The initial disruption to soil usually results in a significant loss of nutrients. Organic matter is oxidised, and the removal of surface cover (litter and protective vegetation) makes the soil more prone to erosion. Stores and cycles of nutrients adjust under the new land use, but in most cases the net loss of nutrients and leakage are greater than under natural conditions. As noted in Section 2.2.4, soil carbon typically reduces to 20-70% of the pre-clearing amount. Restoring this very large stock of carbon is now a key focus in programs for mitigating GHGs around the world, and nationally (e.g. the Australian Government’s Carbon Farming Initiative14).
The removal of native vegetation also results in major changes to the hydrological cycle, including dryland salinity (see Section 2.2.3). The soil also experiences more rapid leaching, and this can change soil properties and processes (e.g. clays may disperse and reduce permeability). A less widely appreciated effect of clearing is that the land surface becomes more uniform—the patchiness of the native system is lost. For example, removing mounds of litter, grass tussocks and rough surfaces leaves a relatively smooth soil surface. This almost invariably leads to more rapid run-off and erosion, less effective water infiltration, and a loss of the micro-environments necessary for many species.
Invasive species impacting on the land environment comprise disease–causing organisms such as, fungi and parasites, insects and other invertebrates, pest animals and weeds.66 Invasive species put pressure on land environmental values in a variety of ways, as well as impacting on biodiversity (see Chapter 8: Biodiversity). Some invasive species, such as rabbits, are long-established pressures. Others, such as foxes, are long established in some environments but new to others; yet others, such as the fungus that causes myrtle rust, are only recently established. The risk of invasive species incursions into Australia is increasing with the growth of international travel and trade.67
Among the numerous introduced fungi affecting plant health, two are of particular concern at a national scale.
The first is Phytophthora cinnamomi, which causes root rot that can significantly alter plant communities and lead to local and perhaps complete species extinctions.68 Its impacts have been recognised since the 1960s. P. cinnamomi now occurs widely across Australia, but its most severe impacts are in vegetation communities in the south-west and south-east of the country. Many genera of endemic taxa have a high proportion of susceptible species—many of which are rare and threatened.69 Consequently, the disease caused by P. cinnamomi is listed as a key threatening process under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act).68
In contrast, the fungus that causes myrtle rust, Uredo rangelii, is a recent introduction. It was first detected in 2010 in central coastal New South Wales, and is now established in most of coastal New South Wales70 and in south-east Queensland.71 Myrtle rust affects trees and shrubs in the Myrtaceae family of plants. This family includes many iconic Australian plant genera, including bottlebrush (Callistemon.), lilly pillies (Eucalyptus.) and affiliates (e.g. Corymbia.), lillypillies (Syzygium.) and tea tree (Melaleuca.). Because of the dominance of the Myrtaceae in the Australian flora and the high mobility of plant rusts, the potential impacts of myrtle rust are profound. However, little is known of the behaviour and impacts of myrtle rust under Australian conditions.72
Some 73 invasive pest animal species (amphibians, birds, fish, mammals and reptiles) have established populations in Australia. In many cases—such as feral cats, foxes, rabbits and wild dogs—these populations are long established and distributed over much of the continent. In other cases, such as foxes in Tasmania, introductions are recent, and populations are still small. The highest concentration of significant pest animal species is along the eastern seaboard, and many coastal and offshore islands suffer significant impacts.73 Some newly established pests, such as the Asian honeybee, may ultimately have significant impacts on ecosystem processes and thus vegetation.74
Pest animals with the greatest impacts on the land environment, in terms of damage estimates, are foxes, feral cats, rabbits, feral pigs, wild dogs, house mice, goats, cane toads, wild horses and camels.75 Their impacts are expressed as environmental damage, such as that caused to soil and vegetation by pigs or camels; as loss of production in agricultural systems; and as loss of biodiversity. Land degradation by goats, pigs and rabbits, and the impacts of cane toads, are formally listed as threatening processes under the EPBC Act.
Invasive weeds present serious threats to Australia’s environmental values. They displace native species, contribute significantly to land degradation, and reduce farm and forest productivity.76 The Australian and state and territory governments have identified the worst of these (currently, 20 species) as Weeds of National Significance for coordinated management action (Table 5.8). The impacts of gamba grass and four other introduced grasses in northern Australia, and of escaped garden plants nationally, have been listed as threatening processes under the EPBC Act. A further 28 species have been identified as being at an early stage of establishment, but with the potential to become significant threats.77 The potential of weeds to transform the land environment is very significant; for example, ‘eleven plant species have the capacity to permanently alter ecosystems across Australia’s rangelands’.41
Table 5.8 Weeds of National Significance, July 2011
|Prickly acacia, blackthorn, prickly mimosa, black piquant, babul
|Pond apple, pond-apple tree, alligator apple, bullock’s heart, cherimoya, monkey apple, bobwood, corkwood
|Bridal creeper, bridal veil creepera, smilax, florist’s smilax, smilax asparagus
|Cabomba, fanwort, Carolina watershield, fish grass, Washington grass, watershield, carolina fanwort, common cabomba
||Chrysanthemoides monilifera subsp. monilifera
||Chrysanthemoides monilifera subsp. rotundata
|Rubber vine, rubbervine, India rubber vine, India rubbervine, palay rubbervine, purple allamanda
|Hymenachne, olive hymenachne, water stargrass, West Indian grass, West Indian marsh grass
|Lantana, common lantana, Kamara lantana, large-leaf lantana, pink-flowered lantana, red-flowered lantana, red-flowered sage, white sage, wild sage
|Mimosa, giant mimosa, giant sensitive plant, thorny sensitive plant, black mimosa, catclaw mimosa, bashful plant
|Chilean needle grass
|Serrated tussock, Yass River tussock, Yass tussock, nassella tussock (NZ)
|Parkinsonia, Jerusalem thorn, jelly bean tree, horse bean
|Parthenium weed, bitter weed, carrot grass, false ragweed
|Blackberry, European blackberry
||Rubus fruticosus aggregate
|Willow (except weeping willow, pussy willow and sterile pussy willow)
||Salix spp.(except S. babylonica, S. calodendron and S. reichardtiji)
|Salvinia, giant salvinia, aquarium watermoss, kariba weed
|Athel pine, athel tree, tamarisk, athel tamarisk, athel tamarix, desert tamarisk, flowering cypress, salt cedar
a May also refer to Asparagus declinatus
b The Weeds of National Significance listed show bitou bush and boneseed separately—these two taxa together are treated as one of the 20 Weeds of National Significance