Evaluation of density-dependent processes and green turtle Chelonia mydas hatchling production at Tortuguero, Costa Rica

Abstract

The role of density-dependent processes in sea turtle populations remains largely unknown. This study quantified density-dependent and density-independent processes that underlie hatchling production in the green turtle nesting population at Tortuguero, Costa Rica, and estimated current mean hatchling output and potential carrying capacity of Tortuguero beach for hatchlings and nesting females. Density-dependent effects of nest destruction by nesting females and coatis were evaluated in the 2000 nesting season along the 28.8 km nesting beach. To quantify factors affecting hatchling production, the fates of nests were monitored in twelve 50 m long study plots. Density-dependent factors included nest destruction by nesting females and predation by coatis, whereas density-independent factors included beach erosion, beach flooding, and below-beach- surface destruction by crabs, ants, microbes, and plant roots. Calculations indicated that between 5 and 6 million hatchlings are currently produced, while a simulation model suggested that at carrying capacity, 6 to 10 times as many hatchlings could be produced by more than 600 000 nesting females. The current mean number of females nesting at Tortuguero is between 3 and 4% of the population that is estimated to nest at carrying capacity. This estimate is consistent with previous estimates that modern day populations of Caribbean green turtles represent only 3 to 7% of pre-exploitation levels. The hatchling production model is applicable to other beaches and sea turtle species, and provides a framework to evaluate recovery goals for sea turtles.