An individual based model of sea turtles: Investigating the effect of temporal variability on population dynamics

Marine turtles show great variability in annual somatic growth and reproductive performance. Fluctuated environmental conditions are closely linked with reproduction, while recent empirical information suggests the existence of a density dependent effect on somatic growth. We developed an individual based model (IBM) to simulate the effects of a varying environment on population dynamics. A density dependent effect on the age at maturity and re-nesting patterns was also examined. The model depicts units as individual animals, aggregated groups and sets of classes to explicitly incorporate behavioral and biological features of species life history. Biological features and procedures of the simulated population were modeled using parameters for the green turtle species (Chelonia mydas) derived from the literature. Temporal variability on reproductive performance was included by considering autocorrelated and randomly sequenced years indicating good and bad conditions. The analysis indicated that temporal variability could have a significant effect on population persistence. Probability of extinction varied according to fluctuated times of age at maturation and re nesting intervals. Density dependence as an additional form of temporal variability has also resulted in an increased proportion of extinction in the simulated populations. We conclude that modelling sea turtle population dynamics by the use of IBM could provide some critical insights into population dynamics, overcoming difficulties that arise from the incomplete knowledge of several aspects of species life history. We further suggest some priorities for future research at both the empirical and theoretical level.