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Density-dependent effects on hatchling production in the green turtle nesting population in Tortuguero, Costa Rica

Density-dependent effects on hatchling production in the green turtle nesting population in Tortuguero, Costa Rica


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.