Habitat fragmentation and its effects on birds and grasshoppers in eucalypt remnants in the Tasmanian Midlands

In the subhumid Midlands region of Tasmania, forty-seven remnants of eucalypt woodland and dry sclerophyll forest as well as six control sites were surveyed for birds and grasshoppers, in order to examine the responses of these groups to habitat fragmentation. A wide range of characteristics of the remnants (including position in landscape, management regime and vegetation) were related to species richness, diversity and density of the two groups as well as to the distributions of individual species. Remnant size, vegetation structure and tree health all showed significant relationships with bird species richness and diversity. Small remnants with open understorey and high levels of dieback showed radically different species composition than larger ones or those with dense understorey. Where the understorey is open the noisy miner (Manorina melanocephala) is present in colonies and is able to effectively exclude almost all other species by concerted aggressive behaviour. Noisy miner colonies were associated with small remnants but were also present at the edges of larger remnants, where proximity to open country and vegetation structure both predicted their presence. Interspecific competition is considered to be the major determinant of species richness and of many species' distributions in the study area. Analysis of remnants which are not dominated by noisy miner colonies found that area and isolation were significantly correlated with species richness and diversity,. although larger remnants did not have more species at a given point. Summer migrants and nomadic species are considered to be more sensitive to habitat fragmentation as a result of the presence of noisy miner colonies in the fragmented habitat. Interspecific competition is considered to be the driving force behind avifaunal trends in the study area. Grasshopper species richness was not related to any of the variables measured, but diversity was higher in remnants in better condition, while density was higher in remnants in poor condition. Common grasshopper species responded to a range of variables. Management of remnants for conservation of avifaunal and grasshopper values is discussed. In the context of avifauna conservation, it is suggested that although larger remnants are more likely to support a healthy suite of bird species, the presence of a dense understorey in smaller remnants can improve their conservation value. Both of these options are likely to also lead to the maintenance of forest- and woodland-dwelling grasshopper species.