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

Abstract

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.

Vertical distribution by demographic groups of ghost crab Ocypode quadrata (Crustacea: Brachyura)

“…Steiner & Leatherman (1981) suggested that ghost crabs become accustomed to increased levels of recreational use of a beach, and perform vertical migrations. However, even the positive effects of recreation such as food left by visitors can cause intense and continuous disturbance that may have negative impacts on the population of O. quadrata in the long term (Lucrezi et al 2009b). According to Turra et al (2005), it is believed that changes in beach morphodynamics alter the availability of food, influencing the size, distribution and abundance of the species.…”

Physical damage to coastal dunes and ecological impacts caused by vehicle tracks associated with beach camping on sandy shores: a case study from Fraser Island, Australia

“…Ghost crabs densities have been shown to be good biological indicators of human habitat disturbance on beaches. The crabs construct burrows in the sand matrix and densities can be easily estimated by counting the number of burrow openings on the sand surface (Barros 2001;Lucrezi et al 2008). A commonly observed response to disturbance caused by 4WDs is a decrease in crab densities in areas affected by cars, partly because vehicles can crush crabs inside and outside of their burrows (Moss and McPhee 2006;Schlacher et al 2007a).…”

Seasonal Burrow Distribution of the Ghost Crab, <i>Ocypode ceratophthalmus</i> (Pallas, 1772), on Sandy Shores of Gujarat, India

“…The burrows of this species have circular or "hole" type openings and therefore counting the number of such burrow entrances is an effective tool to measure the population of Ocypode species on sandy shores (Moss and McPhee, 2006). As Ocypode crabs are form the highest trophic position amongst the invertebrate predators on sandy shores and also play an important role as prey for other vertebrate predators, so changes in the population of Ocypode crab can be used as a ecological indicator to predict the impact of various factors on sandy shore ecosystems (Chan et al, 2006;Valero-Pacheco et al, 2007;Lucrezi et al, 2009;Tureli et al, 2009). Ocypode ceratophthalmus is the most common brachyuran crab species occurring on the sandy shore habitat of Gujarat state (Trivedi and Vachhrajani, 2016) still ecology of local population of the species has not been studied in detail.…”