Long-term trends of shorebird populations in eastern Australia and impacts of freshwater extraction
Introduction
The world is currently undergoing its sixth mass extinction (May et al., 1995), with extinction rates at least several hundred times higher than previously in human history (Pimm and Brooks, 2000). For birds, 20% of the species alive when records began are now extinct (Wilson, 1992). A critical challenge for conservation biology is estimation of population trends (Lawton and May, 1995), especially for species of conservation concern, where further decline might lead to extinction and management is necessary for their survival (Caughley and Gunn, 1996). Even though birds are well studied, few long-term population data exist that separate stochastic variation from population declines. Difficulties in identifying trends are exacerbated for highly mobile species.
Many shorebirds or waders (Charadriiformes) are long-distance migrants. They breed in the arctic tundra and spend the non-breeding season at temperate or tropical latitudes. About eight million migratory shorebirds use the East Asian–Australasian Flyway (Bamford et al., 2006), linking the breeding areas in eastern Siberia and western Alaska with the non-breeding areas in Australia and New Zealand (Battley and Rogers, 2007). In Australia, shorebirds were thought to use primarily coastal habitats but the importance of inland wetlands is increasingly realised (Kingsford and Porter, 1993). These inland wetlands vary considerably in availability reflecting stochastic rainfall and river flows (Roshier et al., 2001). The availability of many of these inland wetlands in Australia has been reduced by dams, water extractions and levee banks, particularly in the southeast of the continent (Kingsford, 2000).
Worldwide, shorebird populations have undergone considerable declines. Of the 237 populations with trend data, 52% are in decline, while only 8% are increasing. For two thirds of the 57 Australian shorebird populations, no trend data exist (Wetlands International, 2006). We investigated changes in shorebirds numbers in eastern Australia over a 24-year period, using systematic waterbird surveys that sampled more than a third of the continent. Waterbirds were counted on about 2000 wetlands along 10 survey bands across the eastern half of the continent (Fig. 1). Trends in shorebirds numbers and wetland area at the 10 most important wetlands were linked to anthropogenic changes to river flows over the past two decades that could have detrimentally affected wetland habitat (Kingsford, 2000).
Section snippets
Methods
Shorebirds were counted as part of annual (October) aerial waterbird surveys across eastern Australia (38°30′S–20°30′S) between 1983 and 2006; methodology remained unaltered throughout this period (Kingsford et al., 1999). Surveys were flown along ten 30 km-wide survey bands (Fig. 1). The survey bands cover 12% (332,360 km2) of eastern Australia (2,697,000 km2). All wetlands larger than 1 ha were surveyed within each band and given a unique number. Smaller wetlands were counted on an ad hoc basis
Results
Between 1983 and 2006, shorebirds were counted on 1447 unique wetlands across eastern Australia (Fig. 1) at least once; not all surveyed wetlands had shorebirds. Of the wetlands with shorebirds, about half (n = 792) the wetlands were small (<1 ha) and counted in one year only, supporting relatively few shorebirds (mean: 9 ± 1.52, range: 1–201). Wetlands that were 1 ha or larger supported on average 338 (±56.87, range: 1–92,842) individuals. The highest population estimate for eastern Australia for a
Discussion
Shorebird populations across a third of Australia are in serious decline, based on data from systematic surveys over 24 years. Between the 1980s until 2006, the mean number of shorebirds declined by 77%. The magnitude and scale of this change has not been reported for the Australian continent and reflects a general trend around the globe, where more than half of all shorebird populations with known trends are declining (Wetlands International, 2006). Generally, estimates of the magnitude of
Acknowledgements
The views and contents expressed in this paper reflect those of the authors and not necessarily the funding bodies. The study received financial support from the Australian Government’s Natural Heritage Trust Fund. Aerial surveys were supported by the Department of Sustainability and Environment in Victoria, the Environment Protection Agency in Queensland, the Department of Environment and Climate Change (DECC) in New South Wales and the Department of Environment and Heritage in South
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