Origins of the upland avifauna of Yapen Island, New Guinea region

Abstract

New Guinea's mountains consist today of the high Central Range, plus ten isolated lower outlying ranges. But during Pleistocene periods of low sea level, when New Guinea's current shallow continental shelf was exposed as dry land, the main island included further outliers that subsequently became cut off as land-bridge islands as rising sea levels submerged the shelf connecting them to New Guinea. We surveyed the upland avifauna of Yapen, the highest of those land-bridge islands. Yapen supports 26 upland species. That number is higher than on nearby oceanic islands of similar elevation, because Yapen in contrast to oceanic islands could acquire species overland during the Pleistocene. However, that number is much lower than on New Guinea's outliers of similar elevation, due to extinctions of many of Yapen's populations following its isolation as an island.Of New Guinea's 193 upland species, some are much more widely distributed on the ten outliers than the rest. Yapen's upland species, and those of the other land-bridge islands, are a small subset of those successful colonists of mainland outliers. Part of the explanation for differential success is that only species whose elevational floors lie well below the summits of the outliers and of Yapen are likely to have survived on or colonised those mountains, all much lower than New Guinea's Central Range. For the remainder, we infer that more than half of Yapen's former upland populations have gone extinct since Yapen's isolation. For those species with poor ability to disperse overwater, abundance is a predictor of survival and continued presence on Yapen—as expected from the inverse relationship between extinction risk and population size.We identify half-a-dozen mechanisms for colonisation by upland species: dispersal overwater when Yapen was an island; regular post-breeding descent to the lowlands; irregular straggling to the lowlands; dispersal through flat lowlands; dispersal over hill bridges; and dispersal during cool Pleistocene phases, when some current upland species had lowland populations. Relict sets of those mostly vanished Pleistocene lowland populations survive on three remnant fragments of southern New Guinea's former Arafura platform: on the Aru Islands, New Guinea's Fly River bulge, and the northern tip of Australia's Cape York Peninsula.