Suchergebnisse

In a sample of 104 closely monitored magpie goose nests, all of 66 successful
nests contained substantial numbers of eggshell fragments
(52–1000+), many embedded deeply in the nest matrix. Detached shell
membranes were also found in all successful nests. All but one of 38
unsuccessful nests contained no shell fragments. The exception contained about
60% of the shell from one egg, but fragments were clearly
distinguishable from shells of hatched eggs because they were contaminated
with egg yolk. Weight of eggshell in individual nests could be related to the
number of hatchlings (excluding failed nests:
r2 = 0.67,
P < 0.0001, n = 39), but
the time required to retrieve and process shell fragments is perhaps too long
to justify routine use to estimate hatchling production for individual nests.
Rather, we suggest that the presence of substantial quantities of eggshell
fragments (>12 g or the equivalent of more than 1 egg) embedded in the nest
matrix be regarded as diagnostic of hatching success. We interpret the clear
discrimination between successful and failed nests as a direct result of
consistent features of magpie goose biology and the breeding environment. The
most important of these are: (i) the prolonged hatching and on-nest brooding
period, during which eggshells from hatched eggs are crushed and their
movement into the nest matrix promoted; (ii) the continuous addition of
vegetation to the nest, often covering shells; (iii) the removal of shells
from damaged (but not hatched) eggs by attending adults; and (iv) the dominant
egg predator being a large snake (the water python, Liasis fuscus) which
consumes eggs whole and hence leaves no shells at the nest site. We suggest
that post-hatch surveys of nest structures should be used to improve
understanding of spatial variation in nest success within colonies, and to
compare success between colonies of different size.

Depredation of artificial ground nests was examined in tropical savanna in northern Australia to assess potential predation pressures on nests of the partridge pigeon (Geophaps smithii), a declining tropical granivore. Predation rates were examined at two sites, Kakadu National Park (which supported a relatively high density of partridge pigeons) and Berry Springs (which had greater habitat fragmentation and comparatively low partridge pigeon density). The effects of distance from road, understorey structure, topography and nest-microsite concealment on nest predation rates were examined. Artificial-nest predation rates were greater at 150 m from roads than <1 m from the roadside. Predation rates did not vary with understorey structure, topography, or level of nest concealment. There was marked variation between sites, with predation levels at Kakadu more than double those recorded for Berry Springs. Discerning predator identity, or even the size of a predator, from marks left in clay eggs proved difficult and was possible for ~35% of predation events. Of these, 42% of predation events involved predators of a size we considered too small to take a natural partridge pigeon nest. We suggest that extrapolation from artificial to natural ground-nest predation rates be undertaken with caution for landscapes such as Australia's tropical savanna, which supports a high diversity and abundance of small potential predators of artificial nests. There was no evidence of predation by birds, and the methodology proved inadequate for identifying predation by feral cats (Felis catus).

We used improved information on fecundity and credible estimates of survival to define the plausible bounds of maximum intrinsic rate of population increase (r
m) in magpie geese and explored the management and conservation implications, given current estimates of off-take. We considered previous estimates of r
m (0.78, based on time-series analysis of aerial count data) and the corresponding predicted maximum sustainable harvest rates (30–39% per annum) to be excessive, thereby risking poor management decisions. In contrast, our estimate of the maximum plausible range of r
m (0.11–0.28) is substantially lower than the previous estimate, and suggests maximum harvest rates of no more than 5–14% of total population size per annum. We consider that it would be unwise to adopt the prior estimate of r
m for management policy because it would risk over-exploitation. Our results embody a more precautionary approach to assessing maximum sustainable yield for magpie geese in northern Australia, and are not confounded by migration. The move from a simple empirical model to a partially mechanistic one reflects a significant improvement in understanding, but must nevertheless be viewed as part of an ongoing process of model refinement and testing.

We developed a population model of magpie geese in the Northern Territory that considered spatial and temporal variation and related sources of uncertainty, building on previous analyses of the plausible rates of increase for this species. The model was used to explore realistic limits to recreational and indigenous harvest and to examine productive, yet risk-averse, management regimes for long-term sustainability. Harvest strategies based on a proportional off-take provided similar yields to a fixed quota system, but resulted in a reduced risk of substantial population decline. Moreover, higher harvests could be supported in model systems that incorporated dispersal phenomena consistent with patterns suggested by the observed distributional and abundance patterns. However, irrespective of harvest strategy and spatial structure, off-take at the levels implied in previously published analyses are clearly unsustainable. These results illustrate the desirability of matching the design of management systems to the heterogeneity of population processes. Management regimes that fail to take account of spatial and temporal heterogeneity could damage the interests of important stakeholder groups and potentially imperil the future viability of the species. However, the costs of disaggregated management systems may be substantial and benefits of investment in them need to be clearly demonstrated. Gaining better appreciation of spatial variation in harvests should be given high priority.

Despite an apparent structural integrity, the savanna landscapes of northern Australia are in flux. Important elements of the fauna have contracted in range and are less abundant than in the relatively recent past. Vegetation patterns are changing as populations of some important woody plants decline in some parts of the savannas while, in other places, different trees and shrubs are substantially increasing in density. These sorts of changes are occurring in lands under all tenures and subject to a variety of management goals and practices, including conservation reserves. Fire, large grazing animals and, more recently, invasive plants have all been implicated as drivers of adverse change. An important general, albeit inadequately tested, theory about apparently widespread faunal decline is that these influences have, jointly or separately, compromised landscape heterogeneity. It has been proposed that resource-rich patches that sustain savanna fauna through seasonal and longer-term peaks and troughs have been reduced in number, become more widely separated in space and time, or have been reduced in quality so that wildlife dependent on the rich patches struggle to use them effectively. The issues connected with these ideas were explored in a conference on fire and savanna management at Charles Darwin University in July 2003. Congruent with the theme of the symposium in which they were presented – Managing for Heterogeneity – the subset of papers presented here is diverse in origins, issues, perspectives and the spatial and temporal scales with which they deal. We consider that they make an important contribution to debate about conservation and development in northern Australia. Not because they answer the important questions, but because they illustrate the need for a shift in emphasis: from tentative exploration of pattern and weak inference about process to a harder-edged examination of the features of savanna habitats that influence their capacity to support viable wildlife populations.

AbstractAustralia has committed to substantial cuts in greenhouse gas emissions (GHGE) achieved through a national emissions trading system, raising important issues for relatively undeveloped regions of Northern Australia and, in particular, Indigenous lands. Can mostly Indigenous and socio‐economically disadvantaged people living in such regions develop institutions to contribute significantly to the mitigation of GHGE, yet pursue regional development? Will national policies adequately recognise the special needs and potential contributions of such communities? These questions and the challenges inherent in them are addressed in this article with reference to a significant initiative involving the community management of landscape fire to reduce annual GHGE from savanna burning. This initiative appears to offer potential for engagement with global carbon markets, but it will need local, national and international support, along with appropriate changes in attitudes and legal arrangements, to ensure an equitable distribution of tangible rewards, while protecting the cultural and related benefits of customary fire use. Copyright © 2008 John Wiley & Sons, Ltd.

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.

Investment in programs that help Indigenous people undertake work maintaining the environmental health of their country has benefits for the environment as well as the physical, mental and cultural health of the Indigenous people involved. For health these findings have direct implications for some national health policies, service provision to homelands, health promotion and Indigenous health research. There are also direct implications for environmental investment in northern Australia and the design and regulation of markets in resource entitlements. Indirectly the findings should be important for economic, employment and education policies as well as those promoting social harmony. Given the range of benefits there is a strong argument for cross‐agency investment in working on country by Indigenous people.