simultaneously (see Ehler and Hall [1982] for details). We noted that the empirical evidence was consistent with the competitive-exclusion hypothesis and that therefore the hypothesis should not be rejected. We did not claim that competitive exclusion has occurred frequently or that our results were conclusive, as some have suggested, and we most certainly did not conclude that all (or most) establishment failures involving multiple-species introductions were due to competitive exclusion. Because too many people have read their own erroneous conclusions into our paper, we feel compelled to set the record straight.