Birgus latro (Linnaeus, 1767)

Cumberlidge, Neil, Caro, Tim, Watson-Zink, Victoria M., Naruse, Tohru, Ng, Peter K. L., Orchard, Max, Rahayu, Dwi L., Wowor, Daisy, Yeo, Darren C. J. & White, Tim, 2022, Troubled giants: The updated conservation status of the coconut crab (Birgus latro), Raffles Bulletin of Zoology 70, pp. 1-21 : 14-15

POPULATIONS OF BIRGUS LATRO AT RISK

Although extensive, the actual geographic range of Birgus latro is rapidly diminishing. For example, B. latro populations are virtually extirpated along continental coastlines from Somalia to Mozambique ( Fig. 4 ; Table 3). In addition, coconut crab populations are low on the coastal East African islands (Unguja and Pemba, Zanzibar) where human populations are high, and B. latro is now restricted to the outlying smaller islands off the coasts of these two larger islands. Crab populations are declining even on uninhabited Zanzibari islands that are visited by fishermen who camp overnight and eat crabs ( Caro et al., 2020). B. latro populations found on the 115 islands and atolls of the granitic and coralline Seychelles Archipelago (459 km ²) have suffered measurable recent and rapid declines of at least 80% over the past 15–20 years ( Poupin et al., 2013). This is accompanied by declining EOOs, AOOs, and fragmented populations, linked to continuing threats from overharvesting and habitat destruction. The result is that in many of these places B. latro is found in only very low numbers, and may even be extirpated in others ( Poupin et al., 2013). One bright spot is Aldabra Atoll (a protected UNESCO World Heritage Site) that supports an abundant B. latro population of more than one thousand terrestrial adults and juveniles. The recent surveys of most of the islands in the Seychelles Archipelago reported on by Poupin et al. (2013), however, indicate that B. latro is rare on Cousine, Aride, the Amirantes islands of D’Arros and Desroches ( Bowler, 1999; Jupiter et al., 2013), Alphonse ( Duhec, 2011), Bijoutier, Coetivy, Assumption, Cosmoledo, Astove, and Farquhar. Although B. latro may still occur in small numbers on Providence and St. Francois islands, Poupin et al. (2013) were unable to confirm this. Birgus latro may even be extirpated on the islands of Desneuf, Mahe, Marie Louise, and Remire, because no crabs were found during recent surveys ( Poupin et al., 2013).

Populations of B. latro on the four main Comoro Islands (2,236 km ²) have suffered rapid population declines of at least 50% over the past 15–20 years, and are now found in only very low numbers on Grande Comore (Njazidja, 1,025 km ²), Mohéli (Mwali, 211 km ²), and Anjouan (Nzwani, 424 km ²) ( Poupin et al., 2013). This species may even be extirpated on Mayotte, where recent searches produced no evidence of its presence ( Bouchard et al., 2013; Poupin et al., 2013). Other metrics include declining EOOs, AOOs, fragmented populations, and declining habitat quality.

Birgus latro populations on the five Glorioso Islands (5 km ²) have suffered rapid reductions in population levels (of at least 80% over the past 15–20 years), and this species, if present at all, is found only in very low numbers ( Poupin et al., 2013). These crabs have been subject to consistent threats from overharvesting and habitat destruction, and are vulnerable to local extirpation. While B. latro may still be present on Grande Glorieuse island, it is extremely rare, and may only persist because of the indirect protection from the military base there, which acts as a deterrent to illegal collecting. On the other hand, this species has been extirpated from Ile du Lys, where it is known only from a single record made in 1882 ( Coppinger, 1884; Miers, 1884) and was not found during recent surveys ( Poupin et al., 2013). Similarly, in two islands in the Mozambique Channel (32.4 km ²), B. latro is extremely rare on Juan de Nova Island (4.4 km ²) and probably extirpated on Europa Island (28 km ²) ( Poupin et al., 2013). Birgus latro is presumed to be extirpated in Madagascar and all four Mascarene Islands (4,536 km ²) because exhaustive surveys of suitable habitat in these inhabited islands have failed to report a single individual. Specifically, B. latro has been extirpated from Mauritius (1,900 km ²), where it was reported to be declining by Charles Darwin in 1836; the Agalega Islands (26 km ²; Poupin et al., 2013); and the Cargados Carajos Shoals (Saint Brandon; 5 km ²; Reyne, 1939). Birgus latro has also been extirpated from Reunion (2,500 km ²) and Rodrigues (110 km ²), though it is possible that they may never have been there in the first place.

Populations of B. latro in many of the 300 islands of the Andaman and Nicobar Islands (8,249 km ²) are small, fragmented, and appear to be in serious decline. Threats include overharvesting, despite social taboos against harvesting and legal protection under the Indian Wildlife Protection Act; and habitat destruction for agriculture and by the 2004 tsunami. The result is that B. latro may now be extirpated on some islands (such as Katchal, Trinket, and Car Nicobar), because recent searches produced no evidence of its presence ( Patankar & D’Souza, 2012). Elsewhere in this archipelago, B. latro is known from only a few locations, and in general there are reductions in both the area and quality of its habitat. The general lack of protective conservation measures for B. latro in the Andaman and Nicobar Islands, together with the continuing threats, indicate that the extinction threat for this species is high ( Patankar & D’Souza, 2012).

Large numbers of terrestrial adult and juvenile B. latro are found on Christmas Island (135 km ²), but their populations may not be stable due to continuing threats. Here the majority (> 70%) of B. latro live in a protected area and receive active conservation measures ( Fig. 7D ). They can be found throughout Christmas Island, both within the National Park as well as wandering outside in the unprotected parts of the island throughout the year. About 20 localities have been recorded according to published research studies, but the small size of Christmas Island means that the recorded localities are very close to each other, and are treated as only two distinct locations according to IUCN Red List protocols (see above). Despite the earlier positive figures for B. latro populations on Christmas Island, these crabs have nevertheless undergone measurable drops between 1979 and 2012 ( Drew et al., 2010; Drew & Hansson, 2014) caused by habitat destruction, invasive species (including the yellow crazy ant), road kills, and harvesting by local residents. It is of concern that these population reductions are taking place despite active conservation protection both in the Christmas Island National Park and along its migration routes outside of the National Park. Without active protection, the extinction risk assessment of B. latro on Christmas Island would quickly meet the threshold for a more threatened category, should new threats or population data demonstrate that the species is likely to decline further within five years.

A number of authors have reported that adult B. latro from Christmas Island in the Indian Ocean grow to a larger size than those from the West Pacific Ocean ( McLaughlin et al., 2007; Oka et al., 2015; Anagnostou & Schubart, 2016; Serosero et al., 2019). It is possible, however, that the smaller size of B. latro in its West Pacific populations is due to the harvesting of large specimens (including ovigerous females) for human consumption, given the high market price of B. latro in parts of Indonesia such as the north Maluku Islands and Sulawesi ( Sulistiono et al., 2009; Gurusu et al., 2017; Serosero et al., 2019, 2021). Studies show that unregulated harvesting in eastern Indonesia that selects by gender has measurably impacted the sex ratio of B. latro populations in some places. For example, although populations of B. latro on Halmahera Island and several other islands in central Sulawesi have similar numbers of males and females (male:female ratio 1.09:1.06), populations in a different part of Halmahera Island have many more females than males (male:female ratio 0.41:1.0) ( Sulistiono et al., 2008a; Gurusu et al., 2017; Serosero et al., 2019), and on Siompu Island off southeast Sulawesi, males outnumber females 3:1 ( Jahidin, 2010).

In the Philippines, B. latro has been reported from Zamboanga del Norte Province and from the islands of Bohol, Cebu, Sulu, Tawi-Tawi, and Basilan, and from Balabac and Cagayancillo Islands in Palawan Province (but not on the island of Palawan itself). Although the Bureau of Fisheries and Aquatic Resources of the Department of Agriculture of the Philippines listed B. latro as a threatened species and has legislated against its collection and sale since 2001, this Fisheries Administrative Order is often superseded, and the crab is still collected for food ( Subang et al., 2020). In the Batanes Province of the Philippines, B. latro populations on the islands of Batan, Sabtang, and Itbayat are declining due to overharvesting and habitat destruction, with the result that these crabs are listed as locally endangered ( Perrucho et al., 2018).

On Teraina atoll (10 km ²) in Kiribati in the Northern Line islands in the Pacific Ocean, B. latro populations are vulnerable to local extirpation, and numbers are falling sharply (down at least 80% over the past 15–20 years) due to measurable habitat destruction and widespread harvesting ( T. White, pers. obs.). Other metrics that support a high extinction threat in Teraina relate to the small size of the atoll: crab populations are found in a single location, with a declining EOO (10 km ²), a low AOO (8 km ²), and a lack of protective conservation measures .

Birgus latro populations in Taiwan are found in Kenting National Park on the island’s southern coast, and on offshore islands Lanyu and Lyudao (Green Island). Crabs are common on Lyudao (15.1 km ²), but most individuals are small (less than a kilogram) and large specimens are very rare. Populations of these crabs have declined by at least 30% over the past 15–20 years due to consistent threats associated with increasing tourism, namely, overharvesting, road kills, habitat destruction from burning forests, and introduced predators. Other factors pointing to a high extinction threat relate to the small size of Lyudao: its populations are effectively a single location when considering the impact of threats, and B. latro has a small and declining range with less suitable habitat to support crab populations. As a result, B. latro has received protection under the Wildlife Conservation Law in Taiwan since 1995, and poachers can be jailed if convicted. A number of conservation studies of B. latro in Taiwan have focused on population genetics using microsatellites ( Gan et al., 2008), larval development ( Wang et al., 2007), and habitat conservation ( Chen et al., 2004, 2006).

On the island of Guam (549 km ²) in the Northern Mariana Islands in Micronesia, declines of at least 30% in B. latro populations over the past 15–20 years have been due to overharvesting, road kills, introduced predators, and widespread coastal habitat destruction along migration routes that deny crabs access to the ocean ( Amesbury, 1980). In the mid-1990s Birgus was sold in markets on Guam for US $20–30 each, but local legislation restricted the sale of juveniles and ovigerous females as the number of crabs decreased. The underlying high extinction threats to the small and declining crab populations on Guam include harvesting coupled with a shrinking range, less suitable habitat, and the fact that the closely clustered populations on this small island effectively represent a single vulnerable location.

Birgus latro populations have only a limited capacity to support harvesting of large adults of both sexes, because this species can take up to nine years to reach sexual maturity and only reproduces once per annum, so replacement and recovery are slow. The selective harvesting of either males or females can skew the sex ratio from the expected ratio of 1:1 (i.e., 50% males:50% females). For example, the selective harvesting of large males (> 4 cm TL) in Japanese populations of B. latro in the Ryukyu Islands skews the sex ratio, resulting in more large females (which are larger) and fewer adult males (which are smaller) in the reproductive population ( Yorisue et al., 2020). This gender and size imbalance may be one of the causes of long-term population decline in this species, because females mate less frequently and with less success if they cannot encounter males that are either equal-sized or larger than themselves (Sato et al., 2010; Sato & Suzuki, 2010; Sato & Yoseda, 2010). Even if a large female does mate with a smaller male, fertilisation is less likely because such matings provide females with lower quantities of sperm ( Sato, 2011; Yorisue et al., 2020).

Figures

Fig. 4. Updated localities where Birgus latro (Linnaeus, 1767) is either extirpated (red, darkest circles) or its continued existence is uncertain (yellow, lightest circles). Note that the scale of the map may imply a wider area of extirpation than is actually known, so reference to the text and Table 3 is recommended to see the exact localities where extirpation is suspected.

Fig. 7. Threats to Birgus latro (Linnaeus, 1767) while crossing roads during breeding season migrations on Christmas Island in the Indian Ocean. A, cannibalism by coconut crabs; B, crab and vehicle in close proximity; C, recording each road kill incident; D, warning to motorists. Photographs by Max Orchard.