Allelopathy and exotic plant invasion

TLDR: It is argued that non-resource mechanisms should be returned to the discussion table as a potential mechanism for explaining the remarkable success of some invasive species.

Abstract: The primary hypothesis for the astonishing success of many exotics as community invaders relative to their importance in their native communities is that they have escaped the natural enemies that control their population growth – the `natural enemies hypothesis'. However, the frequent failure of introduced biocontrols, weak consumer effects on the growth and reproduction of some invaders, and the lack of consistent strong top-down regulation in many natural ecological systems indicate that other mechanisms must be involved in the success of some exotic plants. One mechanism may be the release by the invader of chemical compounds that have harmful effects on the members of the recipient plant community (i.e., allelopathy). Here, we provide an abbreviated compilation of evidence for allelopathy in general, present a detailed case study for Centaurea diffusa, an invasive Eurasian forb in western North America, and review general evidence for allelopathic effects of invasive plants in native communities. The primary rationale for considering allelopathy as a mechanism for the success of invaders is based on two premises. First, invaders often establish virtual monocultures where diverse communities once flourished, a phenomenon unusual in natural communities. Second, allelopathy may be more important in recipient than in origin communities because the former are more likely to be naive to the chemicals possessed by newly arrived species. Indeed, results from experiments on C. diffusa suggest that this invader produces chemicals that long-term and familiar Eurasian neighbors have adapted to, but that C. diffusa's new North American neighbors have not. A large number of early studies demonstrated strong potential allelopathic effects of exotic invasive plants; however, most of this work rests on controversial methodology. Nevertheless, during the last 15 years, methodological approaches have improved. Allelopathic effects have been tested on native species, allelochemicals have been tested in varying resource conditions, models have been used to estimate comparisons of resource and allelopathic effects, and experimental techniques have been used to ameliorate chemical effects. We do not recommend allelopathy as a `unifying theory' for plant interactions, nor do we espouse the view that allelopathy is the dominant way that plants interact, but we argue that non-resource mechanisms should be returned to the discussion table as a potential mechanism for explaining the remarkable success of some invasive species. Ecologists should consider the possibility that resource and non-resource mechanisms may work simultaneously, but vary in their relative importance depending on the ecological context in which they are studied. One such context might be exotic plant invasion.