8 Biodiversity | 3 Pressures affecting biodiversity | 3.7 Clearing and fragmentation of native ecosystems
State of the Environment 2011 Committee. Australia state of the environment 2011.
Independent report to the Australian Government Minister for Sustainability, Environment, Water, Population and Communities.
Canberra: DSEWPaC, 2011.
Overall, national rates of broadscale land clearing have fallen dramatically. However, substantial clearing continues in many areas. In addition, the legacies of past land clearing will continue to operate for some decades as old trees and small remnants of native ecosystems isolated in mostly cleared landscapes die, and recruitment of new plant seedlings, fungi and animals is hindered by changed fire regimes, altered soil properties, exposure to predators and reduced ability to find mates. Addressing these legacy effects will be complex, long term and potentially expensive.
The Assessment of Australia’s terrestrial biodiversity 200815 concluded that:
… broad-scale land clearing has been largely brought under control in the jurisdictions that accounted for most of the clearing in 2002: New regulatory frameworks in Queensland and New South Wales have dramatically reduced the level of approved clearing of remnant vegetation nationally in the past five years. (p. 150)
While this statement remains generally true, the additional analyses reported in Chapter 5: Land reveal a slightly more complex situation:
The annual rate of forest loss in the mapped intensive-use zone over the decade to 2010 averages 1.1 million hectares (range 0.7 million – 1.5 million hectares). This loss has been offset by forest expansion averaging 1 million hectares annually (range 0.6 million – 1.3 million hectares). As a consequence, there was a small net gain of forest in Australia in 2007–10, for the first time since the early 1990s. The overall average net rate of forest change in the area mapped over the decade to 2010 was a loss of around 160 000 hectares annually. As the 2006 SoE report103 noted, ‘regrowth’ vegetation and its environmental values are generally different in many respects from the vegetation that has been cleared.
Consistent with the conclusion by the Assessment of Australia’s terrestrial biodiversity 2008, Chapter 5: Land concludes that:
Rates of land clearing averaged around one million hectares annually over the period 2000–10, and were balanced by the extent of regrowth—although the character and values of the original and regrowth vegetation are often different.
Although average figures suggest that the pressure on biodiversity from land clearing is reducing or stable in many parts of Australia, more detailed data are needed to assess the significance of these changes for biodiversity. For example, relatively high levels of clearing have continued in parts of north-eastern Queensland and in the headwaters of the Murray–Darling Basin, which could have far-reaching impacts.104 Research in the Northern Territory suggests that rates of land clearing there, in conjunction with changed fire regimes and invasive species, have been sufficient to cause major, and somewhat unexpected, changes in biodiversity.105 Although there have been major advances in re-establishing vegetation cover (see Chapter 5: Land), replacing mature native vegetation with regrowth seldom provides the same environmental values as the original vegetation.
Decreases in broadscale clearing have been beneficial for many bird species; however, clearing of northern woodlands and of forests is an emerging threat. Continued clearing of old growth native forest in Tasmania and the legacy of past clearing across large parts of Australia are two major factors degrading the remaining habitat for birds.85
The Queensland Herbarium has collected and published some of the most detailed data on land clearing of any state. These data demonstrate the level of detail necessary to identify where pressures are highest and where interventions might be needed. Past clearing has been uneven across the state—it is concentrated in the south-east, as is most of Queensland’s population. Clearing since 1997 has also been greatest in the south–eastern and southern parts of the state and there is a wide range in extent of clearing, from virtually zero to more than 3.5% annually.104 Clearing of some of the subregions at the top of the Murray–Darling Basin has been much more extensive than the Queensland average (Figure 8.14). Understanding clearing at this scale is important for understanding and managing ecological processes that operate at a range of scales. For example, the clearing in the top of the Murray–Darling Basin is likely to have implications not only for biodiversity in those subregions, but also for freshwater-dependent biodiversity living lower in the Basin.
Fragmentation of habitat is one of the two most frequently cited pressures for EPBC-listed species (the other being invasive species).15Figure 8.15 shows the highly fragmented nature of vegetation systems to the west of the Great Dividing Range in eastern Australia, across much of southern Australia and in the south-west. These areas, not surprisingly, coincide with the areas in which numbers and proportions of threatened species occur (Figures 8.5, 8.6, 8.8 and 8.9).
Source: Australian Government Department of the Environment, Water, Heritage and the Arts15
Figure 8.14 Analysis of the sizes of remnant patches of native vegetation in Australia
Research is starting to provide guidance on how to interpret data on the connectivity of remnant vegetation.106-107 However, we do not yet have enough data to draw strong conclusions, other than recognising that strong environmental pressures (generated by various forces, such as land clearing) are causing habitat fragmentation. Although many of these forces have been reduced (e.g. as land clearing becomes more strongly regulated), there is a strong legacy effect that is likely to see the extent and condition of native vegetation continue to decline for several decades.
Urbanisation is arguably one of the strongest and most expanding forms of human pressure on biodiversity.108 Urbanisation directly removes habitat. It can also reduce habitat quality by fragmenting it, simplifying its structure, or altering the composition of ecosystems embedded in urban landscapes. The effects of urban development on biodiversity are not restricted to urban areas, but may extend into neighbouring landscapes and affect species and ecological processes in adjacent ecosystems.108 Peri-urban development (urban development that spreads into the interface between urban centres and rural land) has been the focus of recent attention, because this is a growing phenomenon with increasing implications for many aspects of land management (see Chapter 5: Land). Peri-urban development brings people who might not have experience dealing with issues like weeds, feral animals, fire, pollution and other factors that affect biodiversity and habitat quality into situations that they are often ill-equipped to manage. These are also situations for which regulations and guidelines are still being developed.109
Areas of urban development coincide closely with many areas of highest species diversity and endemism in Australia (Figure 8.3) and with areas of greatest alterations to habitat and the greatest numbers and proportions of threatened species (Figures 8.5, 8.6, 8.8 and 8.9). This coincidence occurs because people have settled in areas of fertile, productive soils, which tend to occur around the mouths of major rivers. As a result of urban development, not only is biodiversity in those areas reduced, but agricultural industries have been forced to move into less productive land where they have higher impacts.32,109 As Australia’s population grows, our impact on biodiversity will depend on how well the locations and types of urban development, as well as the lifestyles of people living in urban areas, are managed.56,58,108
The impacts of mining on soils and other environmental assets and values are considered in detail in Chapter 5: Land. Mining has direct local impacts on habitat extent and quality, as vegetation is removed to varying degrees depending on the type of mine. Mining can also pollute waterways and disrupt agriculture through competition for land, or by directly affecting the land itself (e.g. through subsidence). The same processes that cause rare minerals to accumulate could also lead to a high endemism of native species (especially plants), which, in turn, could cause conflict between conservation and mining interests.110
Mining currently occupies a very small proportion of the land area of Australia, but the mining industries are growing and their impacts are becoming more extensive. There have been some reports of highly successful rehabilitation of mine sites, which may allow substantial offset of initial impacts in the long term.111