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Impacts of habitat fragmentation on the vertebrate fauna of the tropical savannas of Northern Australia; with special reference to medium-sized mammals

Rankmore, Brooke Rena (2006). Impacts of habitat fragmentation on the vertebrate fauna of the tropical savannas of Northern Australia; with special reference to medium-sized mammals. PhD Thesis, Charles Darwin University.

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Author Rankmore, Brooke Rena
Title Impacts of habitat fragmentation on the vertebrate fauna of the tropical savannas of Northern Australia; with special reference to medium-sized mammals
Institution Charles Darwin University
Publication Date 2006-04
Thesis Type PhD
Supervisor Price, Owen
Brook, Barry W.
0502 - Environmental Science and Management
Abstract Across the Northern Territory, the natural vegetation remains extensive with limited modification. This expanse of relatively unmodified landscape provides an opportunity for conservation planning that is potentially unmatched in the world. However, the intactness of this native vegetation is coming under increasing threat. In this study I investigate the effects of habitat loss and fragmentation on the fauna of Australia's tropical savanna woodlands. In particular, I focus on fauna inhabiting the open eucalypt forests of the Darwin and Daly regions of the Northern Territory, that are taking the impact of current development. Although there is a substantial body of information for temperate Australia on the ecological effects of land clearing and fragmentation, I present the first study of how these processes affect the fauna in the tropical eucalypt forests of the Northern Territory and one of few studies conducted in tropical Australia. In contrast to the restoration works elsewhere in Australia, there is the rare opportunity to enact clearing controls informed with the principles of ecological sustainability, thus ensuring long-term maintenance of the Territory's biodiversity.

The first component of the study investigated the abundance of all vertebrate species in a range of fragments of varying size and isolation, including continuous forest and completely modified sites. The data from this survey were used to investigate patterns in vertebrate species composition and richness, and to determine species-specific relationships with a range of fragmentation and environmental variables. There were 75 species of bird, mammal, frog or reptile recorded frequently enough to analyse. Only a small proportion of these (25%) used modified land, but most (69%) did use corridors. There were no species that could confidently be classified as edge specialists, preferring the interface between woodland and modified land. Fourty-two percent of species analysed were statistically less abundant in more fragmented sites. Three variables were found to have a strong positive influence on the animals that occur in a fragment. They were the area of the fragment, the total amount of woodland within 4 km of the fragment, and the extent of corridors linking the fragment to other large woodland tracts.

The second component of the project focused on the effects of habitat fragmentation on the survival and dispersal of medium-sized mammals. Medium mammals were selected as the faunal assemblage to investigate, as their requirements are believed to broadly encompass the needs of the other species. A mark-recapture study of four mammal species (black-footed tree-rat Mesembriomys gouldii, northern quoll Dasyurus hallucatus, northern brown bandicoot Isoodon macrourus and the common brushtail possum Trichosurus vulpecular) was conducted for two years, within a selection of fragmented and continuous sites from the initial study. The abundance of all four medium-sized mammals was greatest in the fragmented sites. As land clearing and habitat fragmentation is only a recent phenomenon in the study area, these fragments may still be undergoing increased population densities and over crowding, especially for the common brush tail possum and northern brown bandicoot. Despite this higher abundance, survival rates were lower in the fragmented sites with relation to the continuous site. Although, in general, these survival rates were not substantially lower than in continuous areas they do highlight the fact that habitat fragmentation is having an effect on the survival of small mammals in this study.

Of these four species, the black-footed tree-rat was radio-tracked in fragmented and continuous habitats to investigate possible differences in movement, activity area and home range. The size of home range estimates for the black-footed tree-rat in the un-fragmented environment suggest that black-footed tree-rats require large areas (67.3 ± 10.4 ha) when compared to other similar sized mammal species of Australia's tropical savannas. Home range and activity area size decreased significantly under fragmented conditions; however, black-footed tree-rat were able to cross relatively large areas of cleared land. Although they require large areas in order to obtain enough resources, it appears they are able to cope with low levels of habitat fragmentation, using many smaller patches to achieve this.

Although abundance of all four medium-sized mammals was greatest in the fragmented sites, abundance varied widely between fragments. As these differences may be attributed to differences in habitat quality, a more intensive study of denning and food resources preferred by black-footed tree-rats was conducted. Black-footed tree-rat populations were limited by the diversity of food resources and the availability of large trees. Fire history influenced the diversity of the fruiting species present at the site. Sites that were long unburnt had a greater fruiting species diversity and hence were able to support a greater population of black-footed tree-rats. Fire regimes were found to have a direct effect on the habitat quality by influencing vegetation structure and composition and in turn affecting mammal populations.

Most habitat fragmentation studies have been conducted in areas where extensive clearing has resulted in an extreme loss of native vegetation. For these areas, the aim is usually to report on the effects that habitat fragmentation and loss has had on the biota of the region and to suggest the best ways to restore and manage the remaining vegetation. In contrast, the main aim here was to determine the requirements of the region's fauna in order to guide the design of future agricultural landscapes. The Northern Territory has a rare opportunity to enact clearing controls, informed with the principles of ecological sustainability, to help ensure the long-term maintenance of the Northern Territory's native flora and fauna. The configuration of habitat patches capable of maintaining viable wildlife populations can be pre-determined, with guidelines being put into place before vegetation clearance occurs.

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