Population dynamics of the coconut crab Birgus latro on Aldabra Atoll, Seychelles

Abstract

Birgus latro, the largest terrestrial arthropod in the world, has undergone a substantial decline globally over the last decades, with only a few healthy populations remaining where they are actively protected. We aimed to quantify demographic and spatio-temporal dynamics of a protected population of B. latro on Aldabra Atoll (Seychelles). Based on results along 2 transects on the atoll from 2007 to 2016, we calculated an average (±SE) density of 23.7 ± 1.1 ind. ha-1. This suggests a population of around 5000 B. latro in the coastal area of Picard Island, which is likely to be an underestimate due to the cryptic nature of the species. The average density within the study area was stable over the study period, and over the yearly cycle, the population exhibited high spatial and temporal variability in count, size and sex ratio and in association with the lunar cycle. Aldabra’s B. latro population is male-biased (3:1 male:female) and sexually dimorphic, with males being larger than females. Males and females moult at different times of the year, and the breeding season overlaps with the region’s wet season (December-March) and was in synchrony with the lunar phase. B. latro on Aldabra present colour polymorphism in a 4:1:1 ratio of orange:blue:intermediate (mixture of orange and blue). Results confirm that Aldabra is an important refuge for B. latro and potentially a natural source population for other areas in the region. Similar long-term studies on B. latro are encouraged to improve knowledge on the species and to support conservation actions to halt the species’ decline.

Meta‐ and subpopulation estimation with disparate data: coconut crabs in the Western Indian Ocean

“…Density in the Chumbe lodge area (42 crabs/ha), an area of provisioning, matched Christmas Island, Australia, with its high density of 16.3–47.3 crabs/ha (Drew & Hansson, 2014). With few exceptions, densities across subpopulations were <5 crabs/ha and thus much lower than on Aldabra, a protected island in the Seychelles, with an estimated average density of 23.7 crabs/ha (Appoo et al ., 2021). Accordingly, none of the Zanzibari sites, nor the entire Pemba archipelago, came close in population size to those reported for Christmas Island, with 1.257 million coconut crabs (Schiller, 1988), or Eastern Indian Ocean sites of Morotai Island and Sajafi Island in the Malaku Islands, Indonesia with 20 163 and 43 434 individuals respectively (Widiyanti, Marsoedi, & Setyohadi, 2016; Serosero et al ., 2021).…”

Distribution and threats to coconut crabs on Unguja, Zanzibar

“…Birgus latro now has a severely fragmented global distribution and is listed as vulnerable by IUCN (Cumberlidge, 2020). In the Western Indian Ocean, for example, the species is virtually extinct along the African continental coastline, and isolated island populations have all suffered reductions across the Comoro Islands, Glorioso Islands, Mozambique Channel Islands, Seychelles Archipelago, Mascarene Islands and Madagascar (Appoo et al, 2021;Cumberlidge et al, 2021;Poupin et al, 2013). Around the island of Pemba, the northern of the two Zanzibari islands, there still remains a population of approximately 6700 juveniles and adults (neither a mollusc shell) although, there too, populations are fragmented with most individuals found on outlying islands (Caro et al, 2020(Caro et al, , 2023.…”