Bird diversity on shelf islands does not benefit from recent land-bridge connections

Aim Research in island biogeography has long focused mainly on present-day island configurations. Recently, there has been an increasing focus on islands' past histories of land connection, shape and size. Moreover, continental islands (=shelf islands) have received less attention than oceanic islands, and species inventories from extremely small islands are lacking in many datasets. We examine the effects of sea-level rise since the Last Glacial Maximum (LGM) on bird species diversity and composition of tropical shelf islands in Southeast Asia. Location Sundaland. Taxon Birds. Methods We compiled avifaunal island inventories for 94 islands using an exhaustive literature review of historic surveys of larger islands combined with our own comprehensive island surveys from both small and large islands. Using generalised least-squares models with spatial autocorrelation, we assessed the importance of traditional biogeographical parameters including area, maximum elevation, distance from mainland and geographical isolation, along with post-LGM effects of change in island area and duration since isolation. We also compared the species composition on similar-sized shelf islands from two categories-recently submerged and unsubmerged-using non-metric multidimensional scaling. Results Post-LGM effects on species diversity are minimal and insular diversity is instead well explained by present-day island characteristics, such as area, distance to mainland and proportion of land surrounding an island within a 10 km radius (Cox and Snell Pseudo-R-2 = 0.803). Avifaunal diversity is similar across recently submerged and unsubmerged small shelf islands. Main conclusion Avifaunal diversity on tropical shelf islands equilibrates rapidly after isolation, indicating that both extinction and immigration rates are high. In particular, a high immigration rate of dispersive species maintains diversity, especially on small islands. Over-water dispersal is generally restricted to short distances among Sundaic birds. Consequently, the diversity of an island can be maintained by the presence of large or stepping-stone islands near it.

Automated summary

Research on bird species diversity and composition on tropical shelf islands in Southeast Asia found that post-LGM effects on diversity are minimal, with present-day island characteristics such as area, distance to the mainland, and proportion of land surrounding the island within a 10 km radius being more explanatory. Avifaunal diversity is maintained by high immigration rates, especially on small islands, with over-water dispersal limited to short distances among Sundaic birds.