Rarity and extinction risk in coral reef angelfishes on isolated islands: interrelationships among abundance, geographic range size and specialisation

Abstract

Determining the species most vulnerable toincreasing degradation of coral reef habitats requiresidentification of the ecological traits that increase extinctionrisk. In the terrestrial environment, endemic speciesoften face a high risk of extinction because of an associationamong three traits that threaten species persistence:small geographic range size, low abundance and ecologicalspecialisation. To test whether these traits are associated incoral reef fishes, this study compared abundance and specialisationin endemic and widespread angelfishes at theremote Christmas and Cocos Islands in the Indian Ocean.The interrelationships among traits conferring highextinction risk in terrestrial communities did not apply tothese fishes. Endemic angelfishes were 50–80 times moreabundant than widespread species at these islands. Furthermore,there was no relationship between abundanceand ecological specialisation. Endemic species were notmore specialised than widespread congeners and endemicsused similar resources to many widespread species. Threewidespread species exhibited low abundance and somedegree of specialisation, which may expose them to agreater risk of local extinction. For endemic species, highabundance and lack of specialisation on susceptible habitats may compensate for the global extinction risk posed by having extremely small geographic ranges.However, recent extinctions of small range reef fishesconfirm that endemics are not immune to the increasingseverity of large-scale disturbances that can affect speciesthroughout their geographic range.