Multiple metrics of the ghost crab Ocypode quadrata (Fabricius, 1787) for impact assessments on sandy beaches

Our objective was to compare the efficiency of indirect (burrow density, size, depth and occupation rate) and direct (emersion from burrows, tolerance distance from humans and number of foraging crabs) metrics of the ghost crab Ocypode quadrata for urbanization impact assessments. We sampled six beaches during high and low tourism seasons along a gradient of human pressure: high impact (urbanized), medium impact (intermediate) and low impact (non-urbanized). We found that the urbanization index and temperature explained the burrow density and size regardless of the bias related to the burrow occupation rate (similar to 60%) and counting period (diurnal or nocturnal). Moreover, the number of nocturnal foraging crabs explained most (57%) of the dissimilarity among the beaches under different levels of human impact. Thus, the counting of foraging crabs at night was found to be a relevant metric for beach impact evaluation. We strongly recommend integrating the counting foraging crabs with traditional burrow measurements (burrow density and size) in impact assessments of sandy beaches.