Burrow occupation rates and spatial distribution within habitat of the ghost crab Ocypode quadrata (Fabricius, 1787): Implications for impact assessments

Abstract

The most conspicuous characteristic of ghost crabs (Crustacea: Ocypodidae) is the construction of semi-permanent burrows on the sand, widely used as tools for impact assessments. Thus, it is now critical to understand the factors driving burrowing activity and burrow occupation rates, including the effects of natural and anthropogenic disturbances. We tested three hypotheses in the present study with the ghost crab Ocypode quadrata: (i) overestimation of population size by traditional burrow counts depends on disturbance level at local scale; (ii) burrow occupation rate is density-dependent; and (iii) the predominant spatial distribution of burrows within the habitat is uniform. The last two predictions were assessed using a meta-analysis for a macro scale assessment. The comparison of distinct methods for burrow counting confirmed that ghost crab density is overestimated on non-urbanized beaches at local scale. A non-linear relationship between burrow occupation rates and burrow density was found at local and regional scales. When burrow density increases, the burrow occupation rate is also higher. However, from approximately 20 to 30 burrows/100 m2, the occupation rate reduces with increased density. Thus, the overestimation of the ghost crab density on non-urbanized beaches can be partially explained by a density-dependence in burrowing activity probably driven by a stronger intraspecific competition on non-urbanized areas. Finally, the distribution pattern of the ghost crab is predominantly uniform in most of its distribution range, despite of urbanization level and sampling design, hence suggesting intra-specific interactions as a driver of burrowing behaviour. In conclusion, burrow density, occupation rate and spatial distribution within the habitat evidenced at various spatial scales that burrowing activity of the ghost crab is density-dependent and this constitutes an issue never considered before in impact assessments inherent to coastal monitoring and management.