Exposing the Mechanism and Timing of Impact of Nonindigenous Species on Native Species

The nonnative mudsnail Batillaria attramentaria was introduced to the west coast of North America in the early part of this century and has been displacing the native mudsnail Cerithidea californica on a marsh-by-marsh basis. We combined detailed, quantitative field data on this invader and its interactions with the native snail in an individualbased model that allowed us to address both general questions about the mechanism of displacement of native species and more specific concerns about detecting the trajectory and impact of the invasion. In empirically parameterized simulations the native snail was driven extinct within 55–70 yr after introduction of Batillaria, which closely matches direct field estimates. We then tested the relative importance of Batillaria’s demonstrated advantages in parasitism resistance (top-down effect), exploitative competition (bottom-up effect), and mortality rate (demographic advantage) in driving its displacement of Cerithidea. With its demographic advantage maintained, but without its advantages in competition and parasitism, Batillaria still drove Cerithidea extinct within 90 yr. Only when Batillaria’s mortality rate was set equal to that of Cerithidea could the native snail persist indefinitely, demonstrating this factor’s overwhelming influence on the success of this invasion. The difference in mortality between the species was large relative to the other differences, but further simulations showed that the importance of this difference stems not just from its magnitude, but also from the sensitivity of this system to this demographic rate. Identification of the relative importance of mechanisms that contribute to an invader’s success is one of the major benefits of such modeling efforts. To identify empirically measurable quantities that provide the earliest warning of impact on the native species, we tracked many populationand individual-level responses of Cerithidea to Batillaria’s invasion, including population density, biomass, egg production, mean size, proportion of infected individuals, and individual growth rate, as well as availability of shared food resources. We used the empirically observed parameter values and an initial number of Batillaria invaders in these simulations that guaranteed extinction of Cerithidea within 90 yr. Despite a rapid initial increase in invader populations, all metrics for Cerithidea were slow to exhibit signs of impact. Most took at least 25 yr from invasion to exhibit detectable changes, by which time the nonnative species was established at high densities (.3000 snails/m2). Cerithidea egg production was the fastest, most consistent response metric exhibiting declines within 20–25 yr after invasion in ;90% of simulations. Difficulty in finding reliable, early warning metrics has crucial implications for how we should view and conduct monitoring programs and risk assessment analyses.