Loss of wetlandsLoss of wetlands

Loss of wetlands

Over the past 150 years, many wetlands suitable for Australasian Bittern in both eastern and south western Australia have been destroyed, especially due to drainage (Kingsford 2000; Garnett et al. 2011). While occurring throughout the range of occurrence of the Australasian Bittern, this impact would have been especially severe in the formerly extensive swampy landscape of the south-east of South Australia (drained mainly from 1949–1972) and in and near each of the major cities of southern Australia.

Ongoing loss of swampy wetlands suitable for or known to have supported Australasian Bittern, with replacement by urban development and infrastructure, has continued in more recent decades notably in the rapid expansion of the suburbs of Perth, Melbourne and the Gold and Sunshine Coasts, Queensland.

As well as outright loss of wetlands, wetland vegetation has been altered in some parts of the species range. In New South Wales, much of the Gwydir Ramsar wetlands have been cleared and converted from grazing to cropped agriculture, leaving only very small areas of habitat suitable for bitterns (R. Jaensch, pers. comm., 2005). One of the drivers for this trend has been spread of Lippia (Phyla canescens), a mat-forming weed that thrives under conditions of reduced inundation, reducing suitable natural fodder for livestock and forcing landholders to control it by ploughing and cropping. An additional risk for natural wetlands is geomorphic change – natural erosion and sedimentation processes that can isolate wetlands (i.e. Macquarie Marshes Ramsar site, New South Wales). This is primarily a land and water management issue that is exacerbated by river regulation, catchment land use (erosion) and water quality (turbidity).

Because of its comparatively specialised habitat requirements (i.e. densely vegetated wetlands), the species is much more sensitive to habitat loss than many other wetland birds (Garnett and Crowley 2000). Although many sites occupied by Australasian Bitterns are now protected, the species continues to be threatened by ongoing wetland loss and changes to vegetation cover. The encroachment by River Red Gum (Eucalyptus camaldulensis) and subsequent loss of the open Moira Grass Plains from areas like the Barmah-Millewa Ramsar wetlands has the potential to reduce foraging opportunities for Australasian Bitterns. The coastal zone in Australia is still subject to intense and escalating pressure from housing, semi–rural and other developments and the swampy vegetation in some wetlands has declined as wetlands become deeper sumps for increased stormwater from paved and other hard surfaces in expanding urban areas (Jaensch 2004).