Tree canopy defoliation impacts avifauna

Abstract

Tree canopies, critical elements of many ecosystems, are damaged by a variety of processes. We investigate how defoliation of manna gum (Eucalyptus viminalis), a species often over-browsed by koalas (Phascolarctos cinereus), influences winter use of trees by birds at Cape Otway, Victoria, Australia. We conducted bird surveys at 55 trees (‘sites’) that were classified into four defoliation classes. Canopy cover declined and the number of dead limbs increased with more defoliation, but tree size, proximity to trees in other defoliation classes, and sub-canopy vegetation (cover and structure) did not vary between defoliation classes. Species richness of birds was lower and assemblages changed, at highly defoliated sites. Fewer microhabitats (especially those in the canopy) were used by birds in defoliated sites. This suggests that defoliated canopies provide fewer resources to birds than those that are intact. Current management of high-density koala populations focusses on koala welfare. Our study highlights the need to consider the impacts of the defoliation caused by this species to other woodland fauna.