Global Change, Global Trade, and the Next Wave of Plant Invasions

www.frontiersinecology.org © The Ecological Society of America O need only peruse a nursery catalog or visit a local gardening center to realize the enormous array of plant choices available to the everyday American gardener. Unfortunately, this wealth of consumer choices comes at a steep cost. Non-native plants introduced through the horticulture trade often become invasive (Mack and Lonsdale 2001; Reichard and White 2001), which we define here as introduced species whose populations are surviving and reproducing beyond the location of introduction (sensu Blackburn et al. 2011). Although only a portion of species that become invasive cause ecological damage (Williamson and Fitter 1996; Sax et al. 2002), and some have benefited biodiversity (Davis et al. 2011; Schlaepfer et al. 2011), invasive plants as a whole substantially reduce native species abundance and diversity (Vilà et al. 2011) and alter ecosystem function (Ehrenfeld 2010; Vilà et al. 2011). Several wellknown invasive plants in the US were deliberately introduced, including kudzu (Pueraria lobata; planted to stabilize soil), oriental bittersweet (Celastrus orbiculatus; planted for aesthetics), purple loosestrife (Lythrum salicaria; planted for aesthetics), and tamarisk (Tamarix spp; first planted for aesthetics and later to act as wind breaks). Indeed, Mack and Erneberg (2002) estimated that over 60% of established, non-native species in the US were deliberately introduced. Moreover, the introduction process can select for species more likely to become invasive, because traits useful in horticulture – such as rapid establishment, broad climatic tolerance, and high resource allocation to flowers – can also increase invasiveness (Mack 2005). Global change is already aiding the spread of invasive species and increasing their ecological impacts (Dukes and Mooney 1999; Bradley et al. 2010a). As global change proceeds, however, it will influence not just the success of introduced plants but the introduction process itself (Hellmann et al. 2008). Gardeners are poised to plant new species from warmer regions, as earlier onset of spring (Schwartz et al. 2006) and warmer temperatures decrease the requirement for winter-hardiness in ornamental plants (Arbor Day Foundation 2006). Similarly, as human populations increase in the arid and semiarid regions of the world, such as the American Southwest (Mackun and Wilson 2011), demand for drought-tolerant plants is expanding – a trend likely to accelerate in areas where climate change exacerbates drought (eg Seager and Vecchi 2010). At the same time, economic globalization offers opportunities to import new types of plants from previously untapped parts of the world. Here, we review how global changes in trade and climate could influence supply and demand for introduced ornamental plants. We predict the consequences for future plant invasions in REVIEWS REVIEWS REVIEWS