Phytophagous insect community assembly through niche conservatism on oceanic islands

Aim To determine whether a community of phytophagous insects on oceanic islands (the fauna of insects feeding internally on Glochidion trees in south-eastern Polynesia) was assembled predominantly through niche conservatism or adaptive radiation. Location The islands of south-eastern Polynesia (southern Cook, Austral, Society, Tuamotu-Gambier and Marquesas archipelagos) in the Cook Islands and French Polynesia. Methods Internally feeding insects were collected as larvae from 23 endemic species of Glochidion (Euphorbiaceae s.l., Phyllanthaceae: Phyllanthus s.l.) trees on 20 islands in south-eastern Polynesia, reared and identified. Rearing records were compared with host records previously known from the literature and museum collections of closely related taxa in Asia and Australasia. Results Ninety per cent of insect specimens collected fall into five taxa previously known to attack Glochidion in Asia and Australasia (≥ 6000 km distant), indicating a strong role for niche conservatism in the assembly of this community. Three of these taxa, two seed-feeding moths (Gracillariidae: Epicephala; Tortricidae: Tritopterna) and a leaf-mining moth (Gracillariidae: Diphtheroptila) are only known from Phyllanthaceae or Euphorbiaceae s.l. on continents. Two more taxa, another leaf-mining moth (Gracillariidae: Caloptilia) and a leaf-rolling moth (Tortricidae: Dudua), contain many species known only from Phyllanthaceae on continents, and are also very likely to represent examples of niche conservatism. Conversely, many numerically dominant insect taxa known from Glochidion on continents (such as parasitoid Hymenoptera) are not reported from south-eastern Polynesia. This indicates that the insular community represents a subset of those taxa from the continental community, consistent with the well-established Pacific diversity gradient. Main conclusions These findings indicate that niche conservatism can play an important role in the assembly of phytophagous insect communities on oceanic islands, despite the constraints that specialization might be expected to pose on successful establishment. As a result of this niche conservatism, these communities may represent less species-rich versions of continental tropical communities rather than non-analogue insular ecosystems.