The spread of invasive plant species across wildlands is dependent not only on the spatial dispersal of its propagules, but also whether or not these propagules can become established in existing plant communities. Discussions of prevention or any other approaches to invasive plant management are of little consequence if the invasive plant species is dispersing to an environment unsuitable for its survival. Thus, the present discussion focuses on environments that the invasive plant species can survive and thereby invade. The ability of a plant community to limit establishment of invasive plant species can be thought of as "plant community invasibility" or "biotic resistance to invasion". Biotic resistance is influenced by species diversity, functional group diversity, dominance, and disturbance. The contribution of these factors to biotic resistance is their influence on the resource sequestering in relation to resource availability of the existing plant community. Decreasing biotic resistance increases safe sites and other resources that are available for invasive plant species to capitalize on and become established.
Because negative relationships between native and invasive plant diversity have been documented (Elton 1958; Tilman 1997; Knops et al. 1999; Levine 2000; Brown and Peet 2003), diverse plant communities have been assumed to be more resistant to invasion than less diverse plant communities. However, at large spatial scales more diverse plant communities tend to have less resistance to invasion (Lonsdale 1999; Stohlgren et al. 1999, 2003). This discrepancy in the impact of diversity on plant community invasibility may be the result of spatial heterogeneity in the environment driving species diversity in landscapes, thus landscapes with more environmental diversity can support a greater diversity of native and exotic plant species (Davies et al. 2005). Similarly, temporal heterogeneity in climate conditions could increase the opportunities for the establishment of a greater diversity of plant species.
Plant functional group diversity may be a better indicator of plant community resistance to invasion than species diversity. Plant functional group diversity has profound implications to community stability (Hooper and Vitousek 1997; Tilman et al. 1997; Hooper 1998; Dukes 2001; Davies et al. 2007). Davies et al. (2007) and Pokorny et al. (2005) suggested that functional group diversity is critical to plant community resistance to invasion. Plant communities with higher diversity of functional groups used more resources than plant communities with fewer functional groups (Davies et al. 2007).
However, some plant communities may have only one dominant functional group that is important to reducing invasibility (Davies 2008). Thus, dominant plant species may also be important in determining plant community resistance to invasion. In a tallgrass prairie plant community, dominance was of greater importance to determining invasibility than species richness (Smith et al. 2004). Jiang et al. (2007) also reported that species composition was important to determining invasibility. Further supporting this assertion is that disturbances that damaged dominant species increased survival of invaders compared to disturbances that did not damage dominants (Burke and Grime 1996).
Invasibility of plant communities is also influenced by disturbance. Resistance to invasion decreased as disturbances increased bare ground (Burke and Grime 1996). Disturbances have been generally assumed to increase resource availability to invaders and disturbance prevention has been suggested to reduce the potential for exotic plant invasion (Sheley et al. 1999, Clark 2003). However, disturbances may be needed in some plant communities to maintain their resistance to invasion (Davies et al. 2008). Preventing disturbances can result in the loss of important mechanisms that allow plant communities to adapt to external pressure (Groffman et al. 2006).
Invasibility is clearly not controlled by a single casual factor, but by interactions among them. For example, community structure (Smith and Knapp 1999) and composition (Burke and Grime 1996) interact with disturbance to influence plant community invasibility. Relationships between plant community characteristics and invasibility likely differ among plant communities (Jiang et al. 2007). Plant communities that are resistant to invasion are not defined by one measurable characteristic. However, invasion resistant plant communities need to use resources both spatially and temporally to preclude their use by invaders. Resource use can be enhanced by limiting catastrophic disturbances, maintaining appropriate disturbance regimes, and preserving dominant species and major functional groups. Catastrophic disturbances produce opportunities for invasive plant species to establish by removing vegetation (particularly dominant species or major functional groups) that limit the availability of resources to invasive plants. Similarly, the disturbance regime can greatly influence plant community dynamics. Plant communities may be tolerant of periodic disturbances, but intolerant of the same disturbance at a higher frequency of occurrence. Interactions between disturbances can also be critical to determining the response of the plant community (Miller 1982; Collins 1987). Thus, the response to the combination of disturbances and their frequency (i.e. the current disturbance regime) the plant community will be subjected to may be more important than its response to any single disturbance. The dominant species or major functional groups use the most resources, thus, losing them from the plant community can result in a flush of resources for invasive plant species to capitalize on. Therefore, the ability of the dominant plant species or major functional groups to tolerate the current disturbance regime is of critical importance for maintaining biotic resistance to invasion. Management should focus on methods for improving tolerance of the plant community to the existing disturbance regime or altering the disturbance regime to favor survival of dominant plant species and major plant functional groups.
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