Interspecific competition and conservation management of continuous subtropical woodlands

Abstract

Context. Large reserves have potential to provide important refugia for fragmentation-sensitive species as they lack many aspects of habitat degradation associated with fragmented regions. However, large reserves often have a history of deleterious management practices that may affect the restoration of biological diversity. One significant symptom of habitat fragmentation and disturbance in Australia is the increased occurrence of the aggressive noisy miner (Manorina melanocephala). In Queensland, however, even large continuous areas of woodland appear to be dominated by the noisy miner. Aims.We examined the severity of this phenomenon by identifying the interactions between habitat structure, noisy miner abundance and avian assemblages in a private and a public conservation reserve in central Queensland. We investigated potential constraints on restoration of avian diversity including: (1) how changes in habitat structure as a result of grazing by feral animals and burning regimes affect bird assemblages; and (2) how the noisy miner impacts on avian assemblages in these unfragmented woodlands. Methods. Bird surveys and habitat assessments were conducted in 49 sites on three separate occasions. Fire history and intensity of grazing pressure were determined for each site with direct and indirect observations. Sampling for lerp from insects of the family Psyllidae was also undertaken during the survey periods. A Bayesian model averaging (BMA) approach was used to model avian response to each of the habitat variables. Key results. The noisy miner dominated most of the study area, reducing small passerine abundance and species richness. Noisy miners were advantaged where shrub cover was low and feral grazing impacts were evident. Disturbance factors including recent wildfire and heavy grazing strongly reduced small passerine bird richness and abundance. Conclusions. Reducing the abundance of this ‘reverse keystone’ species requires control of feral herbivore populations and modification of fire regimes to achieve a mosaic including patches with a dense shrub layer. Implications. Deleterious interactions with competitive native species, such as noisy miners, are obstacles to bird conservation not only in fragmented landscapes but also in large, continuous woodland areas. Land mangers of protected areas need to be aware of shifts in interactions among native species driven by habitat disturbance, which may ultimately affect conservation outcomes.