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Darwin, Australia

Ward D.P.,Griffith University | Petty A.,Charles Darwin University | Setterfield S.A.,Charles Darwin University | Douglas M.M.,Charles Darwin University | And 3 more authors.
Remote Sensing of Environment | Year: 2014

The Alligator Rivers region is located in the wet-dry tropics along the coastal zone of northern Australia and contains Kakadu National Park, which is recognized under the Ramsar Convention on Wetlands and is a World Heritage listed site. Multiple anthropogenic stressors increasingly affect the floodplains of this region, and baseline information on floodplain inundation dynamics is necessary to manage these threats and develop adaption strategies for sea level rise. This study uses classification tree modeling to combine microwave (ALOS L-band Synthetic Aperture Radar) and optical (Landsat Thematic Mapper, TM 5) satellite data with field-sampled aquatic vegetation and depth logger data to predict the seasonal and inter-annual dynamics of aquatic plant cover and extent of inundation in the region. The USGS Landsat TM 5 image archive was sampled between 1985 and 2011 using three seasonal samples per year to create a comprehensive long-term time series of seasonal and inter-annual floodplain inundation extents. Classification accuracy for the inundation mapping was estimated at 86% based on seasonal depth logger data. The mean extent of inundation at the end of the wet season (March/April) was 1784km2 (range 2283-1309km2), receding on average to approximately 25% of its extent by August/September. Seasonal inundation patterns exhibit an exponential recession of inundation into 'backswamp' areas on the fringes of the floodplains with hydro-periods on the order of 5months. The findings of this work significantly improve our understanding of dynamics in this environmentally and culturally unique area and provide a basis for application in other seasonally flooded environments. © 2014 Elsevier Inc. Source

Setterfield S.A.,Charles Darwin University | Rossiter-Rachor N.A.,Charles Darwin University | Douglas M.M.,Charles Darwin University | Wainger L.,University of Maryland Center for Environmental science | And 4 more authors.
PLoS ONE | Year: 2013

Background:Widespread invasion by non-native plants has resulted in substantial change in fire-fuel characteristics and fire-behaviour in many of the world's ecosystems, with a subsequent increase in the risk of fire damage to human life, property and the environment. Models used by fire management agencies to assess fire risk are dependent on accurate assessments of fuel characteristics but there is little evidence that they have been modified to reflect landscape-scale invasions. There is also a paucity of information documenting other changes in fire management activities that have occurred to mitigate changed fire regimes. This represents an important limitation in information for both fire and weed risk management.Methodology/Principal Findings:We undertook an aerial survey to estimate changes to landscape fuel loads in northern Australia resulting from invasion by Andropogon gayanus (gamba grass). Fuel load within the most densely invaded area had increased from 6 to 10 t ha-1 in the past two decades. Assessment of the effect of calculating the Grassland Fire Danger Index (GFDI) for the 2008 and 2009 fire seasons demonstrated that an increase from 6 to 10 t ha-1 resulted in an increase from five to 38 days with fire risk in the 'severe' category in 2008 and from 11 to 67 days in 2009. The season of severe fire weather increased by six weeks. Our assessment of the effect of increased fuel load on fire management practices showed that fire management costs in the region have increased markedly (∼9 times) in the past decade due primarily to A. gayanus invasion.Conclusions/Significance:This study demonstrated the high economic cost of mitigating fire impacts of an invasive grass. This study demonstrates the need to quantify direct and indirect invasion costs to assess the risk of further invasion and to appropriately fund fire and weed management strategies. © 2013 Setterfield et al. Source

Petty A.M.,Charles Darwin University | Setterfield S.A.,Charles Darwin University | Ferdinands K.B.,Weed Management Branch | Barrow P.,Weed Management Branch
Journal of Applied Ecology | Year: 2012

1.The understanding of large-scale patterns in expanding populations of alien invasive plants can be used to infer the environmental limiting factors, habitat heterogeneity and, ultimately, the range expansion potential of invasive plants. 2.We used multivariate analysis and a novel quantile regression technique accounting for spatial autocorrelation to compare and contrast factors influencing the abundance and distribution of the African grass Andropogon gayanus (gamba grass) at two large-scale invasion sites in the tropical savanna region of Australia. We collected data using aerial and ground surveys and tested for limiting factors using three landscape-scale indices related to soil quality, soil moisture and invasion history. 3.In one site, gamba grass was principally colonising drainage lines and riparian areas. Occupation of these areas was limited in proportion to the distance from the original gamba grass source. In the second site, gamba grass abundance was independent of distance from the original source and was well established in all vegetation communities, although abundance was also limited in higher elevation sites away from drainage lines. 4.Comparisons between these sites with different patterns of invasion enabled the estimation of both the invasion pathways and range expansion potential of gamba grass. Our results indicated that gamba grass spreads from riparian communities to invade upland sites and has the potential to invade 70% of north Australia's upland savanna communities. 5.Aerial surveys comprehensively assessed patterns over a larger area than ground surveys and were much more economical. 6.Synthesis and applications. Large-scale surveys across multiple sites are critical to understanding the dynamics of recent alien species invasions where little is known about the pattern and potential range of spread. The application of quantile regression and aerial surveys shows promise as aerial surveys are efficient at capturing a large amount of data. The novel quantile regression technique we demonstrate here can account for both spatial autocorrelation and noisy ecological data from aerial surveys while returning robust results. We were thus able to demonstrate widespread colonisation of creek lines by gamba grass and recommend that management focuses on detection and eradication along drainage lines in addition to the present focus on transport corridors. © 2012 The Authors. Journal of Applied Ecology © 2012 British Ecological Society. Source

Adams V.M.,Charles Darwin University | Petty A.M.,Charles Darwin University | Douglas M.M.,Charles Darwin University | Buckley Y.M.,Trinity College Dublin | And 6 more authors.
Methods in Ecology and Evolution | Year: 2015

Invasive weeds are a major cause of biodiversity loss and economic damage world-wide. There is often a limited understanding of the biology of emerging invasive species, but delay in action may result in escalating costs of control, reduced economic returns from management actions and decreased feasibility of management. Therefore, spread models that inform and facilitate on-ground control of invasions are needed. We developed a spatially explicit, individual-based spread model that can be applied to both data-poor and data-rich situations to model future spread and inform effective management of the invasion. The model is developed using a minimum of two mapped distributions for the target species at different times, together with habitat suitability variables and basic population data. We present a novel method for internally calibrating the reproduction and dispersal distance parameters. We use a sensitivity analysis to identify variables that should be prioritized in future research to increase robustness of model predictions. We apply the model to two case studies, gamba grass and para grass, to provide management advice on emerging weed priorities in northern Australia. For both species, we find that the current extent of invasion in our study regions is expected to double in the next 10 years in the absence of management actions. The predicted future distribution identifies priority areas for eradication, control and containment to reduce the predicted increase in infestation. The model was built for managers and policymakers in northern Australia working on species where expert knowledge and environmental data are often lacking, but is flexible and can be easily adapted for other situations, for example where good data are available. The model provides predicted probability of occurrence over a user-specified, typically short-term, time horizon. This output can be used to direct surveillance and management actions to areas that have the highest likelihood of rapid invasion and spread. Directing efforts to these areas provides the greatest likelihood of management success and maximizes the return on investment in management response. © 2015 British Ecological Society. Source

Heard T.A.,CSIRO | Elliott L.P.,Weed Management Branch | Anderson B.,Weed Management Branch | White L.,Weed Management Branch | And 6 more authors.
Biological Control | Year: 2010

Macaria pallidata (Warren) and Leuciris fimbriaria (Stoll) (Lepidoptera: Geometridae) are abundant and damaging defoliators of Mimosa pigra L. in the native range of the Neotropics. Both species were assessed for their suitability as biocontrol agents of M. pigra, a damaging invasive weed of northern Australia. Larvae feed on leaves of all ages. Adults are non-feeding, short-lived moths. Generation times are short and fecundity is high allowing rapid population increase. The host specificity of these species was tested using larval development tests on 70 test plant species. Development to adult of M. pallidata occurred on six species other than M. pigra. However, the survival rates were so low that these plants could not sustain a population of this insect species. The maximum survival rate was 1.1% compared to 64% on M. pigra. When the mean lifetime fecundity is considered, a survival rate of 1.1% is the minimum required for population maintenance in the absence of other mortality factors. Open-field trials in Mexico, although not comprehensive, support the conclusion that M. pallidata is specific to M. pigra. Development beyond first instar of L. fimbriaria did not occur on any species other than M. pigra and Mimosa asperata L. The number of eggs laid by M. pallidata was independent of plant phylogeny, but adults of L. fimbriaria laid more eggs on plants more closely related to M. pigra, indicating that not all Lepidoptera show indiscriminate oviposition choices in confined situations. Following the gaining of required permits, M. pallidata was released from 2002 and L. fimbriaria from 2004. Both have established. M. pallidata has been recovered in large numbers from most sites across areas of infestation of the invasive plant, but experiences extreme population fluctuations. L. fimbriaria has been found only at low levels. The rate of parasitism of M. pallidata was 5% with the tachinid fly Carcelia malayana Baranov being the most common parasite. © 2010 Elsevier Inc. Source

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