Reorganization of Bird Communities along a Rainforest Elevation Gradient During a Strong El Niño Event in Papua New Guinea

The El Ni & ntilde;o 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 to, during, and after the 2015 El Ni & ntilde;o event along the elevational gradient ranging from 200 m to 2700 m a.s.l. at the Mt. Wilhelm rainforest in PNG. The abundance of birds in the lowlands dropped by 60% but increased by 40% at elevations above 1700 m during El Ni & ntilde;o year. In the following year, the individual bird species reached mean population sizes similar to pre-El Ni & ntilde;o 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 Ni & ntilde;o values. Thus, at least some terrestrial birds seem to react quickly to the extreme droughts in lowlands and shift to less affected mountain habitats. We recorded upper elevational range limits to shifts by more than 500 m a.s.l. in 22 bird species (out of 237 recorded in total) during El Ni & ntilde;o year, in contrast to their typical ranges. Our study suggests that a strong El Ni & ntilde;o event can have strong but reversible effects on bird communities as long as they have an opportunity to move to more favorable sites through undisturbed habitats. Terrestrial birds reacted quickly to extreme droughts associated with El Nino, and re-established their distribution in few months after the event ended. The most extreme elevational shifts recorded were by more than 500 m elevational. We thus report a strong, yet reversible effect of El Nino on the communities of tropical terrestrial birds.image