Nest site selection by flatback sea turtles: Characterization of nesting beach topography with airborne LiDAR

Protecting sea turtle nesting beaches is a commonly used management strategy for sea turtle conservation as gravid sea turtles are more vulnerable to predation and disturbance during oviposition. Therefore, understanding where sea turtles nest and the characteristics of their nesting sites is fundamental for their conservation. However, the process of nest site selection is not well understood. Beach topography is one factor that is believed to drive nest site selection, but considering that many nesting beaches, particularly in Australia, are large and remote, effectively acquiring beach topographic information is challenging. Airborne light detection and ranging (LiDAR) data can obtain accurate information on beach topography and can be used to understand how topography influences nest site selection. The aims of this study were to (i) characterize beach topography using LiDAR data of Eighty Mile Beach, a remote 220 km long beach in north-west Australia, and (ii) determine which features influenced nest site selection of the flatback sea turtle (Natator depressus) population nesting there. Metrics of beach features that were believed to be relevant for nest site selection were calculated from piecewise linear regression models fitted to beach profiles extracted from transects generated along Eighty Mile Beach. Aerial photographic survey data were used to quantify flatback sea turtle nesting and were converted to presence-absence and nesting density classes per photograph. Classification tree models were used to model the influence of beach topography on the density and presence-absence of nests. Nests were predicted to be present if the highest elevation in the profile was 9 to 12 m (overall accuracy = 60%, detection rate = 80%), though density could not be modelled successfully (overall accuracy = 18%, detection rate = 80%). These results agree with the hypothesis that the silhouette in front of a sea turtle influences nest placement, though moderate model performance suggests that nest site selection by flatback sea turtles at Eighty Mile Beach are also influenced by other factors besides beach topography. The methods used in this study successfully characterized the beach topography of Eighty Mile Beach using LiDAR data and have the potential to be used on remote and large beaches in other parts of the world