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A Case Study of the Coconut Crab Birgus Latro on Zanzibar Highlights Global Threats and Conservation Solutions

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A Case Study of the Coconut Crab Birgus Latro on Zanzibar Highlights Global Threats and Conservation Solutions A case study of the coconut crab Birgus latro on Zanzibar highlights global threats and conservation solutions T IM C ARO,HAJI H AMAD,RASHID S ULEIMAN R ASHID,ULRIKE K LOIBER V ICTORIA M. MORGAN,OSSI N OKELAINEN,BARNABAS C ARO,ILARIA P RETELLI N EIL C UMBERLIDGE and M ONIQUE B ORGERHOFF M ULDER Abstract The coconut crab Birgus latro, the largest terrestrial through local communities capitalizing on tourist revenue, decapod, is under threat in most parts of its geographical a conservation solution that could be applied more generally range. Its life cycle involves two biomes (restricted terrestrial across the species’ range. habitats near the coast, and salt water currents of the tropi- Keywords Birgus latro, coconut crab, conservation recom- cal Indian and Pacific Oceans). Its dependence on coastal mendations, IUCN, Pemba, population size, Tanzania habitat means it is highly vulnerable to the habitat destruc- tion that typically accompanies human population expan- sion along coastlines. Additionally, it has a slow reproductive rate and can reach large adult body sizes that, together with Introduction its slow movement when on land, make it highly susceptible to overharvesting. We studied the distribution and popula- he current wave of species extinctions in the tropics tion changes of coconut crabs at island sites in coastal Tis being driven by habitat loss and human exploitation. Tanzania on the western edge of the species’ geographical Species that are particularly susceptible to habitat conver- range. Our aim was to provide the data required for reassess- sion include those that depend on more than one biome ment of the extinction risk status of this species, which, despite to complete their life cycle, rendering them vulnerable to a indications of sharp declines in many places, is currently ca- wide variety of habitat changes or losses. Examples include tegorized on the IUCN Red List as Data Deficient. Pemba marine turtles that breed on land (such as the leatherback Island, Zanzibar, in Tanzania, is an important refuge for turtle Dermochelys coriacea) and terrestrial anurans that re- B. latro but subpopulations are fragmented and exploited by produce in freshwater (such as the sky blue poison dart frog children and fishers. We discovered that larger subpopula- Hyloxalus azureiventris). Most land crabs (e.g. Gecarcini- tions are found in the presence of crops and farther away dae, Ocypodidae, Varunidae, Grapsidae and Mictyridae) and from people, whereas the largest adult coconut crabs are the terrestrial hermit crabs (Coenobitidae) also occupy two found on more remote island reserves and where crabs are niches because juveniles and adults are entirely terrestrial but not exploited. Remoteness and protection still offer hope for their larvae must develop at sea. This dependence on different this species but there are also opportunities for protection habitats at different times in their life cycle makes it more difficult to assess the best way to conserve such species. Furthermore, long-lived species with long generation times TIM CARO (Corresponding author, orcid.org/0000-0001-6804-8519) Department of Wildlife, Fish & Conservation Biology, University of are particularly susceptible to exploitation because recovery California, Davis, California 95616, USA. E-mail [email protected] from population crashes can take years. HAJI HAMAD and RASHID SULEIMAN RASHID Department of Forestry and Non- The coconut crab Birgus latro (family Coenobitidae) is the Renewable Natural Resources, Wete, Pemba, Zanzibar, Tanzania largest of the land crabs (Laidre, ). Adult coconut crabs ULRIKE KLOIBER Chumbe Island Coral Park, Stonetown, Zanzibar, Tanzania are omnivorous and exclusively terrestrial, being so well VICTORIA M. MORGAN Department of Evolution and Ecology, University of adapted to life on land that they drown in water. Birgus California, Davis, USA latro occupies a vast oceanic range across the Indian and OSSI NOKELAINEN Department of Biological and Environmental Science, Western Pacific Oceans (Reyne, ) where it is found on is- University of Jyväskylä, Jyväskylä, Finland lands and archipelagos, but today it is virtually absent from BARNABAS CARO Berlin, Germany continental landmasses with suitable habitat, probably as a re- ILARIA PRETELLI Department of Human Behavior, Ecology and Culture, Max sult of overharvesting by people and habitat destruction, and Planck Institute for Evolutionary Anthropology, Leipzig, Germany possibly from competition with other crabs (IUCN, ). NEIL CUMBERLIDGE Department of Biology, Northern Michigan University, Marquette, USA The life cycle of coconut crabs involves overland mating migrations to coastal areas where females carrying fertilized MONIQUE BORGERHOFF MULDER ( orcid.org/0000-0003-1117-5984) Department – of Anthropology, University of California, Davis, USA egg masses remain for weeks before depositing their Received April . Revision requested July . eggs into the sea (Amesbury, ; Brown & Fielder, ). Accepted July . Coconut crab larvae spend several weeks in the plankton, Oryx, Page 1 of 8 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International. doi:10.1017/S0030605319000863 Downloaded from https://www.cambridge.org/core. UC Davis, on 22 Apr 2020 at 09:27:55, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605319000863 2 T. Caro et al. carried by ocean currents, where they develop through several larval stages before emerging on to land in coastal areas as juveniles (glaucothoe) that, like most hermit crabs, protect their vulnerable abdomen inside an empty mollusc shell. At this stage, they are susceptible to predation by rats, pigs and ants. While coconut crabs grow, their abdomen becomes hardened and they no longer need the protection of a mol- lusc shell and can eventually attain kg in weight as adults (Fletcher et al., ). Coconut crabs take – years to reach sexual maturity (Fletcher, ) and breeding females are encountered infrequently: only one berried female (carrying eggs) was identified out of female crabs observed during the years of this study. Most populations of coconut crabs have a sex ratio skewed towards males, but the reasons for this are unknown (Drew et al., ). Coconut crabs are restricted to coastal areas for reproductive reasons, and to humid forested habitat that contains rock crevices or cool burrows where they can hide during the heat of the day. Unfortunately, agricultural expansion is encroaching on this coastal scrub habitat, so that it is diminishing in many parts of the species’ range. The coconut crab was initially categorized as Rare on the IUCN Red List and changed to Data Deficient in (Drew et al., ), but there is a growing and widespread con- sensus from numerous studies across the species’ range that many isolated populations are under sustained threat from human exploitation (Fletcher, ). The status of the coco- nut crab is currently being reassessed in the light of the threats to its coastal habitats from expansion of farmland, and over- FIG. 1 Location of islands mentioned in the text and of Dar es harvesting for both local consumption and export (Eldredge, Salaam and Kilwa, Tanzania. ). The massive size and strength of coconut crabs (Oka et al., ) make this charismatic invertebrate an unusual were to monitor exploited populations of coconut crabs on candidate as a flagship species for raising conservation aware- Pemba and its outlying islands, and to monitor protected ness on tropical islands. These crabs could also become an crab populations on Chumbe Island off Unguja, and on umbrella species for reducing the rapid decline of tropical is- Mbudya and Bongoyo Islands close to Dar es Salaam. land coastal forests and scrubland, a biome that is often over- We employed two different methods. The first was to looked by those trying to protect nearby mangrove forests. make a comprehensive island-wide survey of the local Threats to the coconut crab are similar in many parts of its children in every shehia (ward) around the entire coastline vast global range, which includes two major tropical oceans of Pemba, to establish the current distribution of coconut (Eldredge, ), but the intensity of the threats mean there crabs on the island. The second was to measure coconut is insufficient time to gather data from every population. crabs at sites on Pemba and the other three islands to Here we present a case study of the coconut crab in the determine crab population sizes, ongoing changes, habitat Zanzibar Archipelago, to determine the status of these subpo- status, and the principal threats to these populations. pulations and the threats to their existence, and to understand and resolve the threats that afflict this species worldwide. The Zanzibar Archipelago (i.e. Pemba and Unguja) lies off Methods the coast of East Africa in the Western Indian Ocean along the Somalia–Kenya–Tanzania–Mozambique coastline (Fig. ) The distribution and population size of, and threats to, at the western edge of the coconut crab’s range. In coconut crabs on Pemba were assessed using focus groups Tanzania, where the species has no formal protection, of local primary school children (typically – years old) there are populations of coconut crabs on the Pemba because they are the principal collectors of these crabs. The Island archipelago and on isolated islands off the coast of children may have come from several different villages, Unguja (sometimes erroneously called Zanzibar) and Dar but were chosen for their interest in forests and coastal areas es Salaam (Caro & Morgan, ).Theaimsofthisstudy and because they had experience foraging for wild animals. Oryx, Page 2 of 8 © The Author(s), 2020. Published by Cambridge University Press on behalf of Fauna & Flora International. doi:10.1017/S0030605319000863 Downloaded from https://www.cambridge.org/core. UC Davis, on 22 Apr 2020 at 09:27:55, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0030605319000863 Coconut crabs of Zanzibar 3 Focus groups were conducted by two people, who visited accessible by foot).
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