Effects of ENSO-induced forest fires and habitat disturbance on the abundance and spatial distribution of an endangered riverine bird in Borneo

Abstract

Droughts induced by the 1997–1998 El Niño Southern Oscillation (ENSO) event sparked large-scale forest fires affecting millions of hectares on Borneo. We studied the effects of ENSO-induced disturbances on a riverine bird, the critically endangered white-shouldered ibis, Pseudibis davisoni, along the Mahakam River in East Kalimantan, Indonesian Borneo. Island-wide fire hotspots were more likely to occur near rivers and part of our study area was indeed affected by fire. Based on 25 boat surveys (2590 km) that yielded 91 records, we recorded significant changes in abundance and spatial distribution of this bird in our study area. Encounter rates were higher during the pre-ENSO (1992–1996) periods than the post-ENSO (1997–2000) ones and differed between seasons. No ibises were seen post-ENSO along river sections that were affected by the fires, whereas pre-ENSO encounter rates were 2.4 birds/102 km. Encounter rates along those sections that were not affected by the forest fires more than doubled from 2.0 birds/102 km pre-ENSO to 5.6 birds/102 km post-ENSO. This was most probably due to enhancement of displaced birds. In view of the permanent character of the change in spatial distribution, active protection of the remaining stretches of riverine forest is of the utmost importance for the survival of white-shouldered ibis.