Ready or Not, Garlic Mustard Is Moving In: Alliaria petiolata as a Member of Eastern North American Forests

426 BioScience • May 2008 / Vol. 58 No. 5 www.biosciencemag.org T impact of introduced, invasive species on communities and ecosystems is one of today’s most pressing global environmental problems (Wilcove et al. 1998, Mack et al. 2000). Biological invasions are a leading cause of extinction and biodiversity loss (Wilcove et al. 1998), and invasive plants are permanently altering natural communities and their ecological characteristics (Mack et al. 2000). To date, generalizations about the mechanisms for invasive species’ success and the susceptibility of communities to invasion have proven elusive. Consequently, it may make more sense to focus on individual species and their large-scale impacts than on general theories for all invasive species. Garlic mustard, Alliaria petiolata (M. Bieb.) Cavara and Grande (formerly Alliaria officinalis), is one of the most problematic invasive plant species in eastern North American forests (Blossey et al. 2001, Stinson et al. 2006). Currently listed as a “noxious weed” in 6 of the 34 US states it has colonized (Nuzzo 2000), garlic mustard is spreading at an alarming rate (Nuzzo 1993a). Although garlic mustard possesses many of the characteristics of an “ideal weed” as defined by Baker (1974), this invasive, biennial forb is unique in that it is shade tolerant and capable of persisting, indeed thriving, in the forest understory. Also, unlike many other herbaceous invasive plants (D’Antonio et al. 1999), garlic mustard does not necessarily require a disturbance to become established or to proliferate (Meekins and McCarthy 2001). In this article, we review the current understanding of garlic mustard as a highly successful invasive plant in eastern North America. We describe the introduction and early invasion of garlic mustard, and then discuss the mechanisms that appear to explain the widespread success of this non native plant species in eastern North American forests. Finally, we discuss the ecological impacts of garlic mustard invasion (figure 1).