Population density and habitat use of the yellow land crab Johngarthia lagostoma on the remote Trindade Island, Brazil: implications for island conservation

Abstract

Abstract Several land crab species inhabit oceanic islands worldwide, where they often dominate terrestrial arthropod communities, and act as ecosystem engineers. In the South Atlantic, Johngarthia lagostoma is the only gecarcinid species, endemic to Rocas Atoll, Fernando de Noronha, Ascension, and Trindade. Despite its classification as ‘Endangered’, there is a gap in knowledge on the spatial and temporal distribution of this species. Here, we investigated the population density and habitat preferences of J. lagostoma on Trindade Island, Brazil, examining the influence of dune vegetation sectors, diel activity, and environmental variables (elevation, relative humidity, and distance from the sea). Crabs were sampled weekly across three vegetated dune areas during both day and night, over eight weeks. No significant variation in sex ratio or size was detected among areas, although males were consistently larger than females. Crab density varied significantly with vegetation dune sector and diel period. Daytime density was highest in intermediate areas where two plant species co-occurred, and crabs remained sheltered, whereas nighttime density increased closer to the shoreline during feeding activity. Elevation, humidity, and mean carapace width were positively associated with density during the day, explaining 42% of the variation in the use of vegetation for shelter. These findings highlight the importance of dune vegetation and abiotic conditions in shaping the spatial ecology of J. lagostoma on Trindade. By identifying habitat features that support high crab densities, this study provides insights for conservation planning and restoration actions on oceanic islands, emphasising the need to protect vegetated areas as key habitats for endangered species.