Species invasions are escalating worldwide as coastal communities become increasingly connected. Native predators can control or resist invasion by non-native species and refuge habitat can modify those interactions. We aim to understand the distribution of invasive European green crab, Carcinus maenas, in San Francisco Bay (SF Bay) and whether this distribution is associated with predation and refugia. Previous unpublished work in SF Bay found higher abundances of C. maenas in high intertidal elevations with marsh plants and rocky structure. Crustaceans can also find spatial refuge from subtidal predators in higher elevation intertidal areas. We assessed crab distributions using depth-distributed trapping and by trapping inside and outside of high elevation refuge habitat. We also used tethered crabs in areas with and without structural refuge to determine if predation was associated with patterns in crab distribution.
During depth-distributed tethering we found that tethered C. maenas will bury into sediments of low elevation unvegetated mudflats but not in high elevation habitats, where structural habitat likely constrains burial. Therefore, we tested the effects of burying behavior on predation by tethering C. maenas with and without underlying mesh to experimentally impede burying in lower elevations with soft sediment at four sites: Two sites in SF Bay and two sites in British Columbia.
Results/Conclusions
As expected, depth-distributed trapping efforts found higher abundances of C. maenas in high intertidal elevations. However, depth-distributed tethering revealed no clear pattern in predation across elevations and sites. We found that C. maenas is more abundant in rocky habitats than mudflat areas without structure, and there are site-specific patterns in C. maenas use of cordgrass, dependent on habitat composition. However, results with tethered crabs did not show higher crab mortality outside of areas of structural refuge.
The discrepancy in crab use of refuge habitat and low predation may be due to variable crab behavior across heterogenous habitats that may preclude assessment of relative predation. Experiments in which burying was prevented by mesh showed greater mortality by predation than without mesh in SF Bay mudflats suggesting higher potential predation in low elevations than originally thought. Alternatively, fluctuating physical and biological regimes associated with highly altered, urbanized estuaries such as SF Bay may decouple prey responses from predator distributions, causing crabs to concentrate in high elevation refuges regardless of the presence of predators. These results highlight the importance of evaluating the interaction of habitat characteristics, prey behavior and predation to better understand invasive species distributions.