Human threats to sandy beaches: A meta-analysis of ghost crabs illustrates global anthropogenic impacts.

Highlights

• Mitigating threats to sandy beaches requires accurate assessments of condition.

• Ghost crabs are widely used as indicator species in ecological beach assessments.

• Human impacts detected with ghost crabs are globally evident for sandy shores.

• Applied conservation needs experiments that yield defensible cause–effect evidence.

• Priority areas are acoustic and light pollution, debris, climate change effects.

Abstract

Beach and coastal dune systems are increasingly subjected to a broad range of anthropogenic pressures that on many shorelines require significant conservation and mitigation interventions. But these interventions require reliable data on the severity and frequency of adverse ecological impacts. Such evidence is often obtained by measuring the response of ‘indicator species’.

Ghost crabs are the largest invertebrates inhabiting tropical and subtropical sandy shores and are frequently used to assess human impacts on ocean beaches. Here we present the first global meta-analysis of these impacts, and analyse the design properties and metrics of studies using ghost-crabs in their assessment. This was complemented by a gap analysis to identify thematic areas of anthropogenic pressures on sandy beach ecosystems that are under-represented in the published literature.

Our meta-analysis demonstrates a broad geographic reach, encompassing studies on shores of the Pacific, Indian, and Atlantic Oceans, as well as the South China Sea. It also reveals what are, arguably, two major limitations: i) the near-universal use of proxies (i.e. burrow counts to estimate abundance) at the cost of directly measuring biological traits and bio-markers in the organism itself; and ii) descriptive or correlative study designs that rarely extend beyond a simple ‘compare and contrast approach’, and hence fail to identify the mechanistic cause(s) of observed contrasts.

Evidence for a historically narrow range of assessed pressures (i.e., chiefly urbanisation, vehicles, beach nourishment, and recreation) is juxtaposed with rich opportunities for the broader integration of ghost crabs as a model taxon in studies of disturbance and impact assessments on ocean beaches. Tangible advances will most likely occur where ghost crabs provide foci for experiments that test specific hypotheses associated with effects of chemical, light and acoustic pollution, as well as the consequences of climate change (e.g. species range shifts).

Introduction

“I'll try you on the shore”

William Shakespeare: Antony and Cleopatra (1606)

Accelerating environmental impacts on ocean beaches and coastal dunes call for effective environmental planning and biological conservation. These interventions should meet two cardinal criteria: 1.) environmental values and conservation goals must be clearly identified for broad and inclusive segments of the population (Harris et al., 2014, Vivian and Schlacher, 2015); and 2.) management decisions must be based on impact assessments that produce defensible and biologically relevant information (Harris et al., 2015).

A sizeable part of this information comes from documenting the response of organisms (at various levels of ecological organisation ranging from individuals to ecosystems) to human activities or anthropogenic habitat change (Defeo et al., 2009, Schlacher et al., 2007a). On ocean shores, a broad range of animals (e.g. benthic invertebrates, birds, marine turtles) has been used to detect biological effects attributed to an equally diverse spectrum of anthropogenic pressures, ranging from vehicle impacts to urbanisation (e.g. Huijbers et al., 2015b, Marshall et al., 2014, Reyes-Martínez et al., 2015, Walker and Schlacher, 2011).

Ghost crabs (Genera Ocypode and Hoplocypode) are perhaps the most widely-studied invertebrate indicator species on ocean-beaches (e.g. Barros, 2001). Ghost crabs are attractive as ecological indicators for a number of reasons: i) they are widespread in the subtropics and tropics; ii) they occur on both the non-vegetated beaches and in the dunes backing beaches; iii) they are relatively large, often locally abundant, arguably charismatic, and require no specialised tools to sample; iv) their taxonomy is well known and identification not overly difficult; and v) they construct semi-permanent burrows with clearly visible openings at the beach surface (Lucrezi and Schlacher, 2014, Schoeman et al., 2015). It is the fossorial habits of ghost crabs, in particular, that has led to their widespread adoption as ecological indicators, chiefly because estimates of abundance and body size can be made from counts and measurements of burrow sizes without the need to physically collect the organisms (Barros, 2001).

Given the extensive use of ghost crabs as ecological indicators on ocean beaches, a formal review and meta-analysis of this practice is warranted. To this end, here we synthesise the literature and address five broad questions:

• What are the types of human pressures acting on beach-dune systems that have been assessed with ghost crabs?

• What is the magnitude of reported ecological effect sizes for different stressors?

• Which metrics (response variables) have been used?

• What are the gaps in terms of human pressures currently not adequately assessed using ghost crabs?

• What opportunities exist to advance the use of ghost crabs as ecological indicators on ocean beaches?