Community-Level Impacts of Plant Invasions
Although the spread of exotic species is a global phenomenon, few studies have actually quantified the impact of invasive plants on the communities and ecosystems they invade or considered the degree to which different plant groups within invaded communities vary in response to invasion. Moreover, those studies that have examined such impacts have rarely considered whether these effects are reversed with invader removal. For many years, my graduate students and I have used both comparative and experimental studies in coastal and riparian systems to document that three invasive perennial plants species have had profoundly negative impacts on native communities – cape ivy (Delairea odorata), an evergreen vine native to South Africa (Alvarez & Cushman 2002); giant reed (Arundo donax), a large grass native to Asia (Cushman & Gaffney 2010); and iceplant (Carpobrotus edulis), a succulent native to South Africa (Magnoli et al. 2013). In all three cases, these taxa have reduced the richness and abundance or cover of native plant species. However, in many cases, removal of exotics also led to an increase in the richness and/or abundance of other exotic groups. Such results suggest that reducing the dominance of focal exotics may facilitate the proliferation of other, non-target exotics. This present land managers with a challenging dilemma, one that further research may not be able to resolve: Is a system dominated by many invaders preferable to one dominated by only one? This dilemma is probably not unique to our systems, but will arise in any regions where dominant invaders occur in landscapes that already have a well-developed exotic flora.
Although the spread of exotic species is a global phenomenon, few studies have actually quantified the impact of invasive plants on the communities and ecosystems they invade or considered the degree to which different plant groups within invaded communities vary in response to invasion. Moreover, those studies that have examined such impacts have rarely considered whether these effects are reversed with invader removal. For many years, my graduate students and I have used both comparative and experimental studies in coastal and riparian systems to document that three invasive perennial plants species have had profoundly negative impacts on native communities – cape ivy (Delairea odorata), an evergreen vine native to South Africa (Alvarez & Cushman 2002); giant reed (Arundo donax), a large grass native to Asia (Cushman & Gaffney 2010); and iceplant (Carpobrotus edulis), a succulent native to South Africa (Magnoli et al. 2013). In all three cases, these taxa have reduced the richness and abundance or cover of native plant species. However, in many cases, removal of exotics also led to an increase in the richness and/or abundance of other exotic groups. Such results suggest that reducing the dominance of focal exotics may facilitate the proliferation of other, non-target exotics. This present land managers with a challenging dilemma, one that further research may not be able to resolve: Is a system dominated by many invaders preferable to one dominated by only one? This dilemma is probably not unique to our systems, but will arise in any regions where dominant invaders occur in landscapes that already have a well-developed exotic flora.