The tendency for island taxa to lose their long-distance dispersal abilities has been widely-observed across terrestrial birds, plants and insects. The generality of, and mechanisms underlying, these evolutionary shifts are, however, less well-understood. For insects, insularity is typically a poor predictor of flight loss, while support for other environmental drivers, including wind pressure, low temperatures, habitat fragmentation and predator release, varies across habitats and taxa. Here, we test each of these hypothesised drivers on the insect communities of the Southern Ocean and Arctic islands. Flightlessness is a conspicuous trait across these high-latitude islands, although its prevalence and drivers are not well-understood. Using primarily species descriptions, we determined the flight-capability of all described insects in these island communities (1077 species, 33 islands). We examined taxonomic and geographic patterns of flight loss to determine the extent to which convergent selection or adaptive radiation processes drive the diversity of flightless species. Finally, using high-resolution environmental and climate data, we tested each of the proposed environmental drivers against the diversity of flightless insects to determine which conditions best predict the incidence of flight loss on high-latitude islands.
Results/Conclusions
The indigenous Southern Ocean Island (SOI) insect communities have an exceptionally high, and perhaps globally-unique, diversity of flightless insects. Nearly half (47%) of the indigenous SOI insects were monomorphically flightless, whereas flight loss was uncommon among insects that have evolved outside the Southern Ocean region (i.e. introduced to the SOIs, and Arctic island species). We show that flightlessness has evolved independently among the endemic SOI insects over 120 times, indicating extensive convergent selection for flight loss in these communities. Wind pressure and island isolation were consistently the best environmental predictors of flightlessness, explaining much of the variation in the incidence of flightless species and genera across the islands. Our results provide support for Darwin’s hypothesis that strong winds select for dispersal reduction on oceanic islands because flying individuals would be more readily blown to sea than ground-dwelling individuals. Differences in the functional trait diversity of indigenous and introduced species could have significant ecological consequences should flight provide aliens with a competitive advantage in SOI communities in the future.