Introduced and invasive species

Overall status and trends

Thousands of species have been introduced either deliberately or inadvertently into Australia and become established. Introduced species include agricultural, horticultural and forestry plants and crops, and domestic animals that benefit our economy or the livability of our environment. Many of these species have also established feral populations separate from managed domestic populations. Many introduced species are impactful invasive non-native species (i.e. animals, plants, fungi and microorganisms that have been introduced and/or spread outside their natural past or present distribution) that pose huge problems for some commercial sectors, and threaten biological diversity (sCBD 2010). Invasive species that are non-native to Australia or a region also severely impact Indigenous people’s connection to Country, through their direct effect on cultural knowledge and practices (Ens et al. 2015).

A global study by Seebens et al. (2017) found that one-third of all introductions in the past 250 years occurred after 1970, and the rate of introductions is showing no sign of slowing down. Many naturalised noxious weeds or weeds of significance in Australia were deliberately introduced as ornamentals or due to perceived commercial value at the time (e.g. Cook & Dias 2006, Cook 2008, Setterfield et al. 2018). These non-native species may become even more impactful as they benefit from changing climatic regimes and establishment opportunities following bushfires and other disturbances, such as widening of transport routes and clearing for powerlines and firebreaks (e.g. Pickering & Hill 2007, Laurance et al. 2009). A subsequent study by Seebens et al. (2021) predicted that the number of established non-native species was likely to increase by 36% between 2005 and 2050, with particularly high increases for invertebrate taxa. Eastern Australia is among 5 global regions considered most vulnerable to establishment of new invasive species originating mainly from Asia and America via people, trade and tourism (Bellard et al. 2016).

Biosecurity is Australia’s first line of defence in preventing species and disease introductions (see Management of specific pressures). From 2012 to 2017, the number of interceptions of biosecurity risk materials at Australian borders increased by almost 50% to 37,014 per year (CSIRO 2020). Despite these interceptions, the cumulative burden of new and existing incursions is likely to continue to escalate in future years (Figure 44). As at 2018, more than 18,000 vessels, 1.8 million sea cargo consignments, 41 million air cargo consignments, 152 million international mail items and 21 million passengers arrive in Australia annually, and numbers are growing every year (Inspector-General of Biosecurity 2019a). Following restrictions introduced in early 2020 to curb the spread of COVID-19 in Australia, international air passenger arrivals declined by 98%, but freight decreased only by 23% and shipping was much less affected, with some Australian ports now exceeding 2019 trade volumes (Infrastructure and Transport Ministers 2020).

While many biosecurity risks are intercepted at the border, some exotic pests and diseases are found only after entering Australia. For example, plant pest incursions represented more than two-thirds of reported incidents by 2016 (Table 7). However, the rate of new weed detections is in decline, and the introduction rates for some groups, most notably invertebrates, is also on the decline in our region compared with other parts of the world (Seebens et al. 2021), providing indirect evidence of the likely effectiveness of Australia’s biosecurity system.

A National Priority List of Exotic Environmental Pests, Weeds and Diseases (abbreviated as the Exotic Environmental Pest List, EEPL) has been developed to help prevent entry, establishment and spread of exotic pests, weeds and diseases (ABARES 2021e). A subset of 42 species from the EEPL pose the greatest risk to Australia’s environmental biosecurity (ABARES 2020b, ABARES 2021e), of which 32 are high-risk species (Table 8) (see Invasive species management).

The International Union for Conservation of Nature (IUCN) launched the Global Register of Introduced and Invasive Species (GRIIS) in 2018 to support national government responses to biological invasions (Pagad et al. 2018). The GRIIS database provides harmonised, open-source, multi-taxon information about the presence of introduced and invasive species for most of the world’s countries, including Australia, across categories of impacts. Species verified as non-native to a country are assessed for evidence of impact and may be further designated as ‘invasive’. This information, combined with Australian occurrence records from the Atlas of Living Australia, provide a national framework for reporting on the status and trends of introduced species.

There are 2,383 introduced species with at least one occurrence record in the Atlas of Living Australia based on observations up to and including 2020 (Table 9). The actual number of introduced species in Australia is expected to be much higher, because many occurrence datasets are not publicly available due to sensitivities, or are not aggregated in one place. For example, there are known to be more introduced plant species (27,500) in Australia than native species, of which around 2,800 have naturalised (Groves et al. 2005). As a general rule, around 10% of those that have naturalised in the wild are thought to become impactful invasive species (Williamson & Fitter 1996).

Using 1980 as a baseline for the total number of species recorded at least once since 1901, anywhere in Australia and remote territories, the aggregated public data record shows that the number of introduced species increased by more than 17% (i.e. 368 additional species) between 1980 and 2020 (Figure 45). Over this same period, the number of invasive non-native species increased by 24% (i.e. 26 additional species). The species in the ‘other introduced’ category include agricultural crops, and ornamental and garden plants for which there is presently no documented evidence of their environmental impact or expert anecdotes of invasiveness (e.g. widespread, spreading rapidly or present in high abundance).

As expected, the numbers of introduced species are higher where more land is intensively used (Figure 46); however, by 2020, the total number of introduced species recorded since 1980 increased at a faster rate in the extensive land-use zone (63% more) compared with the intensive land-use zone (18% increase) (EcoAssets 2021). This may be due to range expansions following increases in land use in the extensive land-use zone over successive decades, and also greater survey effort. New species introductions recorded in the relatively natural zone increased steadily over the same period. Considering only the trajectory of invasive non-native species, by 2020 the rate of new species added to the public data record since 1980 appeared to be plateauing in the intensive land-use zone (increasing by 26%, 26 additional species), but continued to climb in the extensive land-use and relatively natural zones (increasing by 83% and 121%, respectively).

These patterns are repeated for introductions of both vertebrate animals and plants (Figure 47). The number of introduced plant species increased at almost double the rate of introduced vertebrate animal species in the extensive land-use zone (63% more plant species in 2020 than in 1980 compared with 39% more vertebrate animal species) and relatively natural zone (72% more plants species versus 39% more vertebrate animal species). This trend was less pronounced in the intensive land-use zone, with 17% more plant species in 2020 than in 1980 compared with 20% more vertebrate animal species (Figure 47). These results are not surprising given that there are thousands more plant than vertebrate non-native species outside Australia, many of which could potentially become established within Australia (Pagad et al. 2018, Pyšek et al. 2020).

Introduced species now make up a significant proportion of all species recorded in Australia’s bioregions (Figure 48). The bioregional numbers of introduced species recorded at least once over the past 120 years to 2020, and the subset of invasive non-native species, are shown in Figure 49. The bioregions with the highest number of introduced species include the intensive agricultural land surrounding the major cities, such as the Sydney Basin, South Eastern Queensland, Flinders Lofty Block and Victorian Midlands bioregions (Table 10).

For more detail on insects, see case study: Invasive insects. The Biodiversity chapter has more detail on invasive plants, animals, animal diseases and plant pathogens (see the Biodiversity chapter).

Overall impacts

Most of the environmental impacts on threatened species in Australia are caused by 267 invasive species, of which 230 are non-native to Australia (187 plants, 41 animals and 2 pathogens). Several invasive native species (20 animals, 16 plants and 1 pathogen) have been introduced into regions outside their native range and become problematic (Kearney et al. 2019) (see the Biodiversity chapter). For example, around 20 introduced plant species grown in urban parks and gardens become naturalised in Australia each year, including native plants grown outside their native range (Dodd et al. 2015). In 2001, Parsons & Cuthbertson (2001) detailed 200 impactful (i.e. invasive) weeds in Australia. Applying the 10% rule (Williamson & Fitter 1996) over the 20 years since that publication, there may be around 240 impactful weeds now in Australia (although only 80 such species have occurrence records in the Atlas of Living Australia; Table 9). The number of non-native species in Australia has increased linearly over recent years (Figure 45), compared with most parts of the world where introductions are still increasing exponentially (Seebens et al. (2017). Far less is known about the variety of non-native invertebrate animals in Australia until they start to become a problem (see case study: Invasive insects).

Based on incomplete Australian datasets, Bradshaw et al. (2021) estimated that since the 1960s, Australia has spent or incurred losses totalling at least $389.59 billion (2017 value) from invasive species. The costliest weeds were annual ryegrass (Lolium rigidum), parthenium (Parthenium hysterophorus) and ragwort (Senecio jacobaea), and the costliest animals were cats (Felis catus), rabbits (Oryctolagus cuniculus) and red imported fire ants (Solenopsis invicta). The authors concluded that with better investment, standardised assessments and reporting, and coordinated interventions (including eradications), some of these costs could be substantially reduced. With more complete data, wild radish (Raphanus raphanistrum), which is one of the most widespread and competitive broadleaf weeds of cropping regions (GRDC 2021), is likely to be closer to the top of the list from a loss standpoint (McLeod 2018). For example, the total annual cost of weeds (revenue loss plus expenditure) to Australian grain growers has been estimated at $3.3 billion (Llewellyn et al. 2016), and across all grain, beef and wool industries at nearly $5 billion annually (McLeod 2018). However, impacts due to reductions in value of natural capital assets and associated impacts on human health and wellbeing have been much harder to quantify in monetary terms. In a study of the cost of threat abatement strategies to reduce extinction risk of species listed in the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act), Watson et al. (2021) found that controlling weeds required the most financial resources, making up 69% of the total estimated costs. Invasive species and disease were the most prevalent of threats identified to have high or medium impact (Ward et al. 2021).

In semi-arid and arid regions of Australia, the main invasive grass of environmental concern is buffel grass (Cenchrus ciliaris), which is contentious to manage because it is valued by pastoralists as fodder (Grice et al. 2012, Marshall et al. 2012, Scott et al. 2018). Buffel grass transforms ecology, alters fire regimes, and changes the vegetation density and character of Australia’s arid landscapes (Cook & Grice 2013, Grice et al. 2013, Read et al. 2020). It has replaced the distinctive Triodia hummocks over extensive areas in some regions, and has been called ‘devil grass’ by some senior Indigenous women due to its detrimental impact on wildlife and culture (Read et al. 2020). Buffel grass is predicted to expand its range further south with shifts in arid zone climates over coming decades (Martin et al. 2015).

New plant and animal diseases caused by introduced pathogens are a particular concern for the agricultural and ornamental industries, and can also seriously impact native plants and animals. Under the EPBC Act, the key threatening process ‘Novel biota and their impact on biodiversity’, which was listed in 2013, acts as an umbrella for impactful wildlife diseases (see the Biodiversity chapter). On the EEPL, under the Biosecurity Act 2015, exotic diseases of environmental and social amenity concern are also included (ABARES 2021e). The 2 systems of regulation to address invasive species management are interconnected (see Invasive species management). For example, white-nose syndrome, a fungal disease causing severe population declines in North American bats, could arrive in Australia within the next 10 years and pose a significant threat to wildlife (Turbill & Welbergen 2020). Australia has systems in place through the National Environmental Biosecurity Response Agreement to manage an environmental biosecurity incursion (DAWE 2019, DAWE 2020a) The COVID-19 pandemic is an example of a severe biosecurity event impacting human and animal health, including wildlife (Kiros et al. 2020). The increased incidence of new infectious diseases emerging over the past 50 years signals the need to better integrate human health outcomes with sustainable development (Jones et al. 2008, Rohr et al. 2019, Di Marco et al. 2020).

In a first for Australia, the New South Wales Biodiversity Indicator Program developed and reported on 2 indicators related to invasive non-native species: the degree of exposure, and impact (Froese et al. 2021). The program listed 413 species (360 weeds, 49 pest animals and 4 diseases) that had been identified as threats to native biodiversity and ecosystem quality in New South Wales in at least one of several authoritative sources. Of these 413 species, 83% had become naturalised in at least one population in New South Wales (305 weeds, 36 pest animals and 3 diseases). Experts assessed impacts of 15 weeds and 7 pest animals on local biodiversity. Weeds were found to be more likely than pest animals to cause local extinction of at least one local population of a threatened species, or at least one characteristic species in at least one remnant patch of a threatened ecological community (Froese et al. 2021). The impact assessment was modelled on the IUCN standard for the classification of the impact of non-native species on the environment (IUCN 2020).