Abstract
Most oceanic islands harbor unusual and vulnerable biotas as a result of isolation. As many groups, including dominant competitors and predators, have not naturally reached remote islands, others were less constrained to evolve novel adaptations and invade adaptive zones occupied by other taxa on continents. Land crabs are an excellent example of such ecological release, and some crab lineages made the macro-evolutionary transition from sea to land on islands. Numerous land crabs are restricted to, although widespread among, oceanic islands, where they can be keystone species in coastal forests, occupying guilds filled by vertebrates on continents. In the remote Hawaiian Islands, land crabs are strikingly absent.Here we show that absence of land crabs in the Hawaiian Islands is the result of extinction, rather than dispersal limitation. Analysis of fossil remains from all major islands show that an endemic Geograpsus was abundant before human colonization, grew larger than any congener, and extended further inland and to higher elevation than other land crabs in Oceania.Land crabs are major predators of nesting sea birds, invertebrates and plants, affect seed dispersal, control litter decomposition, and are important in nutrient cycling; their removal can lead to large-scale shifts in ecological communities. Although the importance of land crabs is obvious on remote and relatively undisturbed islands, it is less apparent on others, likely because they are decimated by humans and introduced biota. The loss of Geograpsus and potentially other land crabs likely had profound consequences for Hawaiian ecosystems.
Highlights
Isolation leads to disharmonic biotas on islands, because poor dispersers are absent or under-represented relative to good dispersers [1,2]
Four species of Geograpsus are currently recognized: G. lividus in the Atlanto-East Pacific, and G. stormi, G. crinipes and G. grayi in the Indo-West Pacific region [38]. They can be arranged in a series reflected both in morphology and habitat, with the morphologically and ecologically very similar, supratidal G. lividus and G. stormi at one end, mostly coastal G. crinipes in the middle, and most terrestrial G. grayi at the other end
G. stormi examined show lividustype ridges at the anterior branchial region of the carapace (Banerjee’s character 1); the external maxilliped palps of the two are hairy, and the antero-distal angle of the second walking leg in some G. lividus we examined are as pointy as illustrated for G. stormi by Banerjee
Summary
Isolation leads to disharmonic biotas on islands, because poor dispersers are absent or under-represented relative to good dispersers [1,2]. Low starting diversity and absence of various major taxa generally lead to ecosystems with fewer types of biotic interactions than on continents. This in turn provides ecological release for some colonizers, while preventing the establishment of others. For example several land snail and insect families, as well as species of wide-ranging sea birds, are restricted to islands [7,8]. These taxa appear to be especially vulnerable and suffer massive extinctions following introduction of continental predators [8,9,10]. The low initial diversity of insular biotas has fostered diversification, evolutionary radiations are a hallmark of island life: Galapagos finches and Hawaiian fruit flies are celebrated examples [16], [17]
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