Vehicles versus conservation of invertebrates on sandy beaches:: mortalities inflicted by off-road vehicles on ghost crabs

Sandy beaches face increasing anthropogenic pressures, with vehicle traffic being ecologically highly harmful. Ghost crabs (Fam. Ocypodidae) are conspicuous on many beaches, and they have been used as a bio-monitoring tool to measure the ecological responses to human disturbance. However, the mechanisms causing declines in crab numbers are unknown, yet conservation must target the actual impact mechanisms. Therefore, we quantified the magnitude and mechanisms of off-road vehicle (ORV) impacts on ghost crabs, addressing three key questions: (i) Does abundance of ghost crabs respond to traffic intensity?; (ii) Can burrows protect crabs from vehicles? and (iii) Can mortalities caused by vehicles contribute to population declines? ORV-impacts were measured on North Stradbroke Island (Australia) for Ocypode cordimanus and Ocypode ceratophthalma. Crab densities were significantly lower in areas subjected to heavy beach traffic, suggesting direct crushing by vehicles. Burrows only partially protect crabs against cars: all individuals buried shallow (5 cm) are killed by 10 vehicle passes. Mortality declines with depth of burrows, but remains considerable (10-30% killed) at 20 cm and only those crabs buried at least 30 cm are not killed by ORVs: these 'deep-living' crabs represent about half of the population. After crabs emerge at dusk they are killed in large numbers on the beach surface. A single vehicle can crush up to 0.75% of the intertidal population. While conservation measures should primarily regulate night traffic, our results also emphasise that the fossorial life habits of sandy beach animals cannot off-set the impacts caused by ORVs.