Rates of Spread Lag Phase and Importance of Scale

Biological invasions are characterized by remarkable spatiotemporal dynamics with many species having extended their distribution ranges from within a single region to cover very large areas on several continents within the last century. Invasions are often faster than most natural migrations, for example, those following deglaciation. The dissemination of non-native species is facilitated by intercontinental commerce and travel (e.g., seed contaminants, horticultural trade), dispersal along regional transport networks (e.g., roads, water courses, railways), and their capacity for local colonization and rapid population increase. Many studies describe aerial spread rates of between 3 and 500 km yr~ , but indirect estimates of spread drawn from distribution maps can give much higher values up to 5000 km2 yr_1 as documented for the invasion of B. tectorum in temperate grasslands of North America at the beginning of the twentieth century. A hierarchy of processes operating at different temporal and spatial scales defines the dynamics of biological invasions. At the local scale, simple reaction-diffusion models are sometimes adequate for predicting the spread of new invaders. Average rates of local spread reported for invasive species in the literature range from 2 m yr~ to a maximum of 370 myr~ . Average rates of long-distance dispersal (LDD) are at least two orders of magnitude greater than estimates of local dispersal. This is illustrated by Wedelia trilobata that spread from a single focal area and covered 2500 km of the Queensland coastline in 15 years, thus averaging 167 kmyr~ . LDD events may occur during periods of negligible population increase and appear to bear little relationship to the increase in population size. At regional scales, invasive species rarely move across the landscape as a continuous front and both local and long-distance dispersal define spatial patterns. The frequency and distribution of introduction events shape invasion trajectories, and the stochastic nature of such events means that the longer a species has been present in a region, the more likely it is to be invasive. Understanding invasions demands perspectives from multiple spatial and temporal scales.

Invasion episodes are rarely described from their initiation. Following the introduction into a target region and when the alien occurs in a few isolated locales, there is usually a period of slow or no spread (lag phase). This is followed by a phase of rapid range expansion (exponential phase), and a third phase of filling-in, with little or no range extension. Lag phases of up to 80 years have been reported for herbaceous species and 150 or more years for woody plants. Three hypotheses, not mutually exclusive, relate to the factors that determine the length oflag phase between introduction and exponential increase. The 'gen-otypic' hypothesis suggests that the lag phase is the time needed for the development of genotypes with increased dispersal ability. This hypothesis predicts the length oflag phase to be proportional to generation time and that LDD is an intrinsic attribute of the species concerned. The 'demographic' hypothesis posits that any new population first expands slowly at its margin via short-distance dispersal, and that spread is largely limited by the local availability of suitable habitat. The rapid spread associated with exponential increase becomes more likely with an increase in population size and is initiated by human-mediated LDD that establishes new satellite populations in suitable habitat. The 'extrinsic' hypothesis proposes that lag phases are a result of inclement environmental conditions that give way to exponential population increase as these conditions improve. Changes in soil disturbance, nutrient enrichment, climate, dispersal vectors, and intraspecific interactions result in increased population growth and/or dispersal.

Chance events are crucial in determining the characteristics of invasion. At each step of the invasion process, from arrival of the invasive plant, through its establishment, spread, and persistence, stochastic events mediate interactions between the invader and the target community. Stochastic events that regularly mediate invasion include extreme events (e.g., flood, drought, and fire), nutrient enrichment, altered herbivory levels, access to new vectors (e.g., cars, animals, and rivers), and disturbance. Such events, while fundamental to the outcome of the invasion, are often infrequent and erratic in both time and space. Many studies are not done over sufficient space and time to capture the crucial roles of such factors.

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