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8. Forest pest management options

Box 1. Attributes of potentially invasive species and susceptible ecosystems

Though much is known about the factors that contribute to the introduction and spread of alien invasive species, little is known about the attributes of alien invasive species and susceptible habitats.

Isolated ecosystems, evolutionarily and geographically, are particularly vulnerable to biological invasions while deserts, semi-deserts, tropical dry forests and woodlands, and arctic systems appear to be the least susceptible (McNeely et al., 2001; Perrings et al., 2002). Ecosystems with low species diversity, especially if predators and competitors are absent, seem to be more susceptible than species-rich systems with well-established species interactions (McNeely et al., 2001; Perrings et al., 2002). However, such species-rich ecosystems may be at risk to a greater range of invaders because of their greater diversity of habitats (McNeely et al., 2001). Frequent disturbance, slow recovery rate and fragmentation of communities promote plant invasions (Rejmánek, 1999).

Some ecological factors that may allow introduced species to spread include the following (Pimentel et al., 2000).

• Lack of predators, competitors and parasites.

• Ability of an alien parasite to switch to a new host.

• Ability to be an effective predator.

• Availability of artificial or disturbed habitats.

• High adaptability to novel ecosystems.

• Efficient dispersal.

While the success of alien invasive species is often explained by simply placing an introduced species in a favourable environment, some have suggested that the lack of enemy pressure actually results in a reallocation of resources from defence, as it is no longer needed in enemy-free habitats, to growth and reproduction (Petit et al., 2004; Withgott, 2004). Others have suggested that this may not be the case for all circumstances and there is likely not a simple trade-off between defence mechanisms and growth (Withgott, 2004).

Identification of the life-history traits of potentially invasive species has been attempted with some success, primarily for woody species. For conifer species, invasiveness is associated with: small mean seed mass (<50 mg); short juvenile period (<10 years); and short intervals between large seed crops (Richardson and Rejmánek, 2004). A short juvenile period and short intervals between large seed crops results in early and consistent reproduction. A short juvenile period may also be related to fast growth in general and small mean seed mass is associated with a greater number of seeds produced which are better dispersed, a high initial germinability and a shorter chilling period to overcome dormancy. Examples of alien invasive species with such life-history characteristics include Cryptomeria japonica, Larix deciduas, Picea sitchensis, Pinus contorta, Pinus strobus and Pseudotsuga menziesii.

Another important consideration is the fact that some alien invasive species explode quickly while others may have a long lag period between initial introduction and subsequent population growth and expansion. Some reasons for such time lags include the following (Crooks and Soulé, 1999).

• The area in which the species has been introduced may become more susceptible to invasion.

• The species may have been undergoing rapid yet undetected expansion and some environmental change may trigger invasion.

• Several decades may be needed to build up large enough populations to have significant reproductive potential.

• Newly introduced species may be confined to restricted habitats until mutations favourable for further colonization became available.

Knowing more about which species invade and which ecosystems are more susceptible will improve and refine capacities to prevent, or detect in a timely manner, biological invasions.

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