Landscape position predicts distribution of eucalypt feed trees for threatened black-cockatoos in the northern jarrah forest, Western Australia

Abstract

The Jarrah forest of southwestern Australia supports mineral and timber production and provides important food plants for three threatened black-cockatoo species. To assist the integration of conservation and production objectives, we studied the landscape distribution of two important black-cockatoo food sources, the eucalypts Jarrah (Eucalyptus marginalia) and Marri (Corymbia calophylla), at a mine site in the northeastern Jarrah forest in 2008. Jarrah and Marri were not distributed randomly across the landscape. Stem densities of Jarrah and Marri were highest on lower slopes and on ridgetops respectively, while stem densities for both species were lowest in lowland areas, where stems of Wandoo (E. wandoo) often occurred, The distribution of Jarrah and Marri 'feed trees' used by cockatoos did not follow this landscape pattern, with Marri feed trees showing a distinct association with lower slopes and lowland areas, and Jarrah feed trees more evenly distributed across landscape positions. Multiple logistic regression with biotic (stem densities), topographical (landscape position), and disturbance (e.g. presence of cut stumps) variables indicated that feed trees were most likely to occur on lowlands and lower slopes in the absence of Wandoo. Although Jarrah and Marri occurred at a frequency ratio of 3:1, a significantly higher proportion of Marri trees (13.0%) were used as feed trees than were Jarrah trees (5.2%). These findings suggest that Marri is likely the more important food source at a landscape-scale, but longer-term studies are needed to rule out the possibility that the relative importance of the different tree species varies seasonally and inter-annually.