Invasive species are the most common pressure on species listed under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), affecting 82% (1,257 of 1,533) of threatened taxa in Australia in 2018. In total, 230 invasive non-native species and 37 problematic native species (207 plants, 57 animals, 3 pathogens) are listed as affecting Australian threatened taxa (Kearney et al. 2019) (Figure 33).
Invasive species are consistently identified as the most prevalent threat to Australian fauna and are a primary cause of extinction (Allek et al. 2018, Kearney et al. 2018b, Ward et al. 2021). The management of invasive species is clearly a significant economic burden in Australia (Hoffmann & Broadhurst 2016, Bradshaw et al. 2021). Given the difficulties and costliness of eradicating invasive species, one overriding priority for Australian biosecurity must be to prevent more harmful species arriving and establishing. The Land chapter has details about Australia’s border security efforts (see the Land chapter).
Problematic species can also be native; native species are listed as threats for one-fifth of threatened species. Grazing pressure from macropods (kangaroos and wallabies) is the most prevalent threat, listed for 152 threatened plant and 5 threatened animal taxa (Kearney et al. 2019). Another problematic native species is the noisy miner (Manorina melanocephala). This aggressive bird species has benefited greatly from habitat degradation because they are better able to establish and defend territories that lack structurally complex vegetation. The subsequent increase in their numbers and exclusion of other bird species from woodland remnants have resulted in them being listed as a key threatening process (Westgate et al. 2021).
Other native species become problems when introduced outside their native range in Australia. For example, redclaw crayfish (Cherax quadricarinatus), native to New Guinea, the Northern Territory and north Queensland, was introduced into Lake Poongkaliyarra, a major Pilbara water supply reservoir in Western Australia. The species has subsequently been introduced and established in other freshwater systems in the region, probably through deliberate releases from the lake to provide fishing opportunities in areas where public access is permitted. The potential impacts include the introduction of disease; habitat alterations (e.g. through macrophyte consumption); competition with, and predation on, native fishes and invertebrates; and the displacement of those species (Beatty et al. 2020).
Movement of native species and the occurrence of vagrants from nearby countries is a regular occurrence in Australia but may be increasing as a result of climate change. Increasingly, we may see vagrants as forerunners of climate adaptation – individuals best placed to found new populations beyond their previous range (Davis & Watson 2018). Recently, Bardi Jawi rangers in the Kimberley discovered a colourful pigeon, later identified as the Nicobar pigeon (Caloenas nicobarica), that is normally found on small islands and coastal regions, including the Andaman and Nicobar Islands, India, and east through the Malay Archipelago to the Solomons and Palau. It is classified as Near Threatened by the International Union for Conservation of Nature (IUCN). The captured bird was given a thorough health check and cleared of any pathogens, parasites and potential weed seeds before being transferred to Adelaide Zoo where a breeding group of Nicobar pigeons is kept and displayed (Davis & Watson 2018). The incident highlights the important role that Indigenous rangers in remote areas of northern Australia play in Australia’s biosecurity system (see case study: Indigenous involvement in biosecurity, in the Invasive species management section in the Land chapter).
Invasive plants
Almost 3,000 non-native plant species have become naturalised (grow in the wild) in Australia (Dodd et al. 2015), several hundred of which have been formally listed as ‘invasive’ plants (or ‘noxious’, ‘restricted’, ‘declared plants’ or ‘weeds’) under relevant state legislations, including 32 Weeds of National Significance. This process is continuing; an estimated 20 plant species become naturalised in Australia each year (Dodd et al. 2015).
Invasive plants (see the Introduced and invasive species section in the Land chapter) can displace native plant species, harbour pests and diseases, and transform diverse ecological communities into homogeneous monocultures of limited productivity. Invasive plants can also threaten the integrity of nationally and globally significant sites such as Ramsar-listed wetlands, cultural heritage sites and World Heritage properties. Invasive plant control is estimated to impose an overall average annual cost of nearly $5 billion across Australia, with control in agricultural areas accounting for most costs. Around $300 million in public expenditure is estimated across national parks and Indigenous lands, and on weed research (McLeod 2018).
European blackberry (Rubus fruticosus aggregate) is listed as one of the top 10 invasive species impacting threatened species in Australia (Kearney et al. 2019), and has been declared a Weed of National Significance. It reduces the floristic richness of more than 9 million hectares of waterways and depressions along the east and south-west coasts. The prickly thickets can pose a fire hazard because of the dry material contained within them, and larger animals may become trapped in them. The species also provides food for introduced species such as starlings, blackbirds and foxes, causing further spread.
Invasive grasses have been intentionally introduced into Australia since European settlement (Table 3). Some species have caused profound ecosystem impacts and important conservation challenges (van Klinken & Friedel 2018). Five particularly high-impact species primarily occur across northern Australia: gamba grass (Andropogon gayanus), para grass (Urochloa mutica), olive hymenachne (Hymenachne amplexicaulis), mission grass (Cenchrus polystachios syn. Pennisetum polystachion) and annual mission grass (Cenchrus pedicellatus syn. Pennisetum pedicellatum). Together, these grasses have been listed as a key threatening process because they can alter nutrient cycling and water availability, and subsequently cause ecosystem degradation, habitat loss and biodiversity decline. Of particular importance is their capacity to increase fuel loads, resulting in intense fires that may subsequently lead to permanent transformation of the structure of the ecosystems they invade.
A further 12 species have been identified as high-impact species based on their dominance on land managed for environmental values in Australia (van Klinken & Friedel 2018) (Table 3). A wide range of tropical, subtropical, temperate, arid and semi-arid ecosystems have been transformed by these species. Their impacts are expected to continue to increase, with most still having relatively restricted distributions compared with their potential distribution.
Invasive plant species impact how Traditional Owners use and manage their land. For example, the invasion of buffel grass (Cenchrus ciliaris) across arid Australia has impacted bush food collection and hunting, and access to traditional lands. It has also had cascading negative effects on cultural transmission to younger generations and maintaining cultural practices. Indigenous people of central Australia are now reluctant or unable to conduct traditional fire management due to the increased intensity of buffel grass fires and quick recovery of buffel grass after fires (Read et al. 2020).
Aquatic invasive plants
Invasive exotic freshwater plant species can have serious negative impacts on native communities and freshwater ecosystems. They may suppress native plant communities and have cascading effects on ecosystem functioning. A fundamental attribute altered by invasive plants is light availability, particularly when canopy-producing floating (e.g. water hyacinth – Eichhornia crassipes) or submersed species (e.g. cabomba – Cabomba caroliniana) reduce underwater penetration of sunlight. Invasive plants can also alter freshwater microclimates and water chemistry. Water movement may be disrupted by dense growth of non-native vegetation and sediment can be trapped at higher rates (Mayfield et al. 2021).
There are at least 63 freshwater plant species naturalised in Australia, many of which have been introduced through the ornamental aquarium and water garden trade (Gufu & Leishman 2018). Approximately one-third of naturalised aquatic weed species in Australia are still available for sale either by water garden nurseries or over the internet (Gufu & Leishman 2018).
More than 40% of naturalised freshwater plant species are categorised as either invasive or declared weeds, most being perennial wetland marginal plants. Even though aquatic plants make up a very small proportion of invasive plant species overall, they have widespread geographic ranges and may have disproportionally high impacts on aquatic systems. Six aquatic species are Weeds of National Significance.
Invasive animals
The most commonly cited invasive species affecting Australia’s threatened species is the European rabbit (Oryctolagus cuniculus), which threatens 21% (322) of EPBC Act–listed species (Kearney et al. 2018b) (Figure 34). The feral cat (Felis catus), feral pig (Sus scrofa) and feral goat (Capra hircus) are cited as threatening more than 100 threatened species each (see the Introduced and invasive species section in the Land chapter).
The European rabbit occurs over most of Australia south of the Tropic of Capricorn, as well as some areas in north Queensland. Rabbits affect species and ecosystems by competing with native animals for food, and through gazing, browsing and ringbarking vegetation and preventing regeneration of seedlings. The decline and extinction of many mammal species has been attributed to impacts caused by rabbits, particularly in the arid and semi-arid zones. Rabbits also support populations of pest predators such as feral cats, foxes and wild dogs.
A Korean strain of rabbit haemorrhagic disease virus, known as RHDV1 K5, was released nationally during the first week of March 2017. This was the first time in 20 years that a new rabbit biocontrol agent had been released into Australia. However, the impact of the release of RHDV1 K5 seems to have been limited by the emergence of a competing strain, RHDV2. RHDV2 is a globally widespread rabbit virus that was first detected in Australia in 2015. It has suppressed the rabbit population by an average of 60%, with impacts most pronounced in South Australia and Western Australia (Ramsey et al. 2020).
Feral cats are believed to have been a major factor in the extinction of 30 Australian native mammals species lost since European settlement (see case study: Cats are a major threat to Australia’s biodiversity). Because there is no effective broadscale control method for feral cats, they remain a major cause of decline of many Australian mammals (Legge et al. 2017) (see case study: Novel baits for feral predators). Cats have also been primary agents in the extinction of some Australian birds that were restricted to islands, such as the Macquarie Island parakeet (Cyanoramphus erythrotis) and the Macquarie Island buff-banded rail (Gallirallus philippensis macquariensis). Feral cat control has been a priority of the 2015–20 Threatened Species Strategy.
Wild dogs are also a serious predation risk to a range of native (and livestock) species. For example, dog attacks are a major contributor to koala (Phascolarctos cinereus) mortalities in peri-urban areas of north-eastern Australia (Gentle et al. 2019).
The feral pig is widely considered one of the worst invasive species throughout its introduced range, particularly in the tropical north. Feral pigs have a direct physical impact in natural landscapes as ecosystem engineers, as well as in the cultural landscape as pests. Rooting by feral pigs directly damages the ground and vegetation and impacts plant species richness; increases run-off, erosion and water quality; influences soil chemistry and fungal and microbial life; and slows regeneration. For remote and regional Indigenous communities, pig rooting can affect vehicle access to traditional lands, as well as the values of culturally important wetlands and places. Pigs can also predate on food sources such as yams, roots, tubers and turtles. In northern New South Wales, feral pigs predate on eggs and chicks of the culturally important coastal emu (Dromaius novaehollandiae), which is at risk of local extinctions (less than 50 animals) because of the small size of the population, habitat fragmentation, and inappropriate fire regimes (Heenan 2020).
Feral vertebrate herbivores and livestock (see Production systems) are an important threat to many species and ecological communities, as well as to native vegetation broadly (see Wetlands and billabongs). The problem of feral herbivore management is complex because many have social and cultural value, and some species are considered a resource by landowners, recreational and commercial hunters, and by Indigenous communities. Feral herbivore impacts on biodiversity occur through grazing and browsing of native vegetation, trampling, compaction and rooting of soils, and competition with native herbivores for food and resources. Subsequent changes to structure and composition of vegetation can open pathways for weed invasion, increase fire risks, and affect nutrient and carbon cycling.
Feral goats are a serious threat to biodiversity over large tracts of Australia’s rangelands. Methods of control are well known; however, progress in mitigating impacts has been limited, and the range of feral goats is continuing to expand in places.
Wild deer are present in every state and territory in habitats ranging from temperate forests to montane and arid woodlands, grasslands, tropical savanna and rainforest (Davis et al. 2016). The 6 wild deer species in Australia evolved in a wide range of environments, and can thus affect a wide range of natural and agricultural ecosystems. The environmental impacts of deer are not well known, which hampers the development of effective management responses. The limited ability of current control options to reduce deer populations in all but open environments means eradication is likely to be infeasible, except, perhaps, in small and isolated locations. Management of threatened flora affected by deer grazing is mainly through fencing of sites to exclude herbivores (e.g. Rathbone & Barrett 2017).
The Australian Government has recently appointed the first National Feral Deer Management Coordinator who will support community-led deer control in all states and territories and facilitate co-development of a National Feral Deer Action Plan. A similar model is in place for feral pigs, with the Australian Government providing funding to Australian Pork Limited to support a National Feral Pig Management Coordinator who will facilitate the delivery of feral pig management approaches nationally, regionally and locally scale; undertake stakeholder engagement; and raise awareness of feral pig issues. The coordinator is also leading the development and implementation of the National Feral Pig Action Plan.
Threat abatement plans under the EPBC Act are in place for key threatening processes (see Key threatening processes) arising from feral cats, the European red fox, unmanaged goats, feral rabbits, feral pigs, cane toads, and exotic rodents on offshore islands. A National Wild Dog Action Plan is in place for 2020–30 to provide a nationally agreed framework that promotes and supports a strategic and risk-based approach to wild dog management.
Invasive invertebrates
A large proportion of invasive fauna globally are invasive invertebrates. Border interception records show that the vast majority of interceptions are for invasive invertebrates (see the Introduced and invasive species section in the Land chapter). In 2017 and 2018, the National Border Surveillance program detected 42 pests and diseases of environmental concern; most of these were snails (23.8%) and ants (16.6%), and all but 7 were invertebrates. However, invasive invertebrates tend to receive less attention compared with plants and vertebrates when considering their impacts on natural ecosystems and biodiversity.
Exotic invasive ants, also called tramp ants, are some of the world’s most invasive pests because of their devastating environmental, economic and social impacts. Between 2001 and 2017 in Australia, 20 serious tramp ant incursions occurred, including 16 incursions of red imported fire ants (Solenopsis invicta) (Australian Government Inspector-General of Biosecurity 2019). Red imported fire ants pose a risk to human health because they are aggressive, delivering repeated painful stings that can cause anaphylactic shock. They prey on vertebrates, invertebrates and plants, and compete for food with native herbivores and insects.
The yellow crazy ant (Anoplolepis gracilipes) is listed as one of the top 100 worst invasive species by the IUCN and the Global Invasive Species Database. First discovered in Australia in Cairns in 2001, the ants have been found at more than 20 sites in Queensland and in a large, scattered population in Arnhem Land in the Northern Territory. On Christmas Island, yellow crazy ants have killed millions of red land crabs (Gecarcoidea natalis) and robber crabs (Birgus latro), both of which play an important role in Christmas Island’s forest floor ecology.
The giant African land snail (Achatina fulica) is considered one of the most significant snail biosecurity threats to Australia. This snail can grow to 15 cm or more in length and has a broad diet, with the potential to feed on hundreds of plant species. The species is not currently present on mainland Australia, but has invaded Christmas Island where it occurs in high densities in rainforest. So far, it does not appear to have had major impacts on Christmas Island, feeding mostly on waste or debris material. However, Christmas Island is perhaps an unusual environment in that other land snails are absent and the interactions between the native red land crab and the invasive yellow crazy ant shape the forest understorey and structure, and litter decomposition (O’Loughlin & Green 2017). Introduction of the species to the mainland could lead to very significant impacts on natural and agricultural systems.
Aquatic invasive animals
At least 34 exotic freshwater fish species have established in Australia: 22 from ornamental and aquarium releases, 8 from acclimatisation, 2 from ballast water, 1 aquaculture species (carp – Cyprinus carpio) and 1 failed biocontrol (eastern gambusia – Gambusia holbrooki) (Lintermans 2004) (see the Aquatic invasive species section in the Coasts chapter).
Of the invasive species known to affect Australia’s 22 most at-risk native fish species (see Fish and aquatic species), invasive trout (Salmo trutta, Oncorhynchus mykiss) pose the greatest threat, followed by eastern gambusia, tilapia (Oreochromis mossambicus and Pelmatolapia mariae), eastern rainbowfish (Melanotaenia splendida) and sooty grunter (Hephaestus fuliginosus) (Lintermans et al. 2020). Trout are particularly difficult to manage because they are now widespread in cool freshwater streams in south-eastern Australia, and are a popular recreational fishing target.
Common carp are a major threat to aquatic biodiversity in Australia. They occupy most aquatic habitats, from estuarine lakes to upland streams, and are at their highest densities in south-eastern Australia. When carp exceed a density-impact threshold of 80–100 kilograms per hectare (kg/ha) in wetlands and lakes, increased water turbidity caused by the carp begins to affect aquatic vegetation, native fish and invertebrates. Carp densities in Australia are commonly 200–400 kg/ha and can exceed 1,800 kg/ha in some shallow lakes, far surpassing the density-impact threshold (Figure 36) (Stuart et al. 2021).
The Australian Government has committed $15.2 million towards the development and implementation of the National Carp Control Plan. The plan is being developed to determine the feasibility of using cyprinid herpes virus 3 (the carp virus) as a biological control agent, as part of an integrated landscape-scale control effort to reduce carp below density-impact thresholds and assist ecosystem recovery (McColl & Sunarto 2020).
Animal diseases
Diseases, fungi and parasites can affect the health of native plant and animal species, reducing their ability to reproduce or survive. Wildlife Health Australia lists 41 infectious diseases in Australian animals that carry biosecurity concerns and may have significant impacts on native species or significant zoonotic (i.e. animal–human) impacts. These include 13 viruses, 12 bacteria, 6 fungi, 6 protozoa, 2 internal parasites and 2 external parasites (Wildlife Health Australia (2018)) (see the Introduced and invasive species section in the Land chapter) (see the Marine ecosystem processes section in the Marine chapter).
Disease in fauna can contribute to the decline and extinction of threatened species. For example, chlamydia continues to be one of the main factors threatening the long-term survival of the Vulnerable koala, sometimes causing blindness or urinary tract infections that may lead to infertility (Nyari et al. 2017).
The population of the Endangered Tasmanian devil (Sarcophilus harrisii) has declined by up to 80% since the mid-1990s when the infectious and usually fatal cancer devil facial tumour was first detected. Devil facial tumour has now spread across 80% of Tasmania (Lazenby et al. 2018). Methods for securing healthy populations of devils in the wild either involve translocations of healthy devils to islands, or isolating populations behind barriers. In addition, the Tasmanian Government has established an ‘insurance population’ of Tasmanian devils free from devil facial tumour. This population now contains more than 700 animals housed in 44 institutions throughout Australia, America, Europe and New Zealand (Woods et al. 2018).
Psittacine beak and feather disease is the most common and highly infectious viral disease among parrots. It is found Australia-wide, infecting wild and captive parrots. The disease was listed as a key threatening process in 2001, and experts consider it to be a significant threat to the orange-bellied parrot (Neophema chrysogaster; Critically Endangered), glossy black cockatoo (Calyptorhynchus lathami halmaturinus; Endangered – South Australia), Norfolk Island green parrot (Cyanoramphus cookii; Endangered) and the western ground parrot (Pezoporus wallicus flaviventris; Critically Endangered), and a low to moderate threat to at least 7 other threatened species.
One of the most devastating diseases affecting fauna over recent decades is amphibian chytridiomycosis, caused by the fungal skin pathogen Batrachochytrium dendrobatidis (chytrid fungus). Chytrid-associated declines of Australian amphibians were first observed in Australia in 1979 and continue today; at least 36 species (of Australia’s 238 amphibians) have declined and 7 have become extinct due to the disease. A further 3 species are considered at risk of future declines associated with the continued spread of the pathogen.
Hot and dry environmental conditions appear to limit the distribution of the chytrid fungus throughout much of Australia, largely restricting its impact to the cooler, wetter, mountainous areas of eastern Australia. Species declines have been restricted to eastern Australia, across upland tropical, subtropical and temperate regions (Figure 37). They have been most severe in species in high elevation areas where precipitation is high, and temperatures are moderate to cold. Eight species appear to be recovering, possibly as a result of environmental conditions that have limited chytrid fungal growth (Scheele et al. 2017).
Very little progress has been made on mitigating the impact of chytridiomycosis and stabilising declining species in the wild. As a result, conservation programs to maintain captive populations have been established at several Australian zoos and wildlife parks, targeting a variety of threatened amphibian species (see Ex situ conservation).
Plant pathogens
A large number of plant pathogens occur in Australia. Recent research shows that 117 forest pathogens occur throughout Australia, of which nearly three-quarters are in the phylum Ascomycota (commonly known as sac fungi), including leaf spot, canker and blight (Nahrung & Carnegie 2020). Most of these plant pathogens affect exotic forestry species (e.g. pine, Populus spp.), although about half also have native hosts. Myrtle rust (Austropuccinia psidii) and the root-rot fungus Phytophthora cinnamomi are the 2 plant pathogens with the most significant impacts on native plant species.
Myrtle rust is an invasive species of rust fungus that affects young growing tissues of a wide range of species in the Myrtaceae family, including iconic Australian genera such as Eucalyptus, Corymbia, Melaleuca and Leptospermum. Suitable habitat for myrtle rust includes coastal areas of New South Wales, the Northern Territory, Queensland, Tasmania and Victoria. At least 417 species (or subspecies) have been documented as susceptible to myrtle rust, and at least 1,285 species of Myrtaceae occur in climatically suitable areas for myrtle rust (Berthon et al. 2018).
Although the national-scale impacts of myrtle rust are still not fully understood, myrtle rust has caused dramatic declines in 2 once-common Australian species (native guava – Rhodomyrtus psidioides, and scrub stringybark – Rhodamnia rubescens). Both species have subsequently been listed as Critically Endangered under the EPBC Act; native guava is at very high risk of extinction, with only 1 surviving population – and it does not produce viable seed (Fensham et al. 2020a).
As with the extinction of any species, the loss of native guava from rainforest edges where it was formerly common could have significant long-term effects on the composition and structure of ecosystems (Fernandez-Winzer et al. 2020), and subsequently on ecosystem function. For example, the invasive species Lantana camara occupies the same habitat, and replacement of native guava by lantana will increase the fire hazard at rainforest margins during drought. In addition, more than 100 species of insect-feeding pollinators have been associated with native guava; these interactions between plant, pollinators and insects will be significantly disrupted with unknown, longer-term consequences (Fensham et al. 2020a).
The threat from P. cinnamomi, a soilborne water mould that destroys the roots of affected plants, is well documented across a range of ecosystems. This root-rot fungus is listed as threatening more than 100 EPBC Act–listed species (Kearney et al. 2019). In the Eastern Stirling Range Montane Heath Community in Western Australia, numerous species found nowhere else (endemic taxa) are threatened with extinction due to P. cinnamomi, and, as a result, this community has been assessed as Critically Endangered under the IUCN criteria (Barrett & Yates 2015). P. cinnamomi has also been documented in forests and heathlands in New South Wales, Tasmania and Victoria.
Invasive species and diseases are some of the highest pressures on Australia’s biodiversity, and these pressures look set to continue and increase in the future.
Related to United Nations Sustainable Development Goal targets 15.2, 15.5, 15.8
Invasive species impose a persistent, extensive and sometimes irreversible impact on native species and ecosystems. They are the most cited threat to threatened species, have contributed to a significant number of extinctions of Australian endemic species and dominate national key threatening processes. There is no evidence that the impact of invasive species will lessen in the future.
The impacts from diseases and pathogens are contained to specific species in specific areas and are not yet pervasive across the entire continent. However, a small number of species and ecosystems experience very high impacts from diseases and pathogens over their entire range; some are only being rescued by methods of last resort such as relocation and establishment of insurance populations. There is high potential for ongoing future impacts from new or existing diseases and pathogens.