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An invasive species is 'a species occurring as a result of human activities beyond its accepted normal distribution and which threatens valued environmental, agricultural and personal resources by the damage it causes' (EA 2002a). Invasive species include feral animals, marine pests, weeds, non-native insects and other invertebrates, and diseases and parasites. Invasive species can be native or exotic. They may reduce farm and forestry productivity, threaten native species and contribute to land degradation. Invasive species are acknowledged by the World Conservation Union (IUCN) as the second most significant cause of biodiversity loss in the world, behind habitat loss and fragmentation.
Australia's pest animals have direct impacts on Australia's livestock industries through predation and competition for pasture. Short-term agricultural costs attributed to the main exotic vertebrate pests in Australia have been estimated to total at least $420m per year (Bomford & Hart 2002). A further $60m per year is spent on controlling these vertebrate pests, while the costs of combating associated long-term land degradation are likely to be large. These estimates do not include environmental costs such as threats to the survival of native species.
Pest animals may damage vegetation and soils, foul water or compete with native animals for habitat and food. Along with the processes associated with their presence, several invasive species are listed as key threatening processes under the EPBC Act 1999. Listed threatening processes threaten or may threaten the survival, abundance or evolutionary development of a native species or an ecological community. Competition and land degradation by feral goats and rabbits; predation by feral cats and the European red fox; and predation, habitat degradation, competition and disease transmission by feral pigs are each recognised as key threatening processes (EA 2002b).
The number of bird and mammal species threatened by processes associated with these invasive animals is shown in graph 14.21. These invasive species also threaten a range of plants, amphibians and reptiles. Threat abatement plans have been prepared under the National Feral Animal Control Program and the EPBC Act 1999 for the European fox, cat, rabbit and goat (EA 2002b). These plans focus on strategic approaches to reducing, to an acceptable level, the effects of processes that threaten the long-term survival of native species and ecological communities.
Invasive plants (weeds)
A weed is any invasive plant, native or introduced, that is deemed to be a problem or has the potential to be a problem on any area of land or water. Weeds reduce the productive capacity of agricultural systems (agricultural weeds) and pose a significant threat to natural ecosystems (environmental weeds). Surveys of landholders in Australia show that weeds are the most common land management problem faced by farmers (graph 14.22), and the majority of farmers believe their weed problems are getting worse (Jones et al. 2000). In 1987, weeds were estimated to cost the Australian economy $3.3b annually, through lost agricultural production and control costs (Combellack 1987). Environmental weeds are considered one of the most serious threats to biodiversity and nature conservation in Australia (Williams et al. 2001).
There are over 3,000 weed species in Australia today (National Weeds Strategy Fact Sheet), of which over 370 species have been declared noxious (Lazarides, Cowley & Hohnen 1997). Management of these weeds is coordinated through the National Weeds Strategy, which aims to integrate the efforts stakeholders, including governments, industry, land managers and the general public, to reduce the detrimental impact of weeds on the sustainability of Australia's productive capacity and natural ecosystems.
To help focus national efforts addressing the weed problem, a list of 'Weeds of National Significance' (WONS) has been compiled (Thorp & Lynch 2000). A final 'Top 20' weed species (table 14.23) were selected from an original list of 71 nominated weed species on the basis of their invasiveness and impact characteristics; their potential and current area of spread; and their primary industry, environmental and socioeconomic impacts. Of these top 20 WONS, six were classified as primarily a threat to the environment, another five as primarily a threat to agricultural systems and nine weeds have both environmental and agricultural effects.