Estimating the probability of long-distance overland dispersal of invading aquatic species

Accurately predicting the pattern and rate of spread of invading species is difficult, particularly for species that disperse long distances. Though relatively rare, and often stochastic, long-distance dispersal events increase the maximum rate and geographic extent of invasion. Human activities are responsible for the spread of many exotic species, particularly aquatic species such as the zebra mussel, which are primarily transported within North America by recreational boaters. We estimated spatial and temporal patterns of boating traffic among Wisconsin's inland waterbodies using results of a large, randomized survey of recreational boaters conducted by the Wisconsin Department of Natural Resources. Of the survey respondents, >90% of boaters traveled locally, within a county or to adjacent counties, 8.4% moved >50 km, and only 0.8% moved extreme long distances (>261 km, two standard deviations above the mean of intercounty travel). Extreme long-distance boater movements were correlated positively with greater numbers of registered boaters in source and destination counties, and with greater surface area and numbers of named lakes in destination counties. We compared the observed spatial and temporal patterns of the zebra mussel invasion to those estimated from recreational boater movement by simple diffusion models. Diffusion models underestimated the maximum rate and geographic extent of the zebra mussel invasion and overestimated the invasion of suitable habitats within this extent. Patterns of recreational boater activity in Wisconsin were a better predictor of the observed zebra mussel invasion pattern because they provided probabilistic estimates of invasion at finer spatial resolution. These estimates may be used to manage the spread of boater-dispersed aquatic invaders. To slow the spread of boater-dispersed aquatic invaders such as the zebra mussel, management efforts should target high-frequency, long-distance boater movements, and regions with the greatest volume of source and/or destination boater movement.