Are gullies best for biodiversity? An empirical examination of Australian wet forest types

A relatively common observation in forest environments has been that gullies support higher species richness and individual abundance than elsewhere in the landscape. We completed a detailed case study of birds to contrast species richness and assemblage composition between gullies and other parts of the topography of landscapes in three closely related and spatially adjacent wet ash forest types – those dominated by Mountain Ash (Eucalyptus regnans), Alpine Ash (E. delegatensis) or Shining Gum (E. nitens) – in the Central Highlands of Victoria, south-eastern Australia. We also quantified the influence of a wide range of other measures of stand structure and plant species composition on the bird assemblage and on individual bird species.We identified several unexpected findings. Four in particular were: (1) The absence of gully effects for observed species richness. (2) Assemblage composition differences that were strongly influenced by such factors as forest type and understorey structure but less so by topographic position. (3) The rarity of gully effects for individual species, and in one of the cases where it was identified, a response that was different from the general pattern often recorded by other workers for increased detections in gullies. (4) Major differences in bird responses between different forest types, even though stands dominated by different species of montane ash trees can appear superficially similar in structure and plant species composition.The maintenance of the integrity of gullies is a key general principle for ecologically sustainable forest management in many kinds of forest worldwide, including in montane ash forests. Our findings suggest that although gully environments are important for birds, they were generally not significantly more important than other parts of montane ash forest landscapes. Thus, our results highlight the value for vertebrate biota of forests outside gullies.