Geographic patterns and correlates of the decline of granivorous birds in northern Australia

Abstract

A geographic index of the decline in the distribution and abundance of granivorous birds in tropical northern Australia shows that declines are greatest in Queensland and especially in the south-eastern tropics and in inland areas, and lowest in the north Kimberley and east Arnhem districts. In this paper, we use generalised linear models to investigate interrelationships among an index of decline in 1° by 1° cells and measures of grazing intensity and contemporary patterns of burning, together with the environmental variables of rainfall, vegetation and topographic patterning in the landscape. Grazing intensity was the single strongest human effect but strong correlations between grazing intensity and other human influences suggest that these may have been subsumed within the grazing intensity measure. Impacts of grazing may be worse where pastoral settlement occurred earlier. Topographic variation appeared to be a mitigating effect, suggesting a role for ‘topographic refuges’ from human activities. Relationships among granivore declines, grazing and rainfall are difficult to disentangle using inferential statistics, but a consistent effect is that declines are more severe in areas with greater year-to-year variation in rainfall. We do not suggest that our analyses are conclusive. However, they do support the proposition that better understanding of the causes of decline at finer spatial scales will emerge most strongly in studies that link habitat quality with granivore demography at inland sites of highly variable year-to-year rainfall and with strongly contrasting grazing histories.

Fire, landscape heterogeneity and wildlife management in Australia's tropical savannas: introduction and overview

“…They show that Kakadu has ostensibly achieved one of the characteristics thought to be associated with Aboriginal customary burning regimes, namely increases in spatial heterogeneity of burns at relatively fine spatial scales (1-25 ha) over the period 1980-2000. Moreover, they found greater heterogeneity in topographically variable landscapes, consistent with the suggestion of Franklin et al (2005) that the homogenising effects of fire and associated impacts on resources used by savanna granivores may be mitigated in such places.…”

“…This caution resonates with the contribution from Franklin et al (2005). They show that, over the vast span of the tropical savannas, decline in the distribution and abundance of a large suite of granivorous birds is worse in areas with higher densities of cattle, especially where rainfall is more variable, but ameliorated to some extent in topographically heterogeneous regions.…”

“…This is not to suggest that studies with a biogeographic emphasis based on mining of existing datasets should be discontinued. But in preference to churning of different combinations of low-resolution data seeking insights into the impacts of landscape change, efforts should be increasingly focused on generating improved environmental and other thematic datasets at spatial scales and levels of resolution better matched to understanding of wildlife-habitat relations (Woinarski et al 2005;Hobbs 2005;Franklin et al 2005;Bradstock et al 2005;Price et al 2005b). (2) Future studies of the effects of fire and grazing should focus on organisms that appear to be most vulnerable to shifts in resources of the sort most directly affected by variation in these processes, and seek to test at least crude hypotheses about the nature of the relationships (Woinarski et al 2005;Bradstock et al 2005).…”

“…(3) Studies seeking better understanding of the effects of fire and grazing should be sited to explore wildlife-habitat relationships where the signals are expected to be strongest, including through the use of tightly focused experimental landscape manipulation (Woinarski and Ash 2002;Williams et al 2003;Andersen et al 2003). For example, in the case of granivorous birds, studies are most likely to be incisive in regions where rainfall variance is greatest and at sites with strongly contrasting grazing histories (Franklin et al 2005). (4) Well articulated models (preferably quantitative) should be developed to explore the implications of changed management practice on the hypothesised dependencies of wildlife on particular resources (Woinarski et al 2005;Bradstock et al 2005;Ferdinands et al 2005;Whitehead 2005a, 2005b).…”

Seasonal stress physiology and body condition differ among co-occurring tropical finch species

“…Grazing by introduced herbivores and more frequent, extensive, and intense fires are likely to have decreased the suitability of savanna habitat for grass seed eaters by reducing grass productivity (Franklin et al 2005). …”