Introduction

Invasive plants negatively impact wildlands by decreasing biodiversity, reducing production, displacing native species, degrading systems, and altering ecosystem functions and processes (DiTomaso 2000; Masters and Sheley 2001; Davies and Svejcar IN PRESS). Invasive species are the second leading threat to biodiversity after habitat destruction (Wittenberg and Cock 2001; Randall 1996; Pimm and Gilpin 1989). The negative impacts of invasive plants are a product of their ability to out-compete native vegetation for resources and/or alter conditions to facilitate their dominance to the detriment of native plant species. For example, invasive annual grasses, such as Bromus tectorum L. (cheatgrass) and Taeniatherum caput-medusae (L.) Nevski (medusahead), compete effectively with native vegetation for resources (Hironaka and Sindelar 1975; Mack 1981; Goebel et al. 1988; Young and Allen 1997; Young and Mangold 2008) and increase fire frequency to the detriment of native vegetation (Stewart and Hull 1949; Torell et al. 1961; Whisenant 1990; Young 1992) (Fig. 1). The negative economic impacts of invasive plants species were estimated to be $13 billion a year in the United States in 1994 (Westbrooks 1998). Though not solely focusing on invasive plant species, Pimentel et al. (2000) estimated that nonindigenous species caused environmental degradation and economical losses of about $137 billion in 2000. However, the rapid increase in the area infested with invasive plant species has also escalated the negative impacts of these invaders. Thus, the economic impacts of invasive plant species has probably increased substantially and multiplied every year as invasive species continue to spread and increase their dominance.

The traditional approach to managing invasive plant species on wildlands has been to control already established infestations followed by efforts to restore native vegetation. This reactive approach is expensive and often fails to remove the invasive plant species and/or restore displaced native plant species. Once invasive plant species have established rapidly spreading populations, eradication is rarely an option (Mack et al. 2000; Eiswerth and Johnson 2002). Eradication of invasive species can also produce negative secondary effects if the invaders have filled the functional roles of native species in the ecosystem (Zavaleta et al. 2001). Once an invasive species has established rapidly spreading populations, even the control of the invader is a significant sink of resources and time (Huenneke 1996). Control is often temporary and the invader often reestablishes dominance within a few growing seasons because restoration of native vegetation fails.

Restoration of native vegetation in wildlands infested with invasive plants is rarely successful and prohibitively expensive (Vitousek et al. 1997). Restoration is exceedingly difficult because invasive plants can leave a negative legacy of physical or chemical alterations to the site or a persistent invasive plant seed bank (D'Antonio and Meyerson

2002). These alterations may make it improbable that the native plant community can reclaim the site, even if the invasive species is eradicated (Cronk and Fuller 1995; D'Antonio and Meyerson 2002). Steep and rocky terrain common to wildlands also limits the ability to control invasive plant infestations and restore native plant communities. Terrain can preclude the use of conventional restoration tools and often results in the use of more costly and/or less successful methods. Complete restoration of some wildlands may also be limited by a lack of propagule sources for all the native plant species displaced by the invader. Large amounts of resources have been intensively spent on efforts to restore a few infested wildlands, yet invasive plants continue to spread as emerging populations and expanding established infestations. The conventional reactive approach to invasive plant species management has been largely ineffective at reducing impacts of invasive plants on wildlands, suggesting there is a critical need to adopt a different strategy. A comprehensive strategy with a primary focus of proactively preventing invasive plant establishment and spread will be required to successfully reduce the impacts of invasive plant species on wildlands.

Figure 1. Taeniatherum caput-medusae (annual grass dominating the community) invaded Artemisia tridentata-bunchgrass plant community (left) and non-invaded Artemisia tridentata-bunchgrass plant community (right). Note the lack of diversity in plant species composition and structure in invaded community compared to the non-invaded plant community. Also note the greater fine fuel continuity and quantity in the invaded compared to non-invaded plant community. This fuel loading can result in an increased fire frequency that is determinantal to native vegetation (Artemisia tridentata skeleton at top of invaded community).

Figure 1. Taeniatherum caput-medusae (annual grass dominating the community) invaded Artemisia tridentata-bunchgrass plant community (left) and non-invaded Artemisia tridentata-bunchgrass plant community (right). Note the lack of diversity in plant species composition and structure in invaded community compared to the non-invaded plant community. Also note the greater fine fuel continuity and quantity in the invaded compared to non-invaded plant community. This fuel loading can result in an increased fire frequency that is determinantal to native vegetation (Artemisia tridentata skeleton at top of invaded community).

Adopting an approach to managing invasive plant species that focuses on preventing new infestations and limiting expansion of existing infestations has the potential to preclude the need for restoration on millions of wildland hectares. However, this does not eliminate the need to restore already invaded wildland plant communities, but suggests that with current resource and knowledge constraints that prevention is a more logical priority than restoration in most circumstances. Preventing invasive plant infestations would be more cost-effective and successful than restoration attempts after invasive plant have established infestations (Peterson and Vieglasis 2001; Simberloff 2003; Zavaleta 2000). In fact, the Office of Technology Assessment (1993) estimated that every dollar spent on prevention and early control of invasive plants prevented $17 in later expenses. Prevention is an essential component to developing a successful invasive plant management programs (Sheley et al. 1996; DiTomaso 2000; Davies and Sheley 2007a). Furthermore, the cumulative benefits of preventing the establishment of invasive plant species infestations are exponential. Each infestation prevented eliminates all the future descendant infestations.

However, prevention is probably the most under-utilized strategy for managing invasive plant species in wildlands. The limited deployment of prevention may be a product of the lack of information detailing how to develop successful prevention programs. Most prevention discussions, excluding Davies and Sheley (2007a), are little more than a list of common-sense considerations (e.g. Sheley et al. 1999; Westbrooks et al. 1997). We propose that successful prevention of invasive plant infestations should focus on three primary strategies: 1) limiting the spatial dispersal of propagules (i.e. reducing propagule pressure), 2) maintaining or increasing the ability of wildland plant communities to resist invasion (i.e. biotic resistance), and 3) systematically searching for and eradicating emerging infestations.

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