Combined impacts of natural and anthropogenic disturbances on the bioindicator Ocypode quadrata (Fabricius, 1787)

Abstract

Intense natural disturbances can dramatically affect populations in various ecosystems and existence of human influences can make the impacts of natural disturbance even greater by reducing the resilience of the populations. Here, we determine how the impacts of natural and anthropogenic disturbances alter population structure of a widespread bioindicator, the Atlantic ghost crab (Ocypode quadrata) by examining its burrow density, size and distribution before and after a hurricane on beaches with different levels of human impacts. We predicted that burrow density and size would show a dramatic decline after the hurricane mainly on the most human-impacted beaches. Because the smaller individuals of this species are highly sensitive to desiccation and thus normally occupy the lower parts of beaches, we further predicted that burrow distribution would not show any changes following the hurricane. We found that the density of ghost crab burrows strongly declined, but that this decline was lessdramatic on beaches with higher levels of human impact. However, the decline in the burrow size 4–7 days following hurricane disturbance indicates that the hurricane had greater impact on larger than on smaller individuals. Moreover, we found that the burrow distribution shifted after the hurricane. When the larger individuals were excluded from the system, the smaller individuals occupied the upper parts of the beach, suggesting that intra-specific interactions are the major mechanism determining the distribution pattern of ghost crabs on sandy shores, rather than physiological limitations (desiccation) that have previously been proposed. Our results suggest that, human influences make the populations less resilient against natural disturbances and that natural disturbances have a larger impact on larger individuals of this bioindicator species.