Quantifying predation on sea turtle eggs and emerging hatchlings by ghost crabs and other native predators
Date of Award
Edith Cowan University
Doctor of Philosophy
School of Science
Glenn A. Hyndes
High predation rates on sea turtle eggs and hatchlings, in combination with other pressures, are likely having a significant impact on sea turtle numbers, which is a particular issue for breeding populations of threatened or endangered sea turtle species. Yet, comprehensive assessments of predation on eggs and hatchlings combined are lacking. This study aimed to quantify the levels of predation on eggs and hatchlings by a range of predators, and the importance of eggs and hatchlings to the diets of ghost crabs, which are known predators of early life phases of turtles and have been observed to be highly abundant on turtle nesting beaches on the north-west coast of Australia. The study focussed particularly on two important nesting sites for the loggerhead turtle Caretta caretta in the Ningaloo Coast World Heritage Area, while predation was also assessed at a nesting beach for the flatback turtle Natator depressus on Thevenard Island. Start- and end-of-season nesting inventories, in-situ accelerometers, and infrared videography were used to assess predation on eggs and emerging hatchlings, while ghost crab burrow counts were used as proxy for crab densities to assess the link between ghost crab densities and levels of predation. In addition, complementary methods were used to determine the diets of ghost crabs: gut content analysis (GCA); DNA analysis of gut contents; stable isotope analysis (SIA) of claw tissue; and controlled feeding experiments to determine food preferences and feeding rates on different potential food sources.
This study demonstrated the high pressure that ghost crabs place on sea turtle eggs and hatchlings at some nesting beaches. Ghost crab densities at Gnaraloo Bay were almost twice those at Bungelup Beach and Thevenard Island, corresponding to egg predation levels ranging from extreme to none (79% vs 39% vs 0%, respectively). Accelerometer data suggested that eggs were most vulnerable to predation in the first days after ovipositing and the final weeks before hatching. No hatchlings were observed emerging from nests at Gnaraloo Bay, where egg predation was estimated at 79%. In comparison, 43% of hatchlings were predated at Bungelup Beach, and 30% of hatchlings were predated on Thevenard Island, mainly by ghost crabs, and to a lesser extent by silver gulls. A range of other predators were observed, including native goannas and terns but also the invasive black rat.
The golden ghost crab Ocypode convexa was far more abundant than any other ghost crab species at all locations. GCA showed that this species is a facultative scavenger with an omnivorous diet comprising high contributions of abundant beach-cast leafy brown algae. However, DNA analysis identified C. caretta in ≥ 20% of crab guts, and stable isotope mixing models showed that sea turtle was the most important source of assimilated C and N during the nesting season. Additionally, aquarium-based feeding assays showed that O. convexa preferred sea turtle and fish carrion over leafy brown algae. Thus, ghost crabs play a major role in the consumption of sea turtle eggs and hatchlings, and therefore the transfer of nutrients and energy derived from sea turtles at rookeries on sandy beaches. An estimated ~90% of the energy derived from turtle nests at Gnaraloo Bay is transferred to higher tropic levels through predation, compared to ~53% at Bungelup Beach and ~14% at Thevenard Island. This study highlights the important roles of both sea turtles and ghost crabs in energy fluxes and nutrient cycling at generally nutrient-poor sandy beach ecosystems.
The sometimes extreme rates of mortality of eggs and/or hatchlings via native predators can possibly put the long-term survival of some sea turtle stocks at risk. These findings provide a basis to urgently establish management strategies to limit predation by native predators at turtle rookeries.
Access to this thesis is embargoed until 27th June 2025.
Avenant, C. S. (2023). Quantifying predation on sea turtle eggs and emerging hatchlings by ghost crabs and other native predators. Edith Cowan University. https://doi.org/10.25958/z3r7-g505