The burrow resetting method, an easy and effective approach to improve indirect ghost-crab population assessments

摘要

Sandy beaches are sensitive ecosystems subjected to intense and variable human pressures. Among several potential indicators of beach environmental quality, ghost crabs present a good balance between low costs (burrows are easily identified and counted) and benefits (responding to both chronic and acute threats). However, such indirect assessments may be biased because the occupation rate of burrows differs within and among beaches and over time. Here we propose and analyze a new, simple approach to reduce this bias, the burrow resetting method (BRM). This consists of sampling the same areas twice: the first time corresponding to the traditional method (TM), after which the surface of the burrows are gently covered with sand and reassessed after 12-24 h, corresponding to the BRM. Excavation after BRM revealed a higher accuracy of the proposed method in relation to TM (higher similarity between number of open burrows and number of crabs - 94%), while not injuring the animals. BRM was applied in 16 beaches, classified according to morphodynamics and presence of coastal armoring. Results showed a significant difference, up to 69%, between the TM and BRM regarding abundance, and this difference of estimative between methods varied significantly across beaches. TM overestimation of crab size was higher in beaches without armoring, a critical point in making inferences regarding beach quality from population structure. BRM simplicity, low cost and accuracy elects it to improve temporal and, notably, spatial comparisons of crab abundance and size, a main constraint hindering so far a wider use of ghost crabs as indicator and monitor of environmental quality in sandy beaches worldwide.