Roads, fire and aggressive competitors: Determinants of bird distribution in subtropical production forests

Abstract

Forestry practices such as fuel-reduction burning and maintenance of road networks can negatively impact avian assemblages, both directly by changing habitat structure and indirectly by creating conditions favorable for predators or competitors. The Brigalow Belt forests include some of the largest contiguous areas of native forest in the temperate and sub-humid zones of eastern Australia. Over 1 million ha of these forests are proposed to be converted from forestry to conservation tenure, yet the impacts on the avifauna of current and potential future forest management practices are not known. We investigated the influence of road edges and habitat type and structure on the avifauna of a 356 000 ha forest. Survey sites were either <50 m or >300 m from a road, and in either cypress pine Callitris glaucophylla forest, spotted gum Corymbia citriodora forest with a regenerating cypress pine/buloke Allocasuarina luehmannii understorey or spotted gum forest with an open understorey due to fuel-reduction burning. The avifauna differed significantly among vegetation types but not with proximity to a road, with the greatest differences between cypress pine and both types of spotted gum forest. The noisy miner Manorina melanocephala, an aggressive avian competitor, appears to be the factor mediating these assemblage-level differences. Noisy miners were rare in cypress pine forest but were three times more numerous than any other species in spotted gum forest. Spotted gum forest with a regenerating understorey had fewer noisy miners. Although total bird abundance was highest in open spotted gum forest, the species richness and abundance of passerines smaller than noisy miners was significantly lower in this forest type. Abundance of small passerines was eight times higher in sites where <3 noisy miners were recorded. Only one species, the eastern yellow robin, was influenced by proximity to a road edge. Cypress pine forest is potentially an important refuge for smaller birds. The results suggest that burning regimes that reduce regeneration of the cypress pine and buloke subcanopy in spotted gum forest potentially are exacerbating the problem of noisy miner domination of the avifauna.

Introduction

Most remaining large areas of native forest are managed primarily for human land uses such as livestock grazing and forestry rather than biodiversity conservation (Norton and Kirkpatrick, 1995, Norton, 1996). However, as more species become threatened due to the effects of habitat fragmentation in heavily cleared regions, the conservation importance of these large forest areas is increasing. It is of critical importance to investigate ways of managing these areas that improve their conservation value, particularly to species threatened by habitat loss and fragmentation.

The impacts of forestry on wildlife include not only those stemming directly from harvesting activity, but also from other forest management and maintenance activities. Forestry-related management practices that can impact wildlife include fuel-reduction burning, vegetation thinning, grazing for fuel reduction, maintenance of particular tree species composition and creation and maintenance of a logging road network (Norton and May, 1994). Prescribed fire to reduce fuel loads is one of the most widespread of forestry practices, and in many cases, little is known of its impact on biodiversity (Granström, 2001). Prescribed fire changes stand structure and understorey characteristics, and reduces the amount of dead wood on the forest floor, resulting in substantial changes to habitat structure for forest-dwelling wildlife species (Abbott, 1999, Date et al., 2002, Stuart-Smith et al., 2006).

While edge effects on fauna are well known from studies of fragmentation, internal edges such as those caused by roads bisecting otherwise continuous forest areas have less frequently been the focus of research. Edges can affect species directly due to altered habitat characteristics (Watson et al., 2004), but can also have indirect impacts by facilitating movement of or creating habitat for competitors, predators and parasites (Forman, 1995, Chace et al., 2003, Batáry and Báldi, 2004). Insectivorous understorey birds in particular have been found to be negatively affected by forest edges. Even narrow unpaved roads can reduce abundance of understorey birds in the vicinity of the road edge (Ortega and Capen, 2002, Laurance, 2004) and decrease movement across roads (Laurance et al., 2004).

In Australia, a suite of small passerines has been undergoing widespread population declines (Barrett et al., 1994, SAC, 2000). These declines have typically been attributed to habitat loss and fragmentation in agricultural areas, the effects of which are often exacerbated by subsequent increases in the aggressive native honeyeater, the noisy miner Manorina melanocephala, which severely impacts avian assemblages through interference competition (Barrett et al., 1994, Major et al., 2001, Piper and Catterall, 2003). The Brigalow Belt of southern Queensland is one of the only regions in temperate and subtropical Australia where large continuous areas of forest remain. These areas present a potential opportunity to conserve large populations of small passerines which are declining elsewhere in the continent. However, the majority of the forests of Queensland's Brigalow Belt Bioregion are currently subject to extensive selective logging of both hardwood (mainly spotted gum Corymbia citriodora) and softwood (white cypress pine Callitris glaucophylla) timber species, and most are under extensive cattle grazing licenses. In order to protect the fire-sensitive softwood resources, fuel-reduction burning occurs in adjacent hardwood areas, and an extensive unpaved road network is maintained. Following from the Queensland Government's introduction of the South-East Forests Agreement, which more than doubled the area of native forest under National Park tenure in the South-East Queensland Bioregion (McAlpine et al., 2005), the Western Hardwoods Agreement is currently being negotiated to reduce native forest logging in the west of the state. In part, this agreement proposes to transfer over 1 million ha of public hardwood forest reserves in subtropical inland Queensland, including the Brigalow Belt, to conservation reserves.

However, tenure change alone is insufficient to achieve improved biodiversity outcomes. Ongoing management is required to deal with the legacy of past land uses and ongoing public concerns such as wildfires, as well as to maintain ecological processes and appropriate disturbance regimes. Furthermore, as timber harvesting of softwood species will continue under the proposal, fuel reduction in adjacent hardwood areas will continue to be a consideration. Continuing forest management is likely to necessitate maintenance of at least some roads. Although the proposed tenure changes will mean a change in focus to managing forests for biodiversity, it is difficult to assess the likely influence of this change as the impacts of current management practices on avian assemblages are largely unknown. Furthermore, the relative value of the hardwood and softwood forest areas for small insectivorous birds has not been documented. This information is required in order for informed recommendations to be made regarding future management of the forests, both those that continue to be managed for timber production and those that are moved to a conservation tenure.

A particular challenge faced by conservation managers in the Brigalow Belt forests is that there is a high abundance of noisy miners even throughout large continuous forest areas (Hando and Hando, 1997). This observation is unexpected given research findings from other parts of Australia which suggest that the noisy miner is primarily an edge species, most abundant in fragmented agricultural landscapes (Loyn, 1987, Piper and Catterall, 2003, Chan, 2004). However, the Brigalow Belt forests are impacted by an extensive logging road network, and the responses of birds to internal edges such as those created by roads are not known in the region. It is possible that the creation of internal edges has facilitated noisy miner invasion into the forest interior. Furthermore, the more open habitat structure promoted by prescribed burning regimes could also favor the noisy miner (Grey et al., 1998, Major et al., 2001, Catterall et al., 2002). This study aimed to: (1) determine whether current forest management practices, namely, prescribed burning and creation and maintenance of road edges, are affecting the distribution of noisy miners and forest bird assemblages, and (2) compare the avifaunas of hardwood and softwood forest in order to assess whether conservation of hardwood forests alone is likely to be sufficient to improve conservation outcomes for avian biodiversity in the Brigalow Belt.