Ecosystem Impacts of Plant Invasions and Alternative Stable States

The impacts of biological invasions by non-native plants have been well studied from a variety of angles over the past two decades. Those exotic plants that alter ecosystem processes have been suggested to be especially ecologically important because they can create feedbacks that reinforce their own persistence.  Such feedbacks can move the invaded ecosystem beyond a threshold where ‘natural recovery’ is likely creating a persistent invasion that represents an ‘alternative stable state’.  Despite the widespread inference that plant invaders create alternative stable states, the existence of such states has rarely been tested.  Even when feedbacks between plants and ecosystem properties are shown to exist, they have not been studied over time.  Yet we know from classic succession theory that over time, feedbacks between plants and their environment shift as limiting resources change.  We review several examples of ecosystem altering plant invaders and argue that the existence of alternative states may be rare. Instead ecosystem altering plant invaders may facilitate succession towards either native or novel ecosystems.  Most studies of invasion are relatively short in duration. When studies are made over longer time periods, it may become clear that alternative state concepts may not apply.