The Western Treatment Plant (WTP), managed by Melbourne Water, is Melbourne’s major processor of wastewater. Its 10,500 ha of lagoons, paddocks and other environments, attract tens of thousands of birds, contributing to its recognition under the Ramsar convention of wetlands of international significance. Consequently, and in line with the Commonwealth Environmental Protection ... [Show full abstract] and Biodiversity Conservation Act 1999, any changes in management must be carried out in the context of mitigating potentially adverse environmental impacts. The extensive paddock system (~4,550 ha) of the WTP has been utilised by Straw-necked Ibis Threskiornis spinicollis as foraging habitat for many years. Straw-necked Ibis are listed as one of the key values of the Ramsar site that includes the WTP. Consequently, an important management aim for Melbourne Water is to maintain the numbers of ibis using the site. Many WTP paddocks have now been converted from pasture (previously utilised for sewage treatment) to various agricultural crops. Ibis also use cropped areas for foraging, and although observations indicated that tall (> 45 cm) dense crops were unlikely to be utilised, this had not been investigated rigorously. Most paddocks are now leased by MPH Agriculture Pty Ltd (MPH), with 9% (~400 ha) of the total paddock area reserved for conservation purposes within what is called the Terrestrial Margin.
Long-term monitoring indicated a decline in ibis numbers, and there was concern that changes in land use could affect the amount of suitable habitat for ibis. This study was initiated to further understand ibis foraging preferences and to aid Melbourne Water in maintaining ibis habitat while improving the efficiency of water use at the property. Determining the value of the Terrestrial Margin as ibis habitat not subject to cropping was central to the study.
The study included subjecting the Terrestrial Margin to targeted irrigation during autumn and winter from 2014 to 2017 and exploring how ibis responded to different watering treatments. This information was combined with data from long-term ibis monitoring at the WTP to investigate patterns of habitat use by Straw-necked Ibis.
Ibis foraging data were collected during 46 surveys of the paddock system from March 2013 to June 2017. These were analysed with crop rotation, vegetation height and irrigation records to investigate the impact of land use on the presence, distribution and number of ibis.
• The variety of land uses applied to the WTP paddocks is an important feature of the foraging habitat for Straw-necked Ibis that utilise the site
• Paddocks within the Terrestrial Margin are a major contributor to ibis foraging habitat, particularly when being irrigated
• Irrigation increased the odds of ibis being present by a factor of 9.4, and the average number of ibis by a factor of three
• Paddocks with vegetation heights that were medium (~20-35 cm) or short (<15 cm) were more likely to contain ibis than those with tall (~35-45 cm) or very tall (> 45 cm) vegetation
To maximise habitat suitability at times when agricultural priorities limit ibis foraging habitat at the WTP:
• Continue the current regime of flood irrigation of Terrestrial Margin paddocks from December until the end of June, but particularly April to June to coincide with the peak ibis period.
• In pasture and within the Terrestrial Margin keep, as far as possible, vegetation density low and vegetation height below that of ibis (i.e. below 45 cm).