The effects of isolation, habitat fragmentation and degradation by livestock grazing on the use by birds of patches of Gimlet Eucalyptus salubris woodland in the wheatbelt of Western Australia

Abstract

The numbers of species, and the frequency of occurrence of individual species, in patches of Gimlet Eucalyptus salubris woodland in remnants of native vegetation in the central wheatbelt of Western Australia were recorded over a year. These values were examined in relation to the structural characteristics of the patches and the biogeographic attributes of the remnants (i.e., size and various indices of isolation from other native vegetation). There were five patches in large remnants (>100 ha) and 24 patches in small remnants (0.5?27.0 ha). Most of the small remnants were grazed by livestock and had lost much or all of the shrub understorey. The Galah Cacatua roseicapilla and Port Lincoln Ringnecked Parrot Platycercus zonarius were found in all patches, but the remaining large species of birds (Australian Raven Corvus coronoides,Pied Butcherbird Cracticus nigroregularis, Crested Pigeon Ocyphaps lophotes, and Yellow-throated Miner Manorina flavigula) were found more frequently in patches in small remnants. Conversely, with the exception of the Striated Pardalote Pardalotus striatus, small passerine species were found less frequently in patches in small remnants. Remnant size was significantly negatively correlated with frequency of occurrence of six of the eight common large species, indicating that these species concentrated in patches in small remnants, and significantly positively correlated with frequency of occurrence of two of four small passerine species. Having taken out the effect of remnant area, a stepwise regression procedure was used to see whether other biogeographic attributes of the remnants or habitat structure in the patches influenced the frequency of occurrence of individual species and species richness. As area of native vegetation within a 5 km radius increased so did the frequency of occurrence of the Galah and Port Lincoln Parrot, indicating that local numbers of the species affects their presence. Of the large birds only the Yellowthroated Miner was influenced by patch structural attributes whereas all species of small passerines showed responses to various structural attributes of the patches. Frequency of occurrence of the Striated Pardalote and the number of species of small passerines in a patch decreased with increasing distance to the nearest native vegatation. Overall frequency of occurence of small passerines increased with the number of linear strips of native vegetation linked to a remnant. In this study a majority of the small remnants were too small to support resident birds. The Gimlet patches, if used, would be part of a home range. Loss of the shrub understorey through grazing and loss of canopy cover through tree deaths had a significant impact on numbers of species of small passerines using the Gimlet patches. The conservation value of the patches in small remnants would be enhanced by increasing remnant size and by the linking of remnants to nearby native vegetation.