Multiple and long-term effects of an introduced predatory crab

Despite the importance of invasions, few studies have explored their long-term consequences in marine systems or examined multiple types of population-level effects. Initial effects, however, may not persist over longer time frames; effects have been shown to wane in freshwater systems. We combined 14 yr of field surveys (1993 to 2006) with manipulative experiments to examine the potential for multiple effects of a nonindigenous crab Carcinus maenas on the native shore crab Hemigrapsus oregonensis over time in central California. H. oregonensis abundance was negatively correlated with C. maenas abundance. However, H. oregonensis abundance rebounded to pre-invasion levels once C. maenas numbers declined. Other measured changes include a marked decrease in H. oregonensis body size and an increase in the proportion of H. oregonensis in the high intertidal zone since the arrival of C. maenas. These changes in body size and tidal distribution persisted nearly a decade beyond the peak abundance of C. maenas and after H. oregonensis numbers rebounded. Observed changes in the distribution of the H. oregonensis population correspond to shifts in C. maenas abundance, and experiments support a causal relationship. Stepwise regression suggests a complex and possibly nonlinear relationship between predictor variables and H. oregonensis size and distribution. Overall, our data indicate strong persistent effects on multiple attributes, with a lag in recovery with declining invader abundance, underscoring the potential for long-term effects that are decoupled from year-to-year invasion dynamics.