An Ecosystem Engineered by Crabs
Clipperton Island is a remote, 1.7-square-kilometer ring of coral rubble in the eastern Pacific, enclosing a lagoon. The island's dominant terrestrial residents are millions of bright orange land crabs, Johngarthia oceanica. A 1968 census estimated the population at 11.5 million individuals, creating a landscape almost entirely devoid of vegetation. The crabs are omnivores, consuming seeds, seedlings, and any other organic matter they find, effectively creating and maintaining a coral desert.
The island's enclosed lagoon is a strange, stagnant body of water, cut off from the ocean for approximately 170 years. Its surface layer is almost fresh, while the deep basins—one named "Trou Sans Fond" or "the bottomless hole"—are anoxic and acidic. This harsh chemical environment prevents fish from living in the lagoon, which is instead inhabited by millions of isopods.
The ecological state of Clipperton is an example of a trophic cascade, a phenomenon where the population of one species directly impacts multiple other levels of the food web. The island's history is a series of boom-and-bust cycles driven by human-introduced species.
A History of Ecological Disruption
In the late 19th century, guano miners introduced pigs to the island. These pigs preyed heavily on the land crabs. With the crab population suppressed, weedy vegetation and grasses flourished, eventually covering about 80 percent of the island's surface. The seabird colonies, including Masked and Brown Boobies, were also decimated by the pigs, which ate their eggs and chicks. By 1958, only 150 Masked Boobies and 500 Brown Boobies remained.
That year, ornithologist Ken Stager visited the island and, seeing the devastation, single-handedly eliminated all 58 pigs. The consequences were swift and significant. Without their primary predator, the land crab population exploded, and the crabs consumed the vegetation, returning the island to its barren state. The seabird populations recovered spectacularly; by 2003, Clipperton had the world's largest Masked Booby colony, with 112,000 individuals, and the second-largest Brown Booby colony.
A new chapter of disruption began around 1999-2000, when shipwrecks likely introduced black rats. The rats prey on both seabird chicks and land crabs. By 2005, scientists observed that the declining crab population was once again allowing vegetation to spread across the island, altering the habitat for ground-nesting birds and showing the oscillating balance of this isolated ecosystem.