The impact of land crabs on the near-ocean forests in which they live has long been overlooked, with emphasis placed instead on water levels, salinity, and other abiotic influences. However, a new research synthesis published in Biological Reviews shows that land crab influence is among the most important factors affecting tropical forest growth along coasts, on islands, and in mangroves.
Land crabs come in a variety of species from the tiny Ecuadorian Hermit Crab, which weighs less than an ounce, to the Coconut Crab which, at 38lbs, is the world’s largest terrestrial invertebrate. While many species are similar in appearance to their aquatic relatives, land crabs exhibit an array of unique adaptations, the most notable and universal being an inflatable organ covering their gills which acts as a lung. In addition to an ability to extract oxygen from the air, many species of land crabs can extract moisture from their surroundings (from the soil they burrow into or the food they eat, for example), necessitating their return to the sea only to reproduce.
Most species of land crabs are omnivorous and can have quite an impact on terrestrial vegetation, so much so that many populations have become pests to farmers who grow crops near tropical coasts. Even so, the crabs’ role in the development of coastal ecosystems has only begun to be understood. “Land crabs are found in coastal forests which have been understudied due to inaccessibility in remote areas and habitat loss (deforestation) in developed areas.” says Dr. Erin Lindquist, professor at Meredith College and coauthor of the recently published synthesis. “In addition there has been little groundwork on the natural history of land crabs.”
However, recent research has begun to reveal the true nature of the crabs’ impact on their environment.
“Because their predation on seeds and seedlings has been found to be selective in some studies (preferring some plant species over others), land crabs may promote the growth and establishment of rare species of plants which escape crab mortality,” explains Dr. Lindquist. “In a tropical forest with many species of trees, competition for limited resources (light, water, soil nutrients, etc.) among tree species can limit recruitment (growth and survival). For example, crab predation may minimize the competition between a dominant (common) species and a rare species by reducing the number of seedling recruits of the dominant species. As a result the rare species is released from the competition and experiences higher survivorship (recruitment). Also, when a natural predator with such importance in the food web is removed from an ecosystem, the food web will deteriorate, and other members of the food web may become endangered or extinct. For example, if land crabs were removed from these coastal ecosystems, soil nutrient levels may decrease due to the decreased rates of litter decomposition (land crabs move leaf litter into their burrows thereby increasing litter decomposition and increasing nutrient return to the soil) which will in turn may limit plant growth.”
Land crab influence on local vegetation in now thought to be both direct and immediate, with measurable effects noticeable within a single season. “In years when [the land crab] populations are large, they may prohibit plant recruitment more than they encourage it. However, in years when their populations are small, we expect to see higher rates of plant recruitment.”
In addition, land crabs are an important source of food for many other animal species in near-ocean tropical forests.
Dr. Lindquist recommends that in order to preserve tropical coastal forests, land crab populations must also be protected by encouraging the regeneration of disturbed forests, protecting those forests that are yet pristine, and implementing restrictions on the harvesting of land crabs.
“Given their large ecological role in forested coastal ecosystems, the conservation of land crab populations is critical to the conservation of the forest.”