Land crab species disappeared from Hawaii 1,000 years ago

Geograpsus severnsi was a land crab once common to the Hawaiian islands. They were once major predators of small creatures in the forest. Now, they’re only known from fossil remains. The demise of Hawaiian land crabs occurred around the same time Polynesians first arrived at the Hawaiian islands. These large crabs most likely fell prey to humans, as well as the animals brought to the islands by these first settlers.

Land crab fossils have long been found at numerous locations at the Hawaiian Islands, most commonly on Hawaii, Maui, Oahu and Kauai. But their identities had never been clearly established. It wasn’t until scientists began comparing the fossils’ physical features to other land crab species did they realize that these fossils were of a species new to science.

Oceanic land crabs are found on most tropical islands. Populations in the Pacific are in decline due to a variety of reasons including harvesting for food and predation by introduced animals. One land crab species, Geograpsus grayi, found on many Pacific islands, is closely related to the extinct Hawaiian land crabs.

Hawaiian land crabs were the most land-adapted crab species in the Pacific, their fossils found farther inland and at higher elevations than others. They were the largest of its genus^[1], Geograpsus, and were important predators in the flora and fauna of the Hawaiian Islands, taking seeds, plants, small invertebrates, and small nesting seabirds. Like all land crabs, they returned to the sea to spawn, embarking on mass migrations similar to the well-known land crabs of Christmas Island.

Two clips from National Geographic about the red crabs of Christmas Island. The first video shows migration of red crabs to release their eggs in the ocean. The second has young crabs emerging from the ocean to continue their lives on land. Similar events would have occurred on the Hawaiian Islands before their land crabs became extinct.

But the crabs’ success at colonizing diverse habitats on land was no match for new arrivals to the Hawaiian Islands. Based on radiocarbon-dating of fossil remains, Hawaiian land crabs disappeared not long after the first Polynesians settled on the Hawaiian Islands around 1,000 years ago. The settlers brought animals such as dogs, pigs, and chickens, as well as stowaways like rats and lizards. These newcomers, human and animal, changed the island forever.

Dr. Gustav Paulay, marine malacology curator at the Florida Museum of Natural History and lead author of a paper published in PLoS online, said in a press release,

If these land crabs were alive today, Hawaii would be a very different place. They certainly were major ecological players, as they were very abundant, large, carnivorous omnivores.

Islands have a limited habitat area, so that makes organisms on them inherently vulnerable. Because islands are isolated, major groups of species, like mammals or ants can be absent. Thus, island species evolve in their absence and often can’t cope when such continental predators are introduced.

When people arrive on an island, initially it’s like the Garden of Eden – few people and lots of resources. I can imagine a period when humans and the introduced rats, dogs and pigs would have preyed heavily on the crabs, especially during their mass migrations to release larvae in the ocean.

It doesn’t take much to upset the ecological balance of an island. For instance, at Christmas Island in the Indian Ocean, which is well-known for its land-dwelling crabs, an accidently-introduced ant species, known as “yellow crazy ant,” has destroyed some populations. Said Paulay,

The loss of this [Christmas Island] crab from areas has led to change in the structure of the forest because the crabs controlled litter and ate the seedlings of weeds. We don’t know the full ecological impact of all these land crabs, but we know that on islands, it’s usually substantial.

Helen James, curator of birds at the Smithsonian Institution’s National Museum of Natural History, commented on these findings in the same press release,

A study like this can reveal what the structure of the natural ecosystem was before these human-caused ecological changes, and that’s very important for moving forward with conservation. It highlights the complexity of the ecological changes that took place on the Hawaiian Islands and their severity in causing the extinction of a land crab.

Scientists examining fossil remains of land crabs in the Hawaiian Islands have found a new species that became extinct about 1,000 years ago. Its disappearance coincided with the first arrival of Polynesian settlers to the islands. Hawaiian crabs likely became a food source to humans, the female crabs being especially easy to collect during mass migrations to release their eggs into the ocean. They would have also become easy prey for introduced mammals like dogs, pigs, and rats. The demise of Geograpsus severnsi has significantly changed the ecological balance of the original Hawaiian forests, though the extent of the changes are not yet well-understood.

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