Reorganization of bird communities along a rainforest elevation gradient during a strong El Nino event in Papua New Guinea

The El Nino 2015 event, most extreme since 1997, led to severe droughts in tropical wet Papua New Guinea (PNG), reducing May to October dry season rainfall by - 75% in the lowlands and 25% in the highlands. Such droughts are likely to have significant effects on terrestrial ecosystems, but they have been poorly explored in Papua New Guinea. Here we report changes in bird community composition prior, during and after 2015 El Nino event along the elevational gradient ranging from 200 m to 2,700 m a.s.l. at the Mt. Wilhelm rainforest in PNG. The abundance of birds in lowlands dropped by 60% but increased by 40% at elevations above 1700m during El Nino year. In the following year, the individual bird species reached mean population sizes similar to pre-El Nino years but did not fully recover. Species richness roughly followed the pattern of observed abundance and quickly and fully re-established after the event to the pre-El Nino values. Thus, at least some terrestrial birds seem to react quickly to the extreme droughts in lowlands and shifted to less affected mountain habitats. We recorded upper elevational range limits to shifts by more than 500m asl in 22 bird species (out of 237 recorded in total) during El Nino year, in contrast to their typical ranges. Our study suggests that a strong El Nino event can have strong but reversible effects on bird communities as long as they have an opportunity to move to more favourable sites through undisturbed habitats.