Document: JAM-3-84-7

Using short-term measurements to predict long-term impact of exotic species on native species.

BYERS, J.E.*

University of Washington, Friday Harbor, WA 98250 USA ¹

Abstract:
Variability in the rate of increase and spread of non-native species confounds efforts to predict their long-term impact. The Japanese mudsnail, Batillaria attramentaria, invaded marshes of northern California over a multi-year span earlier this century, and has begun to displace its native ecological equivalent, Cerithidea californica. Due to differences in the year of Batillaria's introduction, variation exists among marshes in the progression of the invasion-from virtual extinction of the native, to equal densities of native and exotic. Therefore, I could examine the snails in a marsh where species densities were equal, but with knowledge of the ultimate timing of the native species' extirpation by the invader gleaned from nearby marshes. Density and demographic data of the two interacting snails over 5 years in 5 independent sites in Bolinas Marsh demonstrate a significantly negative population growth rate in Cerithidea (r = -0.22), while Batillaria exhibited no net change in population size. Multiple regression indicated Batillaria density strongly influenced Cerithidea's rate of decline. This relationship suggests that the present rapid rate of decline likely grew only as Batillaria reached higher densities, explaining why projection of Cerithidea's current rate of decline underestimates its true time to extinction. The relationship further suggests that spatial variation in invader density may be a surrogate for temporal data to anticipate the effects of increases in invader density on native population growth. I present an analytical framework based on coupled Lotka-Volterra competition equations, and concrete steps for parameterization using life table type calculations from field data on individual growth rates and size-fecundity relationships. This technique improves extinction time estimates by incorporating the effect of constantly changing densities on r and also captures the long lag time in invader increase commonly observed in invasion studies.

Keywords: biological invasions; competitive exclusion; estuaries; invasion impact; local extinctions; non-native species; time lags

This abstract is being presented at: 10:45 AM in session:
Oral Session #7: Aquatic Ecology: Shellfish to Snails.