Invasiveness Ranking System for Non-Native Plants of Alaska

IntroductionThe control of invasive, non-native plants is of increasing concern in ecosystem management across the world(Pimentel 2002). The invasion of non-natives into intact ecosystems is recognized by scientists and landmanagers as one of the primary causes of biodiversity loss, ranking second only to outright habitat loss (Pimmand Gilpin 1989, U.S. Congress 1993, Myers 1997, Stein et al. 2000). More than just the native biodiversity isthreatened, however; the introduction of invasive non-natives threatens community structure and compositionand ecosystem processes (Cronk and Fuller 1995, Walker and Smith 1997, Cox 1999). Not all non-native speciesare equally harmful. Most non-native species that are introduced are poorly adapted to their new environmentsand are unable to establish viable populations. Of those that can establish, only a small subset proceed to invadenative ecosystems. Additionally, most introductions involve relatively few individuals and small populations ofany species are much more susceptible to extirpation (see Taylor and Hastings 2005). Establishment is highlydependent on ecological and climatic conditions. As resources for managing invasive plants are limited, the needto evaluate and rank non-native species is a primary concern before expensive management is attempted, so thatthe most threatening species may be addressed first (Wainger and King 2001).Focusing management efforts on those species that have the capacity for rapid expansion in natural settings yetare currently at low population sizes should be of highest concern (D’Antonio and Meyerson 2002). Eradicationeffort rises exponentially with infestation size, and elimination of non-natives is most likely to be successful ininfestations smaller than 100 acres (Prather, et al. 2005). In particular, we recognize the need to evaluate theinvasiveness of non-native plants in the context of their current and possible ecoregional distributions. This needto identify which species have the greatest potential for establishment and spread was highlighted as a necessaryaction in a strategic plan for noxious and invasive plant management in Alaska (Hébert 2001).Non-Native Plants in AlaskaAlaska, the Yukon Territory, Northwest Territories, Nunavut, and northern British Columbia have remained relativelyunaffected by the negative consequences of non-native plant establishment that has plagued most regionsof the world. However, Alaska is at the cusp of facing serious ecological problems associated with introducedplants in both natural and human-altered landscapes (Carlson and Shephard 2007). While most non-nativeplant populations in Alaska are small and largely restricted to anthropogenically disturbed areas, a number ofintroduced plant species have begun to threaten intact biological communities and impact ecological conditions(Carlson and Shephard 2007, Conn et al. in press).A total of 157 non-native plant taxa have naturalized (i.e., form self-perpetuating populations) in Alaska; anadditional 136 non-native taxa are apparently ephemeral (Carlson and Shephard 2007). The naturalized taxarepresent roughly 14 percent of the total flora (Carlson and Shephard in press and see Hultén 1968), with newnon-native species recorded every year. This is not a particularly high percentage relative to most other states, forexample 18 percent of California’s flora (Hickman 1993), approximately 20 percent of Oregon’s flora (Kaye pers.comm.), and 49 percent of Hawaii’s flora, (Randall and Hoshovsky 2000) are non-native. Additionally, mostnon-native plants in Alaska are restricted to the small area of anthropogenic disturbance. Alaska has a populationdensity of only 0.39 people per square kilometer, relative to the national average of 29 per square kilometer. Themajority of infestations occur along the road network, despite the low densities of one kilometer of road per68 square kilometers of land; the national average is one to 1.6 and the state with the second lowest density ofroads is Nevada with 1 to 4 kilometers of road per square kilometer of land (State of Alaska 2004). Additionally,introductions and infestations of non-native plants on agricultural lands are not as serious a concern as in otherregions since only 0.009 percent of Alaska’s land is devoted to agriculture.Despite a lower overall presence of non-native plants and reduced opportunities for introduction by humans inAlaska, a few species are showing alarming signs of spreading into natural areas. In particular, Melilotus alba, white1