Department of Biology, University of Florida, Gainesville, FL 32611, UNITED STATES OF AMERICA.
The role of density-dependent processes in sea turtle populations has been relatively unstudied. This study evaluated and quantified density-dependent and density-independent parameters affecting hatchling production in the green turtle nesting population in Tortuguero, Costa Rica, and incorporated these parameters in a model to estimate current mean hatchling output and potential carrying capacity of Tortuguero. An analysis of spatial and temporal distribution of nests and non-nesting emergences from 1972 to 2000 at Tortuguero showed a consistent spatial and temporal pattern within and among years. Spatial nest distribution conformed to the predictions of the mid-domain hypothesis, whereas temporal nest distribution did not conform conclusively. A first estimate of the mean spatial nesting range of individual green turtles (4.5 miles ± 4.2) on the northern 18-mile beach was derived from the mid-domain model.
Density-dependent effects of nest destruction by nesting females and coatis were evaluated in the 2000 nesting season on the northern 18-mile nesting beach. To quantify factors affecting hatchling production, twelve 50-m long study plots were set up in the dense nesting section of the beach, and the fate of nests laid in these plots was monitored. Density-dependent factors affecting hatchling production were nest destruction by nesting females and predation by coatis when hatching increased. Density-independent factors were beach erosion, beach flooding, predation by crabs and ants, and microbial invasion. A model simulating hatchling production indicated that between 5 and 6 million hatchlings are currently being produced, and that 6 to 10 times more hatchlings could be produced. The model was not sensitive to a 20% increase in coati predation or erosion, but hatchling production decreased with a 20% increase in below-beach-surface mortality and an increase in a nesting female’s radius of destruction. The current mean number of females nesting at Tortuguero is between 3–4% of the population that is estimated to nest at carrying capacity, supporting the estimate that current Caribbean green turtle populations represent only 3–7% 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 a nesting population below historical levels.