Article Review: Predator-to-Prey Relationship of Ghost Crabs and Sea Turtles (and some unfortunate baby sliders)

Sea turtle populations have significantly decreased over several decades from by commercial fishing, poaching,and development of important nesting and nursery sites. In efforts to slow the decline of sea turtles, many conservation practices have been deployed such as tighter restrictions on fishing and development and protection of nests from natural predators.  However, little has been documented for predation rates on turtle hatchlings traveling from nest to sea.  We have all seen, on the Discovery Channel, the newly hatched turtles dashing to the sea from their nests in hopes to survive another day.  Foxes, raccoons, and birds eating till too full to consume any more, allowing a few turtles to make it to the sea. However, one predator and prey dynamic that has little knowledge and large implications is the ghost crab.

The ghost crab, a popular beach dweller, borrowing just beyond tidal wave action and feeds happily on the small turtle hatchlings. To observe and quantify the effects of ghost crab predation on sea turtles, researchers Peterson et. al (2013) in “Conservation implications of density-dependent predation by ghost crabs on hatchling sea turtles running the gauntlet to the sea” explores the effects by observation and clever experimental design.

Peterson et al. first observed an alarming 24% mortality rate from ghost crab predation on 42 loggerhead (Caretta caretta) hatchlings from 10 different nests on a secluded North Carolina beach.

To test the effects of density dependence of predator-to-prey relationship Peterson et. al took varying numbers freshwater slider turtle hatchlings and released them in a high / low abundance of ghost crabs. Freshwater sliders were chosen because unlike sea turtles their populations remain abundant and under no threat.  Also slider turtle hatchlings are nearly the size of loggerhead hatchlings to make a seamless transition for the ghost crabs. These sliders were released in the same type of habitat as loggerhead nests would be found in. Prior to the release of the hatchlings, the experimenters, in the day, mapped out predatory density by counting visible ghost grab borrows with a quadrate to get an accurate assessment of densities.

The article found that for the site with high ghost crab populations nearly 50% of turtles traveling alone were aggressively encountered.  While turtles traveling in groups (8-40 individuals) experienced less aggressive encounters at nearly 45%. In the site where lower ghost crab populations existed, 11% of the lone turtles were affected and 22% of the grouped turtles were affected. In the higher density of ghost crab predators, the lone turtles were at higher risks of predation and at lower densities of predators the grouped turtles were engaged at higher rates.

To lower the chances of predation, traveling to the sea in large numbers gives rise to the theory of predation dilution. Turtles have higher success rates against high densities of predators by overwhelming them with numbers. However, with sea turtle populations declining, turtles are unable to hatch in large numbers while ghost crab populations are remaining stable. These have large implications on the success rate of turtle population sustainability if fewer and fewer turtles return to nest and even fewer make it past the gauntlet to the sea.

One way to apply this article is to turtle conservation is to examine the predation-to-prey relationship when relocating turtle nests. If mammalian predation exceeds 10% on nests on a particular beach, moving the nests to less dense predation populations is considered (Peterson et. al). While nests are relocated, ghost crab densities are rarely considered because they do not predate on the eggs themselves. When relocating the nests, one may space them closer together to increase group emergences under high densities of ghost crab populations. Under low densities of ghost crabs, nests should be spaced further apart to create non-group migrations to the sea. The article should prove extremely beneficial to the conservation and sustainability of sea turtle populations under threat of ghost crab predation.

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Peterson, Charles H.. Conservation implications of density-dependent predation by ghost crabs on hatchling sea turtles running the gauntlet to the sea  MARINE BIOLOGY. Vol. 160 No. 3. 2013 p. 629 - 640.