1.Introduction of coconut crab
The coconut crab Birgus latro (L.), the monospecific genus Birgus within the family Coenobitidae, and probably the largest terrestrial arthropod in the world, is found throughout many of the islands of the Indo-Pacific, including, the Green Island in Taiwan (Fig. 1). It is a typical hermit crab throughout the initial stages of its life. Females release their larvae into the sea in the new moon phase, and the larvae live with the planktonic community for about a month. Then as postlarvae, or glaucothoes, settle to the bottom, and carry a small snail shell, and then emerge onto the shoreline as a part of hermit crab communities (Reese, 1968). When they reach a size of roughly 1 cm across the carapace, coconut crabs give up the shell-carrying habit of their hermit crab ancestors (Fig. 2).
Fig. 1 The coconut crab, Birgus latro
Fig. 2 Life history of the coconut crab, Birgus latro.
2. About Green Island
Green Island is a small volcanic island located off the southeastern coast of Taiwan (Fig. 3). It covers only 16.2 km2, with a population of only 3201 people in 2003. Its climate is warm, wet, and windy, with an average annual temperature of about 23 °C. The plenty ecological resources of Green Island are well preserved due to its unique historical development. Two thousand years ago, aboriginal BeaNam people lived on the island. A mere 200 years ago, Han (Chinese) people began arriving. The basic economy of the island has traditionally been fishing and the raising of deer, but the recent development of tourism has involved many people in this industry (History of Green Island, available at http://www.ttcsec.gov.tw/website/ce31/pic_m.htm). The most important resource of Green Island is the high biodiversity of the surrounding seas. There are records of 176 species of hard corals, 27 species of soft corals, and 602 species of fishes (Chang et al., 1983, 1991; Jeng, 2002), as well as numerous seaweeds, sponges, shells, shrimp, crabs, sea urchins, starfish, etc.
Fig. 3 Map of Green Island.
3. The threaten of coconut crab on Green Island
Tourism development is now viewed as an important aspect of economic activities for enhancing local economies, especially on this island where tourism is rapidly developing; it can provide new employment and revenues and additional tax receipts to local communities, enhance community infrastructure, etc. (Ko et. al., 2002). The development of tourism on Green Island in recent decades has brought many tourists onto this small island, resulting not only in large economic benefits locally, but also in enormous pressures on both the coastal ecology and local society.
On Green Island, coconut crabs have a close relationship with residents. Since their ancestors immigrated from Liuchiu to Green Island 200 or so years ago, the local Han Chinese have used the crab as a source of animal protein for their families, and additionally, children play with the coconut crabs as pets.
However, due to tourism and other activities, environmental pressures have widely impacted the ecological systems of Green Island. For instance, carcasses of coconut crabs and other crabs litter the roads because of passing cars and motorcycles driven by tourists. Up to 2000 tourists visit Green Island per day in the summer season, while the only road circling Green Island is a mere 20 km long. These coastal roads cross the migration pathways of many organisms, and crabs are threatened since most crabs migrate landward from the sea during the mature stage, and then return to the sea during the spawning period. In addition, there are poachers who hunt the crabs illegally. Therefore, the populations of crabs on Green Island are declining at an unprecedented rate.
The coconut crab, is the species most seriously threatened by tourism development. Not only do roads cross their migration routes, but environmental deterioration has also damaged their habitats, e.g., the cutting or burning coastal forests. Moreover, coconut crabs are considered sea delicacies by local residents. As a result, the population of coconut crabs has dropped so low that only experienced local residents can now find them. It has been on the Red list of the International Union for the Conservation of Nature since 1983 (Wells et al., 1983), and is the only protected crustacean species designated by the Wildlife Conservation Law in Taiwan since 1995.
4. To conserve coconut crab on Green Island
Our proposition of using the coconut crab as a target species of conservation efforts is based on its (1) special large body size, (2) special niche, (3) long life history, and (4) the unique life of this migrating species, as adults occur in terrestrial mountain areas. Successfully conserving the habitat of coconut crabs will not only protect the coastal zones and nearby waters, but also land areas. It is an umbrella species of Green Island, and the best indicator for monitoring the quality of this island's ecosystems. In order to restore and manage the vitality of this threatened species, we have implemented three comprehensive lines of works, including artificial culture, habitat protection, and community education.
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