Monitoring human impacts on sandy shore ecosystems: a test of ghost crabs (Ocypode spp.) as biological indicators on an urban beach

Sandy beaches comprise one of the most important coastal resources worldwide, providing habitats to threatened vertebrates, supporting underappreciated invertebrate biodiversity, and delivering crucial ecosystem services and economic benefits to mankind. Monitoring of the natural resource condition of sandy beaches and assessments of the ecological impacts of human disturbance are, however, rare on sandy shores. Because a crucial step in developing beach monitoring is to identify and test biological indicators, we evaluated the utility of using population densities of ghost crabs (genus Ocypode) to measure how beach biota respond to human pressures. Densities of crabs-estimated via burrow counts-were quantified at two sites exposed to high and low levels of human disturbance on an urban beach in eastern Australia. Human disturbance consisted of pedestrian trampling and shoreline armouring which led to the loss of dune habitat. Overall, crab numbers were halved in disturbed areas, but contrasts between impact and control sites were not necessarily consistent over time and varied between different levels of the shore: stronger and more consistent effect sizes were recorded on the upper shore than further seawards. In addition to lowering crab densities, human disturbance also caused shifts in intertidal distributions, with a greater proportion of individuals occurring lower on the shore in the impacted beach sections. The number of visible burrow openings also changed in response to weather conditions (temperature and wind). We demonstrate that spatial contrasts of burrow counts are broadly useful to indicate the existence of a human-induced disturbance effect on urban beaches; we also highlight a number of critical, hitherto unknown, issues in the application of this monitoring technique; these encompass three broad dimensions: (1) a need for standardised protocols; (2) unresolved causal links between observed patterns and putative pressures; and (3) uncertainties of how organisms responds specifically to both natural and human changes of environmental conditions on sandy shores.