Macropods, fire and habitat change in Brisbane Forest Park (South-East Queensland)

Abstract

The aim of this thesis was to investigate the linkages between changes in habitat structure in Brisbane Forest Park and the distribution of macropod populations, particularly eastern grey kangaroo, the red necked wallaby, the whiptail wallaby and macropod species richness. The whiptail wallaby has been reported to have declined and possibly experienced local extinction in Brisbane Forest Park in recent decades. A key hypothesis of this thesis was that the reason for whiptail wallaby declines is alteration of the habitat structure and grass composition due to declines in the frequency of controlled burning. Multi-temporal analysis of aerial photographs shows that habitat structure of Brisbane Forest Park has become increasingly dominated by tree cover. Outcomes of interpretation of tree cover in aerial photographs reveal evidence of an increase in tree and shrub cover over the last 50 years in Brisbane Forest Park and the neighbouring Samford Valley, with a small increase in the Wivenhoe Dam area where whiptail wallabies were observed to be present. Changes in the incidence of fire during the last 30 years may also have had impacts on growth of grasses supporting wallaby presence. Using analysis of regression models, hierarchical partitioning analysis, analysis of variance and chi-squared analyses for eastern grey kangaroos showed grazing, tree cover, grass height, elevation and slope were important in that order. For the red-necked wallaby, tree cover, grazing, grass height, shrub cover, slope and elevation were most important in that order. For macropod species richness, grazing, tree cover, grass height, shrub cover, slope and elevation were most important in that order. The whiptail wallaby was only present at the Wivenhoe study area, with fire within the previous 6 months, grazing (especially by cattle), an open shrub and tree cover, short (<5 cm) grass, and greenness of grass key covariate habitat variables. The research outcomes suggest that fire has an indirect impact on the distribution of eastern grey kangaroos and whiptail wallabies. This is because it affects the life cycles of the grasses eaten by these macropod species so the grass is unable to reproduce. Growth of trees and shrubs closes around grassy areas so that the growth of the grasses preferred by the wallabies is prevented. The research outcomes depended on a community survey and field surveys to determine where macropod species were present in the study area. Photos were shown to residents for identification of wallaby species. The results of these surveys were then used to target macropod fecal pellet surveys, direct searches and habitat surveys for 80 sites extending over a large area of Brisbane Forest Park and adjacent areas of Samford Valley and Wivenhoe Dam. ANOVA, regression models and hierarchical partitioning were used to identify significance relationships between the presence/absence of the eastern grey kangaroo, the red necked wallaby and the count of macropod scats (all species) and key environmental and habitat covariates. The key implication for management is that a less frequent fire regime within Brisbane Forest Park is resulting in a more closed habitat structure that is not suitable for the eastern grey kangaroo, the red necked wallaby and the whiptail wallaby, although the red-necked wallaby appears to least affected by these changes. If a key management objective is to increase macropod presence in Brisbane Forest Park, especially whiptail wallaby presence, then a more frequent burning regime of every 2-3 years is required. This would create a more open-habitat mosaic and encourage the regeneration of grasses such as kangaroo grass (Themeda triandra) preferred by the whiptail wallaby. Kangaroo grass is one of the dominant grasses in Brisbane Forest Park, and is less common when burning every 3-4 years than when it is more frequent. Burning also should be in winter months to encourage kangaroo grass dominance. However, other considerations such as smoke pollution of Brisbane and a community preference for wet eucalypt forest and rainforest may preclude a more frequent fire regime.