Long-term trends of shorebird populations in eastern Australia and impacts of freshwater extraction

Abstract

Shorebirds (Charadriiformes) and their wetland habitats are under threat worldwide. While data exist for shorebird population trends for many parts of the world, two thirds of Australia’s populations have not been assessed. We report the results of a large-scale aerial shorebird survey, sampling about a third of the Australian continent over a period of 24 years (1983–2006). Migratory shorebirds have declined by 73%, Australian resident shorebirds by 81%. Of the 10 wetlands supporting the highest number of shorebirds within the survey bands, eight were inland wetlands and only two coastal, emphasising the importance of inland wetlands for shorebirds. Wetland area decreased significantly at four of the 10 main sites. Annual rainfall showed no trends (1983–2005) but water extraction was substantial for four of the 10 wetlands, contributing to reduced flooding extent and frequency. Loss of wetlands due to river regulation is a significant contributor to the drastic decline in shorebird numbers in Australia, largely unrecognized in international conservation agreements in Australia.

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).