The impact of ghost crab predation on loggerhead sea turtles: environmental influences and management approaches

Abstract

Predation is one of the greatest threats to the eggs and hatchlings of threatened loggerhead sea turtles (Caretta caretta). Management efforts tend to focus on removing mammalian predators, and invertebrate predators have largely been overlooked. Yet ghost crabs (Ocypode spp.) can become prolific predators of sea turtle eggs when they become abundant. Current methods used to quantify egg predation by crabs may underestimate predation, and nest-to-surf mortality of hatchlings rarely quantified. Additionally, ghost crab abundance is difficult to estimate due to their complex behavior shaped by environmental variables, but understanding these factors would aid in effective management. This study, conducted on the Atlantic coast of Florida from 2021 to 2023, explored the effects of environmental variables on ghost crab abundance and how ghost crab abundance affects egg and hatchling predation rates. We found that crabs predated 5-10% of all eggs and attacked 6-13% of hatchlings. Higher crab densities led to increased hatchling predation, particularly among disoriented hatchlings, but egg predation was unaffected by crab density. We quantified crab abundance using two methods, through visual surveys of live crabs and burrow counts. We explored the effects of environmental variables on these indices of crab abundance, and found interactive effects of temperature, moon luminosity, and ambient lighting. Our findings highlight the importance of considering multiple environmental variables when assessing ghost crab abundance and activity, because crab behavior, such as burrow construction and foraging activity, changes with these environmental variables. We found that these two indices were unrelated when comparisons included these other environmental variables, suggesting that it may not be appropriate to use them interchangeably. We attempted to reduce crab density through two methods, live capture and the use of a sod roller to dispatch crabs. While live capture did not significantly reduce crab abundance, the sod roller method had a significant impact, which lowered predation on hatchlings by 75%. Our study demonstrates the importance of invertebrate predators of sea turtle eggs and hatchlings, as well as the importance of including both eggs and hatchlings in predation estimates, when considering conservation strategies for this threatened species.