Southern hairy-nosed wombats: when, where, how many, and why

Abstract

In 2016 the conservation status of the southern hairy-nosed wombat was upgraded from ‘Least Concern’, to ‘Near Threatened’, based on an assessed population decline of up to 30% over the previous 25 years. Conversely, landholders in regions where wombats are present claim that the population has increased over recent decades. To resolve this discrepancy, we conducted a species-wide survey to determine overall wombat numbers and to establish any population trends. To do so, we had to develop reliable means of mapping the wombat distribution and estimating their abundance. We also conducted a literature review to estimate the likely distribution at the time of European settlement. We then used the findings from our survey to determine the factors which influence wombat distribution and abundance at different spatial scales. At the time of European settlement the wombat distribution was split into two main groups separated by Spencer Gulf. The western population extended to Balladonia in Western Australia, while the eastern group covered Yorke Peninsula and the mid-north, Murraylands, and extended along the northern bank of the Murray River to Euston in New South Wales. The population experienced a dramatic decline in the late nineteenth / early twentieth centuries because of human persecution and competition from rabbits. Using field surveys and the analysis of satellite imagery, we found that the wombat population has expanded in geographic range and overall numbers since the 1980s, although the rate of growth has not been uniform across the regions. Remote regions such as the Gawler Ranges and Nullarbor Plain have experienced the highest growth rates, while growth in the Murraylands has been moderate. We estimate the species-wide population at ~ 1.3 million. The population trend is difficult to establish, as earlier surveys did not include some areas in their assessments, including a population group in Western Australia that we surveyed for the first time. The index of 0.43 wombats / active warren, which we calculated by collecting video data on burrow occupancy rates, is similar to the indices that were calculated for the Murraylands in the 1980s / 90s. The environmental factors which shape the wombat distribution are rainfall, rainfall variability, soil texture and vegetation. Wombats are absent from areas with a mean annual rainfall of < 154 mm, with abundance declining when rainfall is < 227 mm. Wombats are unable to construct warrens in areas where the soil clay content is outside the range of 10 - 40% – with a preferred range of 16 – 28%. Wombats also show a preference for open vegetation types, with a lower occupancy rate in closed vegetation types. While the overall distribution has fragmented and declined, abundance is probably higher in some areas due to the clearance of mallee woodlands. The over-riding influence on whether wombats are present is land-use. While 38% of the wombat distribution is in protected areas and 50% on grazing land, wombats are virtually absent from croplands. This explains most of the fragmentary nature of the wombat distribution, and is the main cause of human / wombat conflict.