Threshold dynamics in natural systems can arise due to positive
feedback processes and are thought to be an important mechanism
promoting the invasion of exotic species and can strongly influence
invasive plant management decision-making. However, we rarely if ever
know how to anticipate when or where a threshold is likely to be
crossed in invasive species management. In addition, regional scale
colonization dynamics are rarely integrated theoretically or
empirically in studies of the role of thresholds in invasions, and
while clearly important to the spread of invasive species, we have a
very limited understanding about how colonization might modify
There is clear evidence that ecological thresholds may be very applicable to invasive plant management decision-making (Suding et al. 2004). For instance, substantial shifts in vegetation can occur with little forewarning and once the shifts occur they may be difficult to reverse (Scheffer et al. 2001).
Many invasive plants can influence their environment in ways that create a positive feedback that furthers their own success (Klironomos 2002), another indicator of threshold dynamics. Feedbacks are often the way exotic species achieve and maintain dominance in communities (Levine et al. 2006). Positive feedback can arise when an invasive species changes its environment in a way that favors its own success relative to that of other species, leading to an increase in the invader and even greater environmental change. For instance, many grasses are both more flammable than woody plants and more tolerant of burning. Replacement of shrubs by grasses therefore promotes fire, which further favors the invasion of grasses (D'Antonio and Vitousek 1992), causing rapid conversion of shrublands to exotic grasslands. Likewise, the importance of invasion sequence has been found in many types of communities (Chase 2003). In addition, legacy effects of exotic species can influence the success of restoration years after the exotics have been removed (Eviner 2004). For example, plants have been shown to culture soil microbes (pathogens and mycorrhizae) that either benefit or harm them more than other species (Bever 1994). Positive soil-microbe feedbacks can occur when each plant species grows better on its own soil and worse on soil cultured by other species.
can allow propagule supply to regulate invasion thresholds (Shurin et
al. 2004). While local processes such as competition have been a major
focus of research in invasion, colonization in an invaded landscape is
also a critical process (Hastings et al. 2005). Some species may
produce a lot of propagules, disperse them wide distances, and spread
rapidly. Other species are more limited in their dispersal, only
spreading propagules 1-2m or less on average, and spread more slowly.
These differences in the ability of a species to spread across the
landscape can also be influenced by landscape characteristics such as
reserve size, fragmentation, and regional land use patterns. Thus,
regional dynamics can modify local interactions through propagule
supply, source-sink dynamics, and spatial landscape structure.
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