Hawai`i is the only tropical island group without land crabs.
Those crabs are a classic part of the environment of other warm-climate islands in Polynesia and elsewhere. It’s just another way in which these islands are unique.
But it wasn’t always so. Archaeological digs now show that land crabs once existed here, were quite common, and disappeared shortly after humans arrived—part of that first wave of extinctions that marked the changing Hawaiian environment over the past millennium or two.
The new findings were published under the title, Evolution, Insular Restriction, and Extinction of Oceanic Land Crabs, Exemplified by the Loss of an Endemic Geograpsus in the Hawaiian Islands, by Gustav Paulay of the Florida Museum of Natural History and John Starmer of the University of Florida at Gainesville. Their paper was published in the online scientific journal PLoS ONE, and can be found here.
If we ponder how much today’s Hawai`i differs from that discovered by the first Polynesians, this is an interesting window through which we can peer.
The land crabs may have been part of an entire interconnected web of life that has disappeared in the Islands. They probably were predators, feeding on the eggs of birds whose rookeries are now largely gone, on land snails that are now mostly gone, and on native insects. They had beneficial roles in the environment, helping cycle nutrients in the ground litter, spreading seeds and other things.
The Hawaiian land crab Geograpsus severnsii was once common and found on multiple islands. Its shells have been recovered from Maui and Big Island caves, Moloka`i and Kaua`i sand dunes, from the Barbers Point area of O`ahu, and elsewhere.
These were good-sized crabs, with backs 2.5 inches across, and with huge claws. While, like all crabs they retained an oceanic phase of life during their larval stages, once these crabs returned to land, they were truly terrestrial, sometimes ranging a mile or more inland, and to elevations of more than 3,000 feet.
They grew larger than any other crab in the Geograpsus genus, extended farther inland, and traveled to higher elevations.
The crabs disappear from the fossil record shortly after the arrival of humans in the islands. And their disappearance represents the world’s first extinction of a crab species in the modern age.
“That the first documented crab extinction is of a land crab in the Hawaiian Islands is not surprising. The Hawaiian terrestrial biota suffered a veritable mass extinction following human arrival, and this mass extinction continues today,” the authors write.
Rats, pigs, dogs and perhaps even human predation may have played a role in their extinction.
Their loss was beneficial, certainly, to some prey species, and harmful to other parts of the environment. In the paper abstract, the authors summarize: “Land crabs are major predators of nesting sea birds, invertebrates and plants, affect seed dispersal, control litter decomposition, and are important in nutrient cycling; their removal can lead to large-scale shifts in ecological communities. Although the importance of land crabs is obvious on remote and relatively undisturbed islands, it is less apparent on others, likely because they are decimated by humans and introduced biota. The loss of Geograpsus and potentially other land crabs likely had profound consequences for Hawaiian ecosystems.”
The authors of the study worked with fossil material collected by several scientists who worked across the state. They thanked collectors Mike Severns (for whom the species was named), David Burney, Cory Pittman, Storrs Olson, and Helen James.
© Jan TenBruggencate 2010
^ Gustav Paulay & John Starmer (2011). "Evolution, insular restriction, and extinction of oceanic land crabs, exemplified by the loss of an endemic Geograpsus in the Hawaiian Islands". PLoS ONE 6 (5): e19916. doi:10.1371/journal.pone.0019916.