The population biology of invaders

It is shown that invading species are most likely to establish where levels of competition among the resident species are lowest. Thus, of the higher trophic levels, herbivores are more likely to establish than carnivores or detritivores. Invading plants are most likely to establish in communities where the average level of cover is low. Competition is certainly not the only important process involved in establishment, however. Analysis of invasions by insects released as biocontrol agents against weeds, where exploitation and interference are likely to be negligible, show that the probability of successful establishment is closely related to the insect's intrinsic rate of increase. The main biological causes of failure are generalist predators, with parasites and diseases rather less important. For herbivorous insects, host-plant incompatibility is a major cause of failure in establishment. A general model for the dynamics of an invading species is proposed, which stresses: (a), the importance of distinguishing explicitly between exploitation and interference competition; (b), the potential importance of generalist natural enemies; (c), the role of mutualists; (d), the importance of refuges of various kinds in affecting the probability of establishment. Examples are provided to emphasize how far we still are from real, predictive ability in relation to the likely success of proposed or anticipated invasions.