Responses of ghost crabs and other scavengers to habitat modification of urban sandy beaches

Abstract

Populations in coastal areas are growing at a rate far exceeding that of other areas, resulting in escalating pressures on coastal areas around the world. Sandy beaches make up over 70% of the world coastlines, comprising some of the most highly sought after areas for human activities. Consequently, sandy beaches are some of the most vulnerable habitats to the effects of urbanisation, with beaches now threatened by a wide range of anthropogenic practices. Ghost crabs are widely used as ecological indicators of ecosystem health on sandy beaches, but have rarely been studied on beaches within large urban centres. In this thesis, ghost crab burrow densities were used to assess human impacts on beaches in the highly urbanised estuary, Sydney Harbour. Across 38 beaches in the harbour, mechanical beach cleaning frequency was found to be the most influential predictor of ghost crab distributions in the area, with burrow counts reduced on the most frequently cleaned beaches and highest burrow counts found on beaches that were cleaned no more than three times per week. Mechanical cleaning was associated with the volume of wrack accumulated onshore, with frequently cleaned beaches supporting significantly lower volumes of wrack then beaches that were cleaned less frequently. To examine how organic subsidies on sandy beaches affect crab behaviour, I experimentally manipulated the availability of organic marine matter (wrack and carrion). Ghost crabs demonstrated no short term response to either subsidy (measured by the initiation of new burrows near food source), while vertebrate scavengers demonstrated a rapid response to carrion subsidies only. The presence of vertebrate scavengers in treatment plots reduced the fossorial behaviour of ghost crabs, suggesting biological interactions among invertebrates and vertebrates. The arrival of marine subsidies to sandy beaches facilitates the exchange of resources between the marine and terrestrial zone. Given that these systems can rely heavily on this resource exchange any alteration in the natural supply of organic marine matter, e.g. more frequent beach cleaning in response to urbanisation, could have far reaching implications for both zones.