The establishment and spread of invasive or nonindigenous species has caused concern from stakeholders in affected areas, and has prompted many field and modeling studies. We used stochastic two species, circular three patch dynamic models to investigate the patterns of invasion and impacts upon the affected species. Both persistent and degradable toxicants were incorporated as parts of the model system to act as disturbance regimens. There is a clear series of patterns that result from these simulations. Competition increases population variability, but decreases the number of distinct outcomes possible from the same initial conditions. Isolation of the patch of the introduction was the main determinant of successful establishment through a process we call the beachhead effect. Coexistence of species was often possible in local patches, contrary to the analytical solutions of Lotka-Volterra equations and numerous modeling studies. Contaminants and their resultant disturbances are important as contributors to the stochastic nature of models. The stochasticity leads to a variety of outcomes from some sets of initial conditions. Different outcomes have different probabilities of occurrence and are dependent upon the specific initial conditions of the simulation. A clear pattern that is apparent is the "beachhead effect," where the invasive establishes a population within a relatively remote patch before migrating to the remainder of the landscape. We make predictions and provide specific research hypotheses as to the causes and effects of invasive species establishment, spread, and impacts.
The idea that invasive or nonindigenous species produce a societal cost has become a cultural cliché. Despite the high level of societal concern, the risk assessment process has not been widely used to quantify the risks of invasive species.The study of invasive species has been approached largely on a case-by-case basis, and field studies often have been too idiosyncratic to be used to derive general hypothesis of invasive species establishment (Vermeij 1996). Laboratory and garden studies are hampered by problems of scale, replication, and control (Wardle 2001, Doak et al. 1998. Hypothesized mechanisms for invasive species establishment and spread abound and most may be separated into two general categories: (1) attributes of the nonindigenous species and (2) attributes of the community into which the invasive species has arrived. Studies considering the former are typically searches for lists of common traits among the various species of invasive species, and exceptions to these lists are common (Mack et al. 2000).One of the strongest predictors of plant introductions is if the species has established in another location (Kolar and Lodge 2001). In fish invasions of the Great Lakes, the factors that determined establishment were relatively faster growth, toleration of a wider range of temperature and salinity, and a past history of invasiveness (Kolar and Lodge 2002). Quickly spreading fish have the features of slower relative Environmental Risk and Management from a Landscape Perspective, edited by Kapustka and Landis
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