Biodiversity: Pressures on biodiversity

Independent report to the Australian Government Minister for the Environment and Heritage
(2006 Australian State of the Environment Committee), 2006

5.2 Pressures on biodiversity

Pressures on Australia’s terrestrial biodiversity have been operating over long periods of time and have a legacy (often called an ‘extinction debt’) that will continue for decades to come, even with remedial action. While clearing has been one of the main pressures, it is likely that climate change and urban development, infrastructure, and water extraction will soon dominate. For aquatic systems, the two main pressures are water extraction and habitat loss (see ‘Inland waters’).


Loss of native vegetation continues to be one of the greatest threats to Australia’s biodiversity. Historically, most clearing has been for agricultural production (see ‘Land’), with the result that around 13 per cent of the original vegetation  has been removed since European settlement. Just for forest alone (see ‘Land’), some 17 million hectares have been cleared since 1973 , with 1.5 million hectares of that deforestation between 2001 and 2004. With broadscale clearing controls in most states and territories, the threat is an increase in clearing for urban development on Australia’s richly diverse escarpment and coastal ecosystems.

These broad statistics mask some important trends. For example, some vegetation systems such as hummock grassland are relatively unmodified, while others such as eucalyptus woodlands have 66 per cent of their systems left (Table 9). Natural temperate grasslands have been even more severely affected (Table 10). Apart from limitations to the data, such as the scale of mapping, there is also the continuing inconsistency of vegetation classification systems used across states and territories. This makes it very difficult to consider finer-scale changes in vegetation type across the continent.

Table 9: Estimated changes in vegetation in Australia from pre-1750 to the present (2001–04)*
Major vegetation group Estimated pre-1750 area (km²) Area remaining (km²) Percentage remaining Percentage of remaining vegetation in reserves
Rainforest and vine thickets 53 469 35 200 65.8 54.4
Eucalyptus tall open forest 40 801 35 344 86.6 33.6
Eucalyptus open forest 394 280 272 121 69.0 22.7
Eucalyptus low open forest 4726 3952 83.6 35.1
Eucalyptus woodlands 1 362 263 892 920 65.5 8.1
Acacia forests and woodlands 495 059 408 632 82.5 8.8
Callitris forests and woodlands 40 278 32 296 80.2 6.1
Casuarina forests and woodlands 166 303 149 262 89.8 18.5
Melaleuca forests and woodlands 106 057 99 561 93.9 10.1
Other forests and woodlands 80 772 72 414 89.7 9.9
Eucalyptus open woodlands 498 663 458 905 92.0 6.2
Tropical eucalypt woodlands–grasslands 115 503 112 481 97.4 12.8
Acacia open woodlands 320 981 314 040 97.8 7.6
Mallee woodlands and shrublands 387 230 271 529 70.1 36.8
Low closed forest and tall closed shrublands 25 819 16 278 63.0 30.5
Acacia shrublands 865 845 851 274 98.3 10.0
Other shrublands 157 530 123 464 78.4 18.7
Heath 9256 8071 87.2 44.1
Tussock grasslands 559 850 525 888 93.9 3.0
Hummock grasslands 1 368 861 1 367 973 99.9 9.9
Other grasslands, herblands, sedgelands and rushlands 67 977 64 810 95.3 17.2
Chenopod shrublands, samphire shrubs and forblands 447 239 436 801 97.7 12.6
Mangroves 9664 9325 96.5 33.1
Total 7 578 427 6 562 541 86.6 11.5

Note: * National Vegetation Information System Stage 1, Version 3.0, Major Vegetation Groups

Source:DEH (2006h)

Table 10: Changes in the area of natural temperate grasslands
Bioregion Pre-1750 (ha) Area in 2003 (ha) % remaining
Brigalow Belt South 270 000 25 000 9.26
Flinders Lofty Block 1 500 000 5 000 0.33
Murray–Darling Depression 440 460 1244 0.28
Riverina 2 750 000 26 871 0.98
South East Coastal Plain 60 000 25 0.04
South Eastern Highlands 450 000
Tasmania 80 098 13 617 17.00
Victorian Volcanic Plain 220 073 2291 1.04
Total 5 770 631 96 548 1.67

Note: ha – hectare

Source: Carter et al (2003)

Clearing statistics also give no indication of the condition of various ecosystems. For example, the ecosystems in many arid systems appear to be relatively unmodified, but other pressures, such as grazing, have significantly changed their structure and condition.

Nevertheless, it is possible to say that some species and ecological communities have declined more than others, some regions are being cleared more than others, and the condition and connectivity of vegetation as habitat have declined in many areas. In cleared landscapes there has been a general decline of ecological community functionality and processes. A major concern is that old trees in these landscapes are not being replaced as they die. All of these changes in vegetation condition and extent have major implications for biodiversity .

Loss of vegetation in riparian zones  has been significant for both terrestrial and freshwater biodiversity, with 56 per cent of Australia’s riparian vegetation having disappeared from 172 river basins (ERIN 2005b) (see ‘Inland waters’).

Changed fire regimes

Australia’s fire regimes have been profoundly changed by loss of pre-European Indigenous fire regimes. For example, in most of Australia’s northern savannas there are now more large-scale, late dry season fires (Russell-Smith et al 2003). These fires are hotter and cover larger areas than the early to mid dry season fires that were typical of traditional Indigenous burning practices. In contrast, there are now fewer fires in the Wet Tropics because grazing has removed fuel. The resulting habitat changes  have been widespread. For example, shrubs and trees have replaced grasslands, and rainforests have significantly invaded wet sclerophyll communities. In the 1500+ millimetre rainfall zone there have been widespread and massive habitat changes in the last 100 years, largely related to decreasing fire incidence, with demonstrable change in the last 30 years (Stanton 1995) (see ‘Living in a land of fire’).

Bushfires will occur at some time in most parts of the Australian continent, although they may be very infrequent in some climatic zones, such as those dominated by rainforest or wet eucalypt forest. Between 4 per cent and 10 per cent of the continent (c.32 - 80 million ha) might be burnt in a typical year thereby suggesting an average interval between fires for the continent of about 15 years: in the severe fire season of 1974-75, about 15 per cent of the land area  of Australia was burnt (Luke and McArthur 1978), and in 2002-03, a severe fire year in south-eastern Australia, 7 per cent (54 million ha) was burnt. Years in which bushfires cause the most serious threats to lives and property in Australia are typically serious drought years in southern Australia (Figure 23).

Adopting appropriate fire regimes could be one of the most cost-effective tools for biodiversity management across large parts of Australia. Research has provided fire guidelines for vegetation types that specify, for example, fire should be avoided in New South Wales rainforest, alpine complexes and estuarine and saline wetlands. The guidelines also recommend periods between fires, such as a 25–60 year interval in wet sclerophyll forests, with the period between crown fires being closer to 60 than 25 years.

Total grazing pressure

Total grazing pressure is the combined effect of grazing by all animals. It includes domestic livestock such as sheep, cattle and horses, feral livestock (including goats and camels), native grazing animals (including kangaroos), and, at times, locust populations.

Total grazing pressure is one of the main pressures on biodiversity in Australia. It dramatically reduces the standing biomass of grasses and forbs, and changes the species composition, with the most palatable species suffering the most. In systems where the decline has been significant, even low total grazing pressure can prevent system recovery (Page and Beeton 2000). This is particularly the case in parts of Australia where a combination of extreme overstocking and a 50-year rabbit plague has probably shifted the ecosystem to a new state. Grazing and various agricultural improvement strategies have modified vast areas of grasslands and open grassy woodlands so that, in temperate ecosystems, less than 2 per cent of the original grasslands remain. The changes have altered populations of native animals because the changed environment favours some species to the disadvantage of others.

The proliferation of watering points across many landscapes, especially rangelands, has exacerbated the impacts of total grazing pressure because it allows native animals, feral animals and livestock to survive where they would otherwise die of thirst. The bore-capping programme in the Great Artesian Basin is a positive step in this regard, because it has reduced the number of watering points, and given ecosystems that can withstand only very light grazing pressure the chance to recover (Fisher et al 2004). The effectiveness of the programme is so far measured in terms of groundwater pressure, which has increased in New South Wales, South Australia and the Flinders Zone of northern Queensland. The ecological benefits for rangelands ecosystems are yet to be comprehensively measured and reported, but there is evidence that the 334 mound springs in the Great Artesian Basin will benefit from the demonstrated increases in groundwater pressures (Fensham 2006).

Weeds and feral animals (more information on this topic) 

It is estimated that Australia gains around 20 new pests or diseases each year (CSIRO 2004). Some well-known examples include Cane Toads (Bufo marinus), rabbits, willows and, more recently, Black Striped Mussels (Mytlopsis sallei) and Red Fire Ants (Solenopsis invicta). Historically, feral cats, foxes and rabbits have been a cause of local extinctions and significant reductions in range for native species through a combination of habitat modification and predation. They are a major ongoing pressure. Weeds are an equally significant pressure on ecosystems, with more than 2500 species of introduced plants now established in the wild in Australia. They have invaded every part of the landscape—bushland, rangelands, coasts, rainforests, deserts and farms. About 65 per cent are escaped garden plants, and nurseries still routinely sell many.

Weeds are not necessarily the primary cause of species decline. In many cases, land clearing resulting in habitat destruction, degradation and fragmentation has caused the initial reduction in species numbers and abundance. Environmental weeds then become a threat when invading remaining habitats, especially where these are already fragmented or degraded. Environmental weed invasion is a constantly increasing pressure on these vulnerable ecological communities.

The annual cost of weeds and feral animals has been estimated many times. The most recent estimates indicate that the annual cost probably greatly exceeds the current annual investment in management and control. The cost of weeds to Australian agriculture now exceeds $4 billion a year and almost all the plants involved are foreign (Sinden et al 2004). Half a million dollars a year, for example, is spent trying to keep just one woody weed species (Mimosa pigra) out of the Kakadu National Park (McLeod 2004). Other woody weed species, such as Athel Pine (Tamarix aphylla), Parkinsonia (Parkinsonia aculeata), Prickly Acacia (Acacia nilotica ssp indica) and Rubber Vine (Cryptostegia grandiflora) in Australia’s north and Bitou Bush (Chrysanthemoides monilifera), Blackberry (Rubus fruticosus agg), Boxthorn (Lycium ferocissimum), Broom (Cytisus spp and Genista monspessulana), Gorse (Ulex europaeus), olives, Radiata pine and willow in Australia’s south, are also expanding in range and are difficult and costly to control (NLWRA 2001a).

Many animal species and plants listed under the EPBC Act are threatened by at least one invasive organism. For example, the invasion of Yellow Crazy Ants (Anoplolepis gracilipes) on Christmas Island has led to a reduction in numbers of native Red Land Crabs (Geocarcoidea natalis), and this has resulted in changes to the rates of seedling recruitment and litter breakdown and the recruitment dynamics of rainforest trees. It has almost certainly changed patterns of nutrient availability. This leads to a rapid shift in forest structure and composition or a ‘state change’ in the rainforest ecosystem (Commonwealth of Australia 2005). The expansion of Cane Toads  is similarly of concern (Figure 24).

Changes to the aquatic environment

The combined effect of 50 years of changes to river flows , land use change , water use , over-allocation of water for irrigation , draining of wetlands , and habitat modification have left a legacy of decline in freshwater aquatic biodiversity. Arthington (2002) has reported that altered flow regimes have resulted in the loss of 90 per cent of floodplain wetlands in the Murray–Darling Basin, 50 per cent of coastal wetlands in New South Wales and 75 per cent of wetlands on the Swan Coastal Plain in south-west Western Australia.

The issue is not just the total amount of water, but also the timing and quality of water that stresses aquatic ecosystems. High concentrations of nitrogen and phosphorus  can lead to algal blooms, oxygen depletion, fish kills, and depleted aquatic invertebrate populations. Changing groundwater levels , which contribute to salinity, also affect groundwater-dependent species, such as wetland species that rely on groundwater during key phases of their lifecycles, as well as a range of life that exists in aquifers and about which little is known (Sinclair Knight Merz 2001).

Changes to the physical environment—through dams, weirs and ‘de-snagging’ programmes—have also been a significant pressure. Weirs and dams  change the natural frequency and magnitude of floods, they change water temperature , and they are a physical barrier to movements of fish and other species. The response has been to construct fish-ways  and to ‘re-snag ’ (drop in dead trees), but the scale and scope of these programmes are generally small compared to the scale of the problem (see ‘Inland waters’).