Once abundant, Bermuda’s land crab population has dwindled dramatically. We examine the reasons behind their decline, the impact of biological control on our native species, and ask what is being done and what more we can do to protect the few land crabs we have left.
Up until the early twentieth century, Bermuda was home to fewer people, fewer cars, fewer buildings, no herons, and many more land crabs. Now the opposite is true.
The most notorious of Bermuda’s land crabs is the red land crab, also known as the common land crab. Pre-1976 they were so abundant they would cross South Shore Road in their thousands as they left their burrows in search of the sea. “You were just driving along and there’s this wave of crabs,” remembers Dr Robbie Smith, curator of the Natural History Museum at the Bermuda Aquarium, Museum and Zoo. “You’d slow down, but the wave kept going.”
Because the wave kept coming, people had to drive on regardless leaving, he adds, “a carpet of dead crab shells,” which “stank,” particularly because it was the summer. “They were ubiquitous, and it would happen year after year.” The crabs were considered a pest, ruining farmer’s crops, burrowing into the ground and climbing almost apocalyptically up the walls and windows of south shore homes. People had had enough, and the crabs needed to be culled. Since these crabs had no natural predators in Bermuda, prior to the 1970s the preferred method of control had been poison baits. To reduce the use of toxic substances, however, Dr David Wingate provided an alternative, the reintroduction of their natural predator, the yellow-crowned night heron, which he did, successfully, in 1976.
Dr Wingate chose the yellow-crowned night heron after discovering historical evidence of herons around the time of the first settlers. In his 1982 paper “Successful Reintroduction of the Yellow-Crowned Night Heron as a Nesting Resident on Bermuda,” Dr Wingate, who back then was the Bermuda government’s Conservation Officer, wrote: “Accounts written at the time of Bermuda’s settlement leave no doubt that herons and egrets of several species were resident and breeding on the island.” He goes on to quote accounts from settlers, including Sea Venture survivor Sylvanus Jourdain: “There are also great store and plenty of herons and those so familiar and tame that we beat them down from the trees with stones and staves, but such were young herons.” He later found conclusive evidence of these herons when subfossil bones were identified from three separate collection sites around Bermuda.
While the heron re-introduction was undoubtedly a success in dealing with the problem of the red land crabs, they weren’t the only land crab species being eaten by these birds. The hermit and giant land crabs were also preyed upon, and their numbers have dwindled to such an extent that they both became protected species in 2012. The giant land crab has level 1 protection under the Protected Species Amendment Order (2016) and the land hermit crab has level 2 protection. This means large fines or imprisonment for anyone caught wilfully doing anything that could damage, kill or interfere with these crabs or their habitats. We don’t know how many remain in existence, but it isn’t very many. In July 2020, then biodiversity officer Alison Copeland published management plans for both the giant land crabs and the land hermit crabs. She wrote that in the early 1990s the population of the land hermit crab was estimated to be 150. No formal population estimate has ever been made for the giant land crab, but she says it is thought to be “no more than a few hundred.”
What does this say about the impact of biological control? “It wasn’t an unreasonable decision given how many [red land crabs] we had, and this was a predatory bird we had here before,” says Dr Smith. “It solved a problem for farmers on the south shore. They were really suffering. It worked, unlike other introductions that didn’t work.
“We have learned hard lessons in Bermuda that biological control can have limited success in accomplishing the intended reduction in pest species,” he continues. “This was the case for the three species of introduced land snails imported to control pest snails. The pests remain but our native and endemic snails seem to have suffered catastrophically.
“We introduced three species of anole lizards to control flies, but we still have flies.
“Then, we introduced the kiskadee to control the lizards and now we have an abundant noisy bird that eats just about anything else besides the lizards and is a predation risk for our endemic skink.”
Biological control aside, there are many other reasons why land crabs are vulnerable and they are almost all man-made. These, writes Copeland, are habitat loss and alteration due to development and destruction of coastal habitats by human and natural causes, invasive species, pollution, traffic and lack of public awareness. In the case of the land hermit crab, which inhabits the shells of other animals, it also includes the lack of mollusc shells, particularly their preferred home of West Indian top shells, for shelter. The West Indian top shell is also protected and carries larger fines or prison terms for anyone caught taking them. Like the crabs’ original predator, these snails were wiped out by early Bermudians who took them for food. They were re-introduced in 1982 at Nonsuch Island and have since spread along the south shore.
In Bermuda, the land hermit crab inhabits vegetated upland coastal sites, saltmarshes, sand dunes and rocky islets. They are, therefore, found along the south shore and the east end, where the majority of West Indian top shells are located. These shells are also becoming more common on the north shore and the west end, and more recently a small group of land hermit crabs showed up on Trunk Island. The giant land crabs primarily inhabit coastal mangroves where they dig burrows. They, therefore, suffered hugely when coastal habitats, especially mangroves, were destroyed during parts of the twentieth century, writes Copeland: “The construction of the airport and associated US military base between 1941 and 1943 reportedly wiped out approximately one third of Bermuda’s total mangrove acreage as well as a giant land crab colony within Castle Harbour.”
More recently, giant land crabs have been observed around Kindley Field, BIOS, Walsingham Bay, Stocks Harbour and Coopers Island. The remaining giant land crabs are distributed across Bermuda “in fragments” with the largest known population in the mangrove swamp at Hungry Bay Nature Reserve and another large population in the mangroves along Wreck Road in Sandys. “The most crucial way to conserve giant land crabs,” points out Copeland, “is to conserve the mangroves they call home.” And the good news is that work is underway at a national level to do just that: “Bermuda’s mangroves were recently completely mapped by the Department for Environment and Natural Resources. The Bermuda Ocean Prosperity Programme Marine Spatial Plan seeks to strengthen protection of mangroves, and four mangrove restoration projects have been started on Trunk Island, Lagoon Park, Hungry Bay and Stocks Harbour,” says Dr Smith.
Another problem for both hermit and giant land crabs is access to the sea. Both female hermit and giant land crabs “walk” to the sea to shed their eggs. These then develop as planktonic larvae before returning to land as young crabs. The construction of buildings, roads, fences and walls has separated land crabs from resources such as food, shelter, mates and safe ocean access, and Dr Smith recommends that anyone owning a coastal property consider “establishing or maintaining corridors to the coast through built structures like retaining walls and pools.” He also recommends their working “to improve their land by removing invasive trees and plants and limiting the use of poisons and the accumulation of trash and debris.”
“There is no accurate data for the number of land crabs remaining,” continues Dr Smith, “but given the relative low populations of land hermit crabs, the giant land crab and potentially the common land crab, it is possible that they could die out.
“Currently we do not fully understand the ecological roles these crabs play but as they are native species, we are obligated to protect them, even if we are not sure what they are doing.”