Biogeography and conservation of Mitchell grasslands of Northern Australia

  • Alaric Fisher

    Student thesis: Doctor of Philosophy (PhD) - CDU


    Maintenance of biodiversity in many Australian landscapes will be best achieved by the integration of nature conservation and sustainable land management practices, in addition to formal systems of conservation reserves. Implementation of off-reserve conservation management requires an adequate understanding of ecological values and processes and the biodiversity costs and benefits of prevailing land use practices. This study is concerned with the Astrebla (Mitchell grass) tussock grasslands and related sparse woodlands and shrublands that occur on heavy clay soils in arid and semi-arid northern Australia. Covering an area of approximately 420 OOOkm2, they are one of the most important of the Australian rangelands and virtually their entire extent is given over to grazing of cattle and sheep on native pastures. Consequently, Mitchell grasslands are poorly represented in the national reserve system and there is a lack of ecological knowledge, particularly regarding the fauna, to inform decisions about conservation management. The aim of this study was to address this knowledge gap, in order to determine the most effective approach to conservation management in this environment.

    Biogeographic context
    To assess the distinctiveness of Mitchell grassland communities at a sub-continental scale, biogeographic regionalisations of the Northern Territory were developed by classification of 0.5° x 0.5° cells according to their plant and vertebrate composition. The cells corresponding to Mitchell grasslands on the Barkly Tableland formed a distinct zone in regionalisations for vascular plants, all vertebrates taxa and some vertebrate groups (birds, landbirds and reptiles), even at coarse levels of classification (5-6 groups) within the Northern Territory. The area of Mitchell grasslands in the western Northern Territory was less sharply defined as a biogeographic zone. Large geocoded databases for plant and vertebrate records in the Northern Territory were also analysed to identify species with a high fidelity to Mitchell grassland communities. 61 plant species and 17 vertebrate species had more than 50% of their records from Mitchell grassland communities. One feature of the Mitchell grasslands is a number of endemic reptile species, including one discovered during this study.

    Vegetation and fauna
    A systematic biological survey of Mitchell grassland communities in the Northern Territory was undertaken to investigate the distribution and environmental relations of the vegetation and fauna. Vascular plants, vertebrates and ants were sampled at 107 sites from 12 locations, selected to sample the environmental variation and geographic range of Mitchell grassland communities in the Northern Territory. For comparative purposes, a small number of sites were also sampled in woodlands on non-clay soils. Classification and ordination were used to examine patterns of species composition and generalised linear modelling was used to relate summary richness and abundance variables, as well as the abundance of individual species, to environmental variation.

    The biological survey confirmed that Mitchell grassland communities support a distinctive flora, vertebrate fauna and ant fauna which, rather than comprising merely a subset of a widely distributed semi-arid zone species, has a significant component of relatively habitat-specific species. For each taxon, there were a small set of species that were frequent and abundant in the study region and a large proportion of species that were recorded very infrequently. The vertebrate and ant faunas are notable for a very low local and regional species richness. The ant fauna also differed markedly from that reported from other Australian rangelands in terms of biogeographic profile and functional group composition, with an unusually high proportion of ant abundance made up of Opportunist species.

    There were some commonalities between the distribution patterns of plant, vertebrate and . ant species within the Mitchell grasslands communities. At a regional scale, there was a gradual turnover in species composition in response to a latitudinal climate gradient, with the intrusion of Torresian species into the northern, high-rainfall areas, and Eyrean species into the southern margins of the biome. At a local scale, species composition was influenced by variation in substrate characteristics. Gravel rises that occur patchily throughout the clay plain support a distinct assemblage of species, as do woodlands and shrublands associated with run-on areas. Vertebrate richness and composition is also strongly determined by vegetation structure. Within each major taxon, the relative abundance of many species was also related to the relative greenness (NDVI) score for the sample site, which was developed as a measure of the influence of temporal variation in seasonal rainfall. A notable feature of the distribution patterns of plants and ants was the strong 'location effect', whereby compositional similarity was greatest between sites within a location. For ants and plants, compositional similarity between sites also decreased with geographic distance, independently of quantified environmental differences.

    A minimum-set selection algorithm was used to investigate the implications of the distribution patterns of plants, ants and vertebrates for conservation management. The majority of Mitchell grassland species could be represented in a small number of selected sites. However, because there were a high proportion of singleton or very infrequent species, a large number of sites were required to achieve reservation targets of 90-100% of species. Reservation of a high proportion of species is more likely to be achieved by a number of small reserves with a wide geographic distribution than one or few large reserves. No single taxon (ants, plants, vertebrates) can be safely used as a surrogate for the other taxa in designing the most efficient reserve network, as there is only limited assemblage fidelity between taxa.

    Waterpoint and grazing distribution
    In many arid rangelands, the distribution of grazing activity is determined by the location of waterpoints. Measurement of cattle dung, footprints and grazing at the sample sites were combined into an index of recent cattle activity, which was strongly related to distance from water. Artificial waterpoints are regularly distributed throughout the extent of the Mitchell grassland communities in the Northern Territory. Spatial analysis showed that 52% of the total area of these communities on the Barkly Tableland are within 4km of water and 97.4% within 10km of water. Prior to pastoral development, only 43% of the area was within 10km of natural permanent water. As a consequence of the current distribution of waterpoints, virtually the entire area of Mitchell grassland is subject to grazing by livestock.

    Effects of pastoral use on the Mitchell grassland biota
    In order to examine the effects of pastoral landuse on the biota, sites were sampled along gradients of grazing intensity and a comparison was made between sites outside and inside Connell's Lagoon Conservation Reserve, from which stock had been excluded for 15 years.

    The impacts of pastoral use on the Mitchell grassland biota appear to be less pronounced than have been reported for some other Australian rangelands, although I was unable to sample any 'reference' areas that have never been subject to cattle grazing. The effects of grazing pressure, at the levels sampled by this study, on the vegetation were subdued. Higher grazing pressure was associated with a decreased frequency and cover of perennial grasses and an increased frequency of annual grasses and some herbs. Grazing pressure also influenced vegetation composition and species with increaser and decreaser responses were identified. There was no evidence from this study that some plant species persist only in the most lightly grazed areas, although a higher intensity of sampling would be required to confidently provide such evidence. One of the major effects of grazing in this environment may be the regular and substantial annual reduction in ground layer cover.

    The impacts of pastoral use (due to livestock grazing and the provision of waterpoints) on the vertebrate and ant fauna of the Mitchell grasslands were also subdued, although not insignificant. Grazing pressure and distance from water were weakly related to the local richness of birds, reptiles and ants, but the most obvious effect was on the relative abundance of species, so that sets of increaser and decreaser species could be identified. Increaser vertebrates were primarily birds, particularly ground and aerial insectivores, that have benefited from the provision of free water and reduction in ground layer cover due to grazing. Decreaser response patterns were most pronounced in reptiles and bird species dependent on maintenance of substantial ground cover. Ant species appeared to be more sensitive to variation in land use intensity than vertebrates and there was some differentiation amongst functional groups in the magnitude and direction of the response, although not to the extent that functional group composition could serve as a useful indicator of grazing effects. For both vertebrates and ants, residual ground layer cover at the end of the Dry season appears to be an important factor in determining the local abundance of many decreaser species.

    A total of 16 vertebrate species, 21 ant species and 25 plant species were identified in this study as decreasers. Of most significance to conservation management, many of the decreaser species are also those with a high fidelity to Mitchell grassland communities (15 plant species, 10 vertebrate species and six ant species). It was estimated that total population sizes of some vertebrate decreaser species on the Barkly Tableland have declined by between 21% and 73% compared to pre-pastoral levels. The sparse historical record also suggests that at least one species, the Flock Bronzewing, has been substantially disadvantaged by pastoral use.

    Conservation management
    Approximately 1.1% of the total area of Mitchell grasslands in northern Australia is contained within conservation reserves, although only 0.5% of the total area of these communities in the Northern Territory is reserved. The majority of species inhabiting Mitchell grassland communities in the Northern Territory are likely to be adequately conserved within the current framework of extensive pastorallanduse. However, the results of this study suggest that a priority for conservation management is the species identified as decreasers, particularly those species that also have a high fidelity for Mitchell grassland communities. This management priority is not site-specific, in that most of these species occur widely within the Mitchell grasslands. I suggest that regional populations of decreaser species can best be maintained, particularly during dry periods, by retaining a network of little-grazed 'conservation areas' (that are either remote from permanent water or fenced to exclude stock). Additionally, there are some site-specific conservation values in the Northern Territory Mitchell grasslands associated with wetland areas that are important habitat for waterbirds and migratory birds, and rare plant species.

    The Barkly Tableland was used as an example to consider the potential economic costs of developing a network of 'conservation areas'. The relationship between grazing activity and distance from water can be used to calculate the current or potential value for cattle production of any selected parcel of land. Ideally, a network of 'conservation areas' would be designed with a geographic and environmental distribution that encompasses the regional variation in species composition within the Mitchell grasslands, and may also include sites with specific conservation values (such as populations of rare plants). Because of the current level of infrastructure development, this would require that the area of little-grazed land in some regions of the Barkly Tableland be increased by fencing or the manipulation of waterpoint distribution. All conservation scenario have significant economic costs, including establishment costs associated with fencing and the opportunity costs of not developing water-remote areas However, the estimated cost of developing a network of 'conservation areas' within existing pastoral leases compares favourably with the cost of acquiring and managing additional conservation reserves.

    The results of this study suggest that biodiversity conservation can be integrated with extensive pastoral use in the Mitchell grasslands and that, with appropriate land management strategies, biodiversity values can be maintained over extensive areas. As a result of the study, there is now sufficient understanding of the spatial patterns of distribution of the biota, and of the impacts of pastoral use, to develop the framework for a regional conservation plan for Mitchell grassland communities in the Northern Territory.
    Date of AwardOct 2001
    Original languageEnglish
    SupervisorJohn Woinarski (Supervisor) & Keith Christian (Supervisor)

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