A case study of the coconut crab Birgus latro on Zanzibar highlights global threats and conservation solutions

Abstract

The coconut crab Birgus latro, the largest terrestrial decapod, is under threat in most parts of its geographical range. Its life cycle involves two biomes (restricted terrestrial habitats near the coast, and salt water currents of the tropical Indian and Pacific Oceans). Its dependence on coastal habitat means it is highly vulnerable to the habitat destruction that typically accompanies human population expansion along coastlines. Additionally, it has a slow reproductive rate and can reach large adult body sizes that, together with its slow movement when on land, make it highly susceptible to overharvesting. We studied the distribution and population changes of coconut crabs at 15 island sites in coastal Tanzania on the western edge of the species' geographical range. Our aim was to provide the data required for reassessment of the extinction risk status of this species, which, despite indications of sharp declines in many places, is currently categorized on the IUCN Red List as Data Deficient. Pemba Island, Zanzibar, in Tanzania, is an important refuge for B. latro but subpopulations are fragmented and exploited by children and fishers. We discovered that larger subpopulations are found in the presence of crops and farther away from people, whereas the largest adult coconut crabs are found on more remote island reserves and where crabs are not exploited. Remoteness and protection still offer hope for this species but there are also opportunities for protection through local communities capitalizing on tourist revenue, a conservation solution that could be applied more generally across the species' range.

Distribution and threats to coconut crabs on Unguja, Zanzibar

“…More optimistically, however, coconut crabs survive chiefly on outlying islets with 14 of the 22 islets having crabs living there (Figure 2). Just like Pemba, crabs are principally found on remote outlying islands (Caro et al, 2020) again showing that the coconut crab distribution is as fragmented on Unguja as it is on Pemba. To reduce further fragmentation, it would be very beneficial to formally list Birgus latro as an endangered species in Tanzania and therefore legally subject to full protection.…”

“…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.…”

“…Two recent studies have explored the anthropogenic factors that affect these subpopulations although they have generated disparate results. In the first study, employing three measures of subpopulation viability taken from 15 sites, Caro et al (2020) found that the mean number of crabs seen per night was positively associated with presence of agricultural crops, whereas the mean number of crabs seen per person‐hour was positively associated with presence of crops and island remoteness. Crab average weights were greater at formally protected government sites, and lower in areas where crabs were hunted and eaten.…”

Practical guide to coproduction in conservation science

“…Indeed, the majority of problems DoF faces require interdisciplinary inputs. For example, to conserve coconut crabs, knowledge of the distribution of the species, its ecological predictors, and the behavior of its principal predator, humans (Caro et al., 2020), is needed. Even studies not directly related to the needs of the DoF (e.g., on the evolution of color polymorphisms [Nokelainen et al., 2018] the global conservation status of the species [Cumberlidge et al., 2022], or invertebrate conservation [Caro, 2022]) can be used creatively to elevate the profile of the DoF by attracting crowds to their stall at Pemba's annual agricultural fair.…”

Can behavioural ecologists help establish protected areas?

“…The Department then featured live coconut crabs in an island-wide exhibition of its conservation activities. Such collaborative actions (see below) were initiated as a consequence of a basic research study [42]. Finally, wildlife films that feature all types of species' behaviour and ecology are regarded as having enormously increased the public's affiliation with the natural world and wish to conserve it [43].…”