Deliberate introduction Deliberate or intentional introduction refers to the purposeful movement by humans of a species outside its natural and dispersal range. This group includes species that are introduced for use as crops, ornamentals, agricultural practices, biological controls or as game species. Some examples of the pathways for deliberate introductions are provided below.
Plants introduced for agricultural, horticultural or forestry purposes Many new plants have been introduced into Australia for use as agricultural plants and form the basis of the successful cropping industry. Most introduced crop species that remain domesticated are non-invasive and do not pose a threat to biodiversity, however there are some examples (e.g. Brassica napus) of species that have been introduced for agricultural purposes that have escaped into the environment and pose a threat to native species. A key example is the introduction of gamba grass (Andropogon gayanus) into the Northern Territory in 1987 as a feed source for cattle. Gamba grass is a large, perennial, highly productive introduced grass which forms tall (to 4 m), dense stands (Csurhes, 2005). These stands cure later than native annual grasses and provide increased fuel loads which promote intense, late, dry season fires.
Gamba grass was first introduced into Australia in 1931 by the CSIRO Division of Land Research but was not widely released as a pasture grass until 1983 when commercial quantities of the seeds became available (Csurhes, 2005). The spread of gamba grass has been accelerated by transportation of seed in hay, road maintenance activities and a succession of above average wet seasons during the mid to late 1990s. The success of gamba grass as an invader can be attributed to its ability to colonise a range of habitats from wetland margins and riparian corridors through to open woodlands and more closed forests (Flores, 1999; Flores et al., 2005).
Gamba grass invasion can increase fire frequency as land managers need to burn more frequently to reduce the large fuel loads. It also has the potential to support fires more than once per year due to its high tolerance to fire and its ability to resprout shortly after fire (Bowden, 1963). Frequent, intense, late dry-season fires have a detrimental effect on native habitats and ecosystems. Fires in gamba grass-dominated ecosystems produce flame heights that are higher than typical fires, causing canopy fires which affect the whole tree. Research has shown that these fires can lead to a 50% reduction in tree cover over 12 years (Ferdinands et al., 2006). The increased fire risk due to gamba grass has also resulted in significant economic costs associated with having fire fighters permanently stationed ready for wildfire due to gamba grass supporting fires 5–10 times more intense than those fuelled by native grasses (Setterfield, 2009).
Across the world many Pinus species have been introduced outside their natural range as important plantation timber species, many of which have become invasive (Richardson et al., 1994). In Australia, Pinus radiata is considered invasive and can invade outside of the plantation forests into threatened ecological communities including grassland and shrublands (Lindenmayer and McCarthy, 2001; Williams and Wardle, 2007; Haby et al., 2010).
Soil remediation Tall wheat grass (Lophopyrum ponticum) is a perennial grass growing to 2.2 m high. Native to the Balkans, Black Sea, Asia Minor and southern Russia, tall wheat grass grows naturally in very salty soil. It was first introduced to Australia in 1935 for reclamation of saline soils and during the last decade has been widely planted on saline soils as a pasture species for domestic livestock (Booth et al., 2009). This has resulted in a very high propagule pressure, particularly in western Victoria. Tall wheat grass has now become a serious invasive grass in temperate Australia (Booth et al., 2009).It is a serious threat for saline and subsaline wetlands, including numerous Ramsar-listed sites (Carr et al., 1992), and also threatens estuaries, riparian environments and grasslands in south eastern Australia. Tall wheat grass can change the composition of ecological communities by out-competing and eliminating native plant species and preventing recruitment of all species; it also degrades faunal habitats and alters ecosystem functions associated with fire and potentially also with hydrology and nutrient cycling (Booth et al., 2009).
Medicinal and herbal introductions Plant species introduced into Australia for medicinal and herbal purposes have escaped and become invasive weeds. St John's wort (Hypericum perforatum) is a perennial herb native to Europe, Asia and North America (Briese et al., 2000). It was introduced into Australia for medicinal and ornamental purposes in the late 19th century and planted into gardens, from where it has escaped (Briese et al., 2000). St John’s wort crowds out native species and reduces habitat. It invades grassland, Eucalyptus and Callitris woodlands and is often found along river banks, travelling stock routes, water catchment reserves and roadsides (Briese et al., 2000).
Commonwealth Plant Introduction Scheme Historically in Australia a number of plant introduction schemes have operated (Cook, 2008) in an attempt to introduce plants to improve Australian agriculture, increase pastoral productivity and maximise Australian human population density (Cook and Dias, 2006). One such scheme, the Commonwealth Plant Introduction (CPI) Scheme (1930s-2000) introduced 145 000 accessions of greater than 8 200 species in 70 years (Cook and Dias, 2006). During the 20th century the introductions by the CPI Scheme facilitated a continent-wide research program that, in an attempt to improve the landscape, also greatly transformed it (Cook and Dias, 2006). While some of the introductions were beneficial to Australian agriculture, many such as Nassella spp., Acacia spp. and Tamarix spp. became weeds that have had a detrimental effect on the environment (Cook and Dias, 2006).
Aquarium plants The introduction of imported aquarium plants into Australian waters, either by accident or intentional release of species into the environment by aquarium owners, or by the intentional release of plants so they can spread and be harvested for the aquarium trade, has serious impacts for the environment. Cabomba (Cabomba caroliniana) was first recorded in Australia in 1967 after introduction from South America as an aquarium plant (Mackey, 1996). Plant fragments found their way from aquaria into open water sources, possibly through the dumping of aquarium water. The weed has spread widely, forming dense underwater thickets that result in clogging of waterways, reduced fish stocks, and reduced native plant species (WoNS, 2009). Cabomba is listed as a Weed of National Significance (WoNS), one of the 20 worst weeds in Australia.
Water hyacinth (Eichhornia crassipes) was introduced to Australia in the 1890s from South America as a decorative plant for fish ponds and aquaria (Calvert, 1998). Once introduced into local waterways it spread rapidly. It can block waterways, deeply shading the water, out-competing native plants and starving the water of oxygen. As a result, native species die from lack of sunlight, and fish suffocate.
The invasive seaweed caulerpa(Caulerpa taxifolia), a native to tropical and sub-tropical regions (Phillips and Price, 2002), was first identified in New South Wales waters in 2000 (Creese et al., 2004, p 110). The invasive caulerpa identified around the world and in Australia has been called the ‘aquarium strain’ as it is thought to have been a selectively bred clone for marine aquaria. This strain of caulerpa is invasive in temperate waters as it tolerates cold water, unlike tropical species (Glasby and Gibson, 2007). In areas where it has been introduced it reproduces rapidly and can cover large areas, out-competing species such as native seagrasses (I & I NSW, 2009). Additionally, all species of caulerpa produce toxic substances resulting in their avoidance by grazing species such as sea slugs, which might otherwise control them (I & I NSW, 2009).
Ornamental plants Bridal creeper (Asparagus asparagoides) was introduced from South Africa in the 1800s as an ornamental plant. This vine was used widely in gardens and for floral arrangements. Bridal creeper is highly invasive and can be found impacting native vegetation in temperate and Mediterranean climate areas across Australia (WoNS, 2009). The plant spreads aggressively and can climb up trees three metres in height, smothering native vegetation. The vine also changes the chemical composition of the soil, inhibiting germination of native species and allowing other introduced weeds to invade the area (WoNS, 2009). The seeds are also spread to new locations by birds, foxes and rabbits, which forage on the abundant red berries.
Scotch broom (Cytisus scoparius) is thought to have been introduced to Australia from Europe in the 1800s, possibly as an ornament plant. It escaped from gardens and is now a major woody weed of temperate areas (Parsons and Cuthbertson, 2001b, pp 470-471). Scotch broom has spring loaded seed pods that spread seeds widely. In local regions where it establishes it takes over large areas. Scotch broom fixes nitrogen, and smothers and out-competes native plants while providing habitat for invasive vertebrate species such as foxes, rabbits and wild pigs (Parsons and Cuthbertson, 2001b, p 107).
African olive (Olea europaea L. subsp. cuspidate) was introduced into Australia as a hedging plant from southern Africa in the early-mid 19th century and is now a well established invasive woody weed in New South Wales, particularly in the western Sydney and Hunter Valley regions (New South Wales Scientific Committee, 2010). African olive is long lived and forms dense canopies that shade the ground and prevent native shrubs and grasses from germinating. The subspecies is adapted to a wide range of soils and produces heavy crops of black fruit every year that are readily dispersed by birds. In 2010 ‘The Invasion of Native Plant Communities by African Olive Olea europaea L. subsp. cuspidata’’ was listed as a KTP under New South Wales legislation (New South Wales Scientific Committee, 2010).
Prickly pear (Opuntia stricta), widely considered to be the worst invasive weed to be introduced to Australia, was first thought to have been introduced from the United States of America as an ornamental plant. Records from the early 1800s show specimens being grown in gardens in Scone, NSW, and Warwick, Qld, as it was considered to be a good fruiting hedge (Tanner, 2009). Prickly pear was thought to be a good standby feed for stock during drought and was planted in paddocks. As the popularity of the prickly pear spread, it was transported around the country. With the suitable climate and lack of natural controls the prickly pear population exploded. In 1880 it was recognised as a major weed and legislation was introduced to control it (Freeman, 1992). By the early 1900s prickly pear had taken over many parts of the landscape and people were even encouraged to kill native species, such as emus and crows that feed on the fruit, to stop the spread (Tanner, 2009). It was not until 1926 that the solution to the problem was found with the introduction of a biological control, the cactoblastis moth (Cactoblastis cactorum), whose caterpillars devour prickly pear (Freeman, 1992). Cactoblastis was an overwhelming success and is recorded as one of the greatest biological control success stories in the world.
Mammals and birds released for hunting purposes European rabbit(Oryctolagus cuniculus), red fox (Vulpes vulpes) and six species of deer (Axis axis, A. porcinus, Cervus elaphus,C. timorensis, C. unicolour and Dama dama) were introduced into Australia during the 1800s by Acclimatisation Societies for hunting (Frith, 1973, pp 162-173; Moriarty, 2004). Deer are now widely spread throughout Australia and occur in habitat that ranges from rainforest to arid woodland (Moriarty, 2004). In 2004 it was estimated there were 200 000 wild deer in Australia (Moriarty, 2004). Deer are impacting on the natural environment and native species by trampling and destroying plants, increasing grazing pressure and ring-barking young trees. Deer also foul waterholes, cause soil erosion and assist the spread of weeds (NSW DECCW, 2004).
Pheasants, quail, grouse, partridges and guinea fowl were introduced to Australia to address the shortage of traditional game birds (Frith, 1973, pp 182–184). Some species established wild populations however, unlike mammals, numbers decreased over time to small populations (Frith 1973, pp 182–184).
Mammals introduced for agricultural purposes Many mammal species have been introduced into Australia as domestic livestock for agricultural purposes and have escaped or been released into the environment and established wild populations. These species include cattle, buffalo, pigs, goats, camels, horses, donkeys and ostrich.
Pigs (Sus scrofa) were brought to Australia on the first fleet for agricultural purposes for the colony but escaped into the wild, establishing in all non-arid regions in Australia (Norris et al., 2005). Feral pigs cause significant damage to the environment and adversely affect rare flora and fauna including turtle eggs, insects, frogs, geckos, birds and fish fingerlings (Norris et al., 2005). They spread weeds, destroy vegetation and disturb the ground while feeding, causing soil erosion, altered drainage and habitat. Pigs wallow in water holes, fouling them and making them unsuitable for native species. Pigs have no natural predators and breed rapidly, out-competing native species for habitat and food.
Biological Control Thousands of cane toads (Rhinella marinus) (formerly known as Bufo marinus), a native of central and South America, were released in Queensland in the 1930s to control French's cane beetle (Lepidiota frenchi) and the greyback cane beetle (Dermolepida albohirtum). The beetle larvae eat the roots of sugar cane and kill or stunt plants, however cane toads proved to be unsuccessful in controlling cane beetles (DEWHA, 2005a). Since its introduction to Australia the cane toad has spread south and west across the continent and now occurs in Queensland, the Northern Territory, Western Australia and New South Wales (DEWHA, 2005a). Cane toads eat a wide variety of prey, breed opportunistically, have a far greater fecundity than native species and develop rapidly, particularly where there are warmer waters. They are considered to be an extreme generalist with a tolerance for a broad range of environmental and climatic conditions and are able to potentially occupy many habitats (DEWHA, 2005a). There is considerable concern over the impact of the cane toad on native species and, in particular, invertebrate communities, through predation and competition. Most significantly, they possess highly toxic chemical predator defences and many scientific and anecdotal reports exist of deaths of native predators that have attempted to consume cane toads (DEWHA, 2005a).
The plague minnow or gambusia (Gambusiaholbrooki) was introduced into the Sydney Royal Botanic Gardens from the United States of America in 1925, as part of a biological control program for mosquitoes (Pyke and White, 2000; NSW Parks and Wildlife Service, 2003). The species is now widely dispersed and occurs in most freshwater habitats in south-eastern Australia, and parts of Queensland, the Northern Territory and Western Australia. Gambusia have been extremely successful invaders; they disperse over a wide area and can occupy diverse habitats and environmental conditions (NSW Parks and Wildlife Service, 2003). Internationally gambusia has been ineffective at controlling mosquitoes and its introduction has had a harmful impact on native fish worldwide. Gambusia impact on native species including frogs, fish and macroinvertebrates. They prey on some native frog species, particularly the nationally listed endangered green and gold bell frog (Litoria aurea), and are thought to be a contributing factor in the decline of this species in New South Wales (Pyke and White, 2000). There is no known effective method of control for gambusia and once introduced to an area they are almost impossible to eradicate, particularly in connected waterways (NSW Parks and Wildlife Service, 2003). Aquaculture/mariculture and fishery releases Fish managed in aquaculture or mariculture can escape and transfer disease into wild stocks. Atlantic salmon (Salmo salar) introduced into Tasmania are reported as being infected with a number of viruses that can transfer to wild populations including amoebic gill disease (AGD) caused by the amoeba Neoparamoeba pemaquidensis (Munday et al., 2001). First introduced into Tasmania with Atlantic salmon in 1986, AGD has the potential to infect wild populations if stock escape from managed farms (Munday et al., 2001).
Species imported as fishery stock that are then released have a direct impact on the environment though habitat modification, predation, competition and disease transfer. European carp (Cyprinus carpio), a native of Eastern Europe and central Asia, was introduced into Australian waterways as a sport fish in the 1800s but was contained until an introduction in the 1960s and has now invaded one million square kilometres (Koehn, 2004). European carp have been reported to achieve high biomasses of 3144 kg per ha and individual densities of up to 1000 per hectare (Koehn, 2004). European carp destroy or reduce the value of habitat for native fish by increasing water turbidity and destroying aquatic plant beds (Roberts et al., 1995; Harris and Schiller, 2001, pp 229–258), resulting in reduced photosynthetic production and inhibiting visual-feeding fish (Koehn, 2004). European carp out-compete native fish by being able to reproduce more rapidly, being able to spawn at lower water temperatures and having a very rapid larval growth rate (Koehn, 2004).
Similarly, rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were released into Australian waters in the 1800s for sporting purposes. Trout now successfully breed in many waterways and lakes in southern Australia and restocking regularly occurs. Trout have a negative impact on a number of native fish, frog and invertebrate species (Cadwalladar, 1996: Arlington and McKenzie, 1997), particularly the nationally listed critically endangered spotted tree frog (Litoria spenceri). This frog species breeds in upland streams and has declined particularly in streams where trout are present. Studies have demonstrated that trout fingerlings feed heavily on tadpoles of the spotted tree frog (Gillespie, 2001).
Pets escaped or released into the wild Pets, including cats, dogs and birds, are important and widespread in Australian society, however not all pets have remained domesticated and many have escaped or been released into the wild. The first cats are thought to have arrived in Australia after swimming ashore from Dutch shipwrecks (Abbott, 2003). Feral cats prey on a wide range of native species including small mammals, birds, reptiles and rodents (Norris et al., 2005). Cats can breed rapidly with first breeding at 10 months and under good conditions can breed continuously at any time of the year, having up to three litters a year. Feral cats can be found in all environments. Males usually occupy a home range of 10 km2 but this may be larger if food supplies are scarce. Feral cats are carnivores and can survive with limited access to water as they utilise moisture from their prey (DEWHA, 2004).
Aquarium fish released into the natural environment have been a major source of introduced fish species in Australia. It is reported that 22 of the 34 alien fish species that have established in Australia are the result of people intentionally or accidentally releasing aquarium species into waterways (Lintermans, 2004).
Similarly, imported parrots such as the rose-ringed (ring-necked) parakeet (Psittacula karmeri) have been known to escape from captivity. If this species was to become established in the wild it would compete with native species for habitat and could pose a threat to the environment and agriculture, similar to impacts it has had in its native India. Additionally, imported parrots may have been the origin of the psittacine beak and feather disease in Australia that is having a large impact on native psittacine species (DEWHA, 2005b).
Escaped, released or stolen from research and exhibitor/conservation species, zoos and private collections Exotic species kept in zoos, private collections and research institutes are a potential risk to native species from animals escaping, being stolen or intentionally released. There have been a number of recorded cases where exotic species such as squirrels, red panda and snakes have escaped from captivity (Invasive Species Council, 2003).
Illegal trade or keeping of animals A significant threat to Australia’s biodiversity is the illegal importation and trade of exotic species such as snakes, lizards, amphibians, fish and birds prohibited in Australia. If introduced many species, particularly reptiles, could establish in Australia with disastrous effects on native species, such as preying on and out-competing them. Species such as the American corn snake (Elaphe guttata), which have been recorded as being illegally imported, could threaten native species (Frith, 1973; QDPI&F, 2007a). Snakes that are illegally imported could also harbour many exotic diseases that could have a severe impact on native species such as Inclusion Body Disease or Ophidian Paramyxo Virus (Jacobson, 1993).
Releases to ‘enrich’ the native flora and fauna Many species were introduced into Australia as a result of the activities of acclimatisation societies. These societies were established in an attempt to transplant elements of Britain into the new colony (Curtin, 1997) and to increase agricultural development (Cook and Dias, 2006). In 1861, the Acclimatisation Society of Victoria was established (Gillbank, 1986) and was supported by many of the colonists who felt that the Victorian environment needed to be improved and made more like England. The society’s purpose was to import animals, plants and game of economic or recreation value from Britain and Europe and to establish them in Australia. Over a number of years the society organised the importation of cashmere goats, alpacas, pheasant, deer, hare, sparrows, larks, thrushes (Frith, 1973, p 137–195; Gillbank, 1986), starlings, Indian myna and blackbirds (Rolls, 1969, pp 302–316). The society was disbanded in the late 1860s (Gillbank, 1986), but not before doing irreversible damage to the environment.
Arrival without human-aided assistance Species can arrive in Australia without human aided assistance, for example vagrant birds, insects, seed and spores aided by the wind and in the gut and on the feathers of migratory birds. Most vagrant birds are individuals that are lost and die or depart again posing no threat. Some bird species have invaded and become established in Australia, including the cattle egret (Ardea ibis) that fills a niche provided by grazing livestock (Maddock and Geering, 1994). Another incursion of a potentially novel biota occurred in December 2007 when four Canada geese (Branta canadensis) arrived on the south coast of NSW, probably from New Zealand where they have invaded and become a minor problem (Dawes, 2008). Canada geese in Australia would compete with native species for habitat and have also been shown to spread weeds and have the potential to hybridise with native species as has occurred in other countries (Dawes, 2008).