Rebound of a native prey species following management of an invasive crab

Background/Question/Methods

Alterations to community composition, habitat structure, or ecosystem function can take years to recover. The rate at which impacted communities and their habitats rebound after removal of an established species has rarely been explored. As a first step in examining community changes following the management of a non-native species, we tracked the recovery of a native crab population following intense removal of the non-native European green crab, Carcinus maenas. Among other changes to Bodega Harbor’s soft sediment community that occurred immediately after colonization by C. maenas, the population of the native shore crab, Hemigrapsus oregonensis, experienced a huge decline in abundance. This decline was accompanied by decreased body size and a change to more H. oregonensis using higher intertidal areas, and these changes persisted.  We therefore set out to test whether the prey species would rebound in population size, body size, and habitat use, and determine the rate of rebound, if we lowered predation pressure by removing C. maenas. In 2006, 14 years after its invasion, we started removing C. maenas from Bodega Harbor and removed thousands of these crabs. We monitored predation rates on tethered H. oregonensis and the numbers and size of crabs in traps.

Results/Conclusions

Predation pressure was greatly decreased upon removal of most of the C. meanas in the harbor as indicated by a significant increase in survival of tethered H. oregonensis with C. maenas removal. This decrease in predation pressure allowed a rapid rebound of the prey species. One year after we began removal, our catch of H. oregonensis increased four-fold and the average body size of H. oregonensis increased by about 15%. While exact numbers have fluctuated, this rebound has persisted. Our results suggest that C. maenas removal resulted in a rapid return to the pre-invasion size and abundance of the native crabs due to reduced predation pressure. A rapid return to prior demographics may be a general and expected outcome from removal of key predators when additional species have not invaded or increased in number to affect rebound in the prey species.