6.4 Terrestrial invasive species
Invasive species remain one of the greatest threats to biodiversity in NSW. Over half of all the key threatening processes listed relate to invasive species.
Once invasive species become widely established, few can ever be eradicated, and controlling them must focus on strategically limiting their impacts on biodiversity.
The main vertebrate pests in NSW have been present for the last century, with many widespread across the State. Predation by foxes and cats is implicated in the decline or extinction of numerous small- to medium-sized animals. Herbivores, particularly rabbits and feral goats, are responsible for overgrazing of native vegetation and land degradation.
Some 1350 exotic plant species have become established in NSW, more than 300 of which are significant environmental weeds.
New pest species continue to become established in the environment. Combining prevention, early detection and eradication is the most cost-effective way to minimise the impacts of new pests.
NSW indicators
Indicator |
Status of indicator |
BIO 6
Introduced terrestrial species |
Status: Invasive species are a major threat to biodiversity and over half of all key threatening processes listed relate to invasive species. Trend: The total number of exotic species is increasing, particularly plants, so the trend for this indicator is deteriorating. Information quality: Systematic data or a framework to assist statewide monitoring is not available. What data exists is regional and inconsistent, so data quality for statewide assessment is considered poor. Statewide data on the impacts of invasive species is also limited. Response(s): The main responses are better coordination efforts across jurisdictions to target control efforts for species listed as key threatening processes, and research into more effective and target-specific control methods, such as biological control. |
Introduction
Invasive exotic pests are among the greatest threats to biodiversity throughout Australia. In NSW, they have been identified as a threat to 657 (or 70%) of the 946 species, populations and communities listed under the Threatened Species Conservation Act 1995 (TSC Act) (Figure 6.1). This is more than any other process including the destruction and disturbance of native vegetation (Coutts-Smith & Downey 2006).
Over 2800 species of exotic plants and 81 species of exotic vertebrates have established wild populations in Australia (Bomford & Hart 2002; Groves et al. 2003). The impacts of the red fox have been particularly conspicuous: the spread of foxes across Australia coincided with regional declines and extinctions of a raft of native fauna, while their persistence has caused the failure of many efforts to recover native species (Christensen 1980; Friend 1990; Short et al. 1992; Dickman 1996; Short 1998).
Invasive weeds have been estimated to cost $4 billion per year in lost production, weed control and other impacts, while estimates for the cost of invasive animals stand at $720 million (McLeod 2004; Sinden et al. 2004).
Current status and trends
Figure 6.1: Numbers of species, populations and ecological communities(a) threatened by selected terrestrial invasive species(b)
Source: Modified from Coutts-Smith & Downey 2006
Notes: (a) Threatened species, populations and ecological communities listed under the TSC Act
(b) The invasive species selected are generally those listed as key threatening processes (except lantana).
Numerous exotic plants and animals have established wild populations in NSW. Once a species becomes widespread, complete eradication is seldom, if ever, possible. Management of widely established exotic species must therefore be prioritised to focus on those areas where their impacts on biodiversity and agriculture are the greatest (Braysher 1993; Mahon 2000). In contrast, the management of recent arrivals seeks to eradicate them completely where possible. Preventing the introduction of new exotic species is the most cost-effective method of minimising the impacts of invasive species on biodiversity.
Invasive exotic plants (weeds)
Over 1350 non-native plant species are naturalised in NSW, with more than 300 of them likely to have significant impacts on the environment. In 2006, there were 286 noxious weeds on the list held by the NSW Department of Primary Industries (DPI).
Many naturalised plant species are the result of deliberate introductions. Approximately 65% of naturalised plants were imported initially for ornamental or agricultural purposes (Groves et al. 2005). This threat continues with 28% of identified weeds still available for sale in NSW (Coutts-Smith & Downey 2006) and most weeds are already well-established in gardens or agricultural land.
In NSW, exotic plants have adverse impacts on 341 species, 14 populations and 64 ecological communities listed as threatened by the TSC Act (Coutts-Smith & Downey 2006). This is 10 times greater than previously identified for the whole of Australia (Vidler 2004).
The biodiversity threatened by bitou bush and boneseed in NSW was assessed during the preparation of the threat abatement plan for this weed (DEC 2006). The study identified 158 native plant species as being threatened by bitou bush invasion.
The distribution and abundance of weed species varies over time. Control programs on the South Coast have contained the spread of bitou bush and lantana and significantly reduced their abundance in some areas. In contrast, outbreaks of some aquatic weed species, such as salvinia in the Hawkesbury–Nepean River, continue to occur because of underlying problems in the management of river systems.
Invasive exotic terrestrial vertebrates (pest animals)
Pest animals have had a devastating impact on biodiversity. Predation by feral cats and red foxes has contributed to regional declines and the extinction of a range of native species, particularly among small- to medium-sized ground-dwelling and semi-arboreal mammals, ground-nesting birds, and freshwater turtles (Dickman 1996). Overgrazing by introduced herbivores, including rabbits and goats, has had significant impacts on native vegetation and has greatly increased soil erosion. These impacts have been particularly severe in the semi-arid rangelands of Western NSW. Introduced herbivores may also have an adverse impact on native animals through competition and the degradation of habitat.
Table 6.9 summarises the main vertebrate pests in NSW and their impacts on native biota.
Table 6.9: Main terrestrial vertebrate pests and their impacts
Pest |
Main areas found |
Main impacts |
Feral cats |
Throughout NSW |
Predation of wildlife.
Evidence of impacts greatest for small mammals (< 200 grams) in open habitats and ground- or low-nesting birds, especially in western NSW. |
Red foxes |
Throughout NSW: Abundant in fragmented agricultural lands and in urban areas. Rare within larger areas of continuous forest in north-eastern NSW. Although widespread, monitoring programs as part of the Fox Threat Abatement Plan indicate that some impacts may be decreasing through targeted control programs. |
Predation of wildlife.
Evidence of impacts greatest for medium-sized (450–500 g) ground-dwelling and semi arboreal mammals, ground-nesting birds and freshwater turtles. |
Wild dogs |
Populations of wild dogs mainly occur on lands along the Great Dividing Range and the coastal hinterlands. |
Prey on a broad range of other exotic and native species, including rabbits, rodents, macropods, koalas and terrestrial wetland birds.
Interbreed with dingos with the proportion of pure dingos among wild dogs is unknown.
Balancing the conflicting objectives of dingo conservation and livestock predation by wild dogs is a major challenge. |
Feral goats |
Widespread in western NSW. Also found in moderately sized populations in escarpment areas of the Great Dividing Range. |
Suppression of native vegetation, especially shrubs and trees in western NSW, through overgrazing and land degradation.
Suppression of native herbivores, especially in rocky habitats, through competition, loss of habitats and land degradation. |
Rabbits |
Throughout NSW. Can reach very high densities in western NSW in good conditions (after rain) but abundance has declined following the release of rabbit calicivirus and ongoing outbreaks of myxomatosis. Observations indicate that rabbit populations may be beginning to increase again in some areas. |
Suppression of native vegetation, especially shrubs and trees in western NSW, through overgrazing and land degradation.
Suppression of native herbivores through competition, loss of habitats and land degradation. |
Feral pigs |
Widespread in western NSW. Also found in many parts of the tablelands and eastern coast of NSW. Populations fluctuate with seasonal conditions and are significantly reduced during droughts. |
Suppression of native vegetation, especially in riparian habitats, through land degradation. |
Wild deer |
Patchy distribution in eastern NSW |
Suppression of native vegetation and competition with native herbivores. |
Source: DPI and DEC data 2005
Most of the widespread animal pests in NSW were introduced deliberately within 100 years of European arrival for agriculture, sport or as household pets. However, a number of vertebrates introduced more recently also have the potential to expand their ranges significantly and inflict new damage to ecosystems:
- Several species of deer have established populations, mainly in eastern NSW. They have the potential to spread throughout forested areas of the East Coast and ranges.
- The cane toad is a major pest in Queensland and is established on the NSW North Coast as far south as the Clarence River near Grafton. Isolated breeding colonies have been confirmed near Brooms Head and Port Macquarie. The coastal area from Yamba to Port Macquarie is considered the most vulnerable to colonisation by cane toads.
- The red-eared slider turtle has been observed in the Hawkesbury–Nepean and several other rivers in NSW. Its future impact on aquatic ecosystems is unclear.
Invertebrate pests
Many introduced invertebrates have established in NSW, although little is known about their impacts on biodiversity. However, invertebrates have fundamental roles in ecosystem processes and thus the introduction of new species has the potential for severe consequences on entire ecosystems.
A recent invertebrate arrival is the yellow crazy ant, which was detected near Iluka on the North Coast in 2004. The ant poses a significant threat to biodiversity as it has the potential to displace native fauna and kill other invertebrates, reptiles, amphibians, hatchling birds and small mammals (O'Dowd et al. 2003; Gerlach 2004; Lester & Tavite 2004).
Introduced pathogens
The impacts of translocated native and exotic microorganisms on biodiversity are poorly understood, although they have the potential to be significant causes of disease in native organisms (Smith et al. 1999). Two important microorganisms are cinnamon fungus, which causes dieback in native trees, and the chytrid fungus, which causes skin infections and death in native amphibians. The spread of these micoorganisms is probably being assisted by human activities, through the transport of infected soils by vehicles, walking shoes and camping equipment (Nadolny 1995; NPWS 2002). 'Infection of native plants by Phytophthora cinnamomi' and 'Infection of frogs by the amphibian chytrid fungus causing the disease chytridiomycosis' are listed as key threatening processes under the TSC Act.
Response to the issue
Noxious weeds are listed by the Minister for Primary Industries through orders under the Noxious Weeds Act 1993. Pest control orders, requiring landholders to eradicate pests on their lands, are also issued by the Minister under the Rural Lands Protection Act 1998. Orders are currently in place for feral pigs, wild dogs and rabbits. The Non-Indigenous Animals Act 1987 regulates the importation, movement and keeping of exotic species in NSW.
Pest management priorities for the conservation of biodiversity will be identified in the Priorities Action Statement for threatened species, populations and communities, and for key threatening processes (see Biodiversity 6.3). Priorities are also identified in individual threat abatement plans. The Priorities Action Statement will identify future priorities for preparing threat abatement plans and ensure the efficient use of resources to manage widespread invasive species, focusing on those areas where the impacts are likely to be greatest.
These plans provide a framework for the strategic control of widespread invasive species. The threat abatement plan for predation by the red fox, for instance, identifies the threatened species at greatest risk and the sites where fox control is most critical. Undertaking collaborative fox control programs across all land tenures at these sites is central to the success of the plan. Critically, the Fox Threat Abatement Plan establishes monitoring programs to assess the response of priority threatened species to fox control measures (NPWS 2001). A plan for feral goats is also being prepared.
The first threat abatement plan for a weed in Australia (bitou bush) (DEC 2006) has been prepared, and control and monitoring programs are in place. The assessment process to identify the species at risk from bitou bush is also being used to assess the threat from other weeds, including lantana, bridal creeper and ground asparagus (Downey 2006).
Combining prevention, early detection and eradication is the most cost-effective way to minimise the impacts of new pests. Preventing the introduction of new species is managed primarily at the national level. The Australian Quarantine and Inspection Service regulates the importation of animals, plants, and animal and plant products, ensuring they are unlikely to pose a threat and are free of exotic diseases and pests.
NSW Government agencies work together to prevent the introduction of new pests into the wild and to respond rapidly when new incursions occur. The response of agencies to new pests will be coordinated through an invasive species plan for NSW.
New pest incursions have been detected regularly in NSW but targeted control programs have prevented many from becoming established. New weeds, exotic birds, invertebrates and aquatic species are the most likely organisms to establish populations in NSW. Government agencies and local councils are working collaboratively with nurseries and markets to develop guidelines and protocols for imported nursery plants.
A notable success in the control of new incursions has been preventing parthenium weed from becoming established in NSW. In Queensland parthenium infests 18 million hectares and is estimated to cost more than $20 million annually in lost production. Another eradication program commenced after the detection of yellow crazy ants at Goodwood Island near Iluka in August 2004. Initial results from the program have been promising: a detailed survey of the island in February 2006 found none of the ants. However, ongoing monitoring of the site will be required for several years.
The Invasive Animals Cooperative Research Centre (CRC) and the CRC for Australian Weed Management conduct research into more cost-effective and humane pest and weed control techniques and products. Biological control methods are of particular interest because they have the potential to deliver continent-scale control of widespread exotics. There have already been some successful programs, including myxomatosis and calicivirus (haemorrhagic) disease for rabbits, the rust Puccinia myrsiphylli for bridal creeper, and an array of insects for Paterson's curse, salvinia and water hyacinth.
The statewide natural resource management target for invasive species is 'By 2015 there is a reduction in the impact of invasive species'. There is a need for data on a statewide basis to provide a full picture of progress toward the target, although some specific monitoring programs have commenced. These include high-risk fauna responses to fox control, monitoring of vegetation communities in response to the control of bitou bush, and changes in the statewide distribution of selected weeds and pest animals.
Future directions
The preparation and implementation of threat abatement plans, such as those for the fox and bitou bush, provide a model for better strategic management of widespread invasive species to protect biodiversity. The incorporation of monitoring programs is essential to provide feedback on the effectiveness of on-ground programs.
Further research, particularly on biological controls, is required to enable broadscale control of widespread species.
With the growth in global trade, it is inevitable that new and potentially invasive species will continue to arrive in NSW. Prevention and eradication of new arrivals is critical to avoid further additions to the current array of widespread invasive species.
Response to unplanned arrivals could be improved by more clearly defining the responsibilities of relevant public authorities. It is imperative to establish a comprehensive and integrated framework across all levels of government to ensure consistency of approach in all phases in the introduction and spread of invasive species.
The preparation of AusBIOSEC, the Australian Biosecurity System for Primary Production and the Environment, is an opportunity to coordinate actions to reduce the environmental and agricultural impacts of invasive species. NSW is actively participating in the development of this strategy.
Further measures should be developed to prevent introductions of potentially invasive species by the agricultural, horticultural and aquarium industries, and the wider community. The use of native species should be encouraged wherever possible.
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