14 Jun Australia’s Invasive Species
Australia is home to many plants and animals that have been introduced by humans either intentionally or by accident. Some of these have become invasive, meaning they have spread and multiplied to the point where they damage the environment, threaten the continued existence of our own native plants and animals, or create significant problems for agriculture. Invasive animals, often called feral animals, and invasive plants or ‘weeds’ are two of the biggest environmental problems facing Australia today. We would commonly call them pests!
What are invasive species? Invasive species are animals, plants, parasites or disease-causing organisms that establish outside their natural range as a result of human activities and become pests. Many of the most damaging invasive animal species were originally introduced either for sport, as pets, or as livestock and pack animals. Some, such as the cane toad, were introduced to control other pests and became pests themselves. Others, such as black and brown rats and the house mouse, arrived accidentally. Invasive plants are introduced in a variety of ways, for example as crops, pasture and garden plants and to prevent erosion. Some have established so well that they have spread to the bush, where they have thrived.
A 2021 report titled Fighting plagues and predators: Australia’s path to a pest and weed-free future, highlights a looming wave of new extinctions and outlines two futures for Australia, one based on an unsustainable ‘business as usual’ approach and the other based on implementing targeted actions that will help save our unique biodiversity. The report pegs the conservative cost of damage caused by invasive species in Australia — predominantly weeds, feral cats, rabbits and fire ants — at $390 billion over the past six decades and around $25 billion a year and growing.
More than eight in 10 nationally-listed threatened species are endangered by invasive species. More than 70% of Australia’s native animals are found nowhere else on earth, so a loss to Australia is a loss to the world. Invasive species also undermine agriculture leading to increased food and fibre prices.
Let’s look at a few of these invasive species and learn how they became pests.
Photo credit: JJ Harrison, via Wikimedia Commons
Australia has had a problem with European rabbits since their introduction to the continent in the late 19th century. In 1859, European rabbits (Oryctolagus cuniculus) were brought into to Australia and released so they could be hunted. Thomas Austin, a wealthy settler who lived in Victoria, Australia, had 13 European wild rabbits sent to him from across the world, which he let roam free on his estate. From this one backyard sanctuary, it took only around 50 years for these invasive rabbits to spread across the entire continent.
European rabbit numbers have become so large that they destroy crops and land, leading to soil erosion. They also negatively affect agriculture and plants by overgrazing. Not only do the rabbits wreak havoc on Australian croplands, but they also contribute to the decline of native plant and animal species. Besides their lack of natural predators on the continent, their success is aided by quick breeding: They can birth more than four litters a year with as many as five kits (baby rabbits) per litter. Now, it is estimated that approximately 200 million feral rabbits inhabit Australia.
Photo credit: Wikimedia Commons, froggydarb
Cane toads (Rhinella marina) are native to South and Central America. They are extremely hardy animals and voracious predators of insects and other small prey. These qualities led to their introduction into Australia as a means of controlling pest beetles in the sugar cane industry before the use of agricultural chemicals became widespread. It was in June 1935 that entomologist Reginald Mungomery travelled to Hawaii where the toads had been introduced from Puerto Rico. He captured a breeding sample and returned to Gordonvale near Cairns, where a special enclosure had been prepared for them. By August, the toads had successfully reproduced in captivity and 2400 were released in the Gordonvale area
Since then, cane toads have spread well beyond Queensland into coastal New South Wales, the Northern Territory’s Top End and the Kimberley region of Western Australia. They are now moving westward at an estimated 40 to 60 km per year. It was in February 2009, that they crossed the NT/WA border which is over 2000 km from the site they were released 74 years before.
Cane toads can live for at least 5 years in the wild, and up to 15 years in captivity. They mate at any time of year depending on available food and permanent water, and their eggs are laid in long, gelatinous ‘strings’ with the developing tadpoles appearing as a row of small black dots along the length. A single clutch can contain up to 35,000 eggs. Under ideal conditions, toadlets may reach adult size within a year.
Cane toads are considered one of the worst pests in Australia, producing a poison that can kill native Australian animals that try to eat them. We know of two predators of the cane toad – the Keelback Snake (Tropidonophis mairii), a non-venomous species native to northern Australia. Keelbacks can eat cane toads without lethal effects, whereas many other snake species would be killed. Also immune to the cane toad poison are estuarine or saltwater crocodiles. Salties can eat cane toads without too much ill effect. They have no other known predators and have had a serious impact on northern Australian ecosystems.
There is no broadscale way to control this pest but scientists are developing a better understanding of the impacts they have on the environment and the ways in which assets, such as rare and vulnerable wildlife, can be protected
Photo credit: Wikimedia, Antweb.org
Photo credit: NSW Government, Belinda Keen
Red imported fire ants or ‘fire ants’ (Solenopsis invicta) are invasive ants that cause serious social, economic, and environmental harm. They are aggressive and have a severe, burning sting.
Fire ants were first detected in Brisbane, Queensland, on 22 February 2001. It is thought that they arrived up to 20 years earlier. How they entered Australia is not known but it is likely that they entered via shipping containers from America. Imported cargo poses a risk as pests can ‘hitchhike’ undetected. There are many types of ant species found in Australia, but only Solenopsis invicta qualifies as an invasive fire ant.
Fire ants have the potential to inhabit most areas of Australia, which could have disastrous consequences for our environment. They have caused significant environmental problems in countries where they are established.
Fire ants can affect our environment as they:
- feed on fauna that nests or feeds on the ground, including insects, spiders, lizards, frogs, birds and mammals
- can displace or eliminate some native species
- eat and damage seeds, possibly causing major ecosystem changes over time
- predate or disturb the insects and animals that pollinate native plants, which may also cause long-term changes to the vegetation of our bushland areas
Following the discovery of fire ants in South East Queensland in 2001 a nationally cost-shared effort commenced to eradicate them. The National Fire Ant Eradication Program is Australia’s largest biosecurity eradication initiative. More info: https://www.fireants.org.au/
The most commonly known members of the opuntioid cacti group referred to generally as prickly pears, Opuntia species were first introduced into Australia with the first fleet, via Brazil, purportedly to establish a cochineal dye industry. The red dye was commonly used for the red coats worn by British soldiers.
The first batch of prickly pear plants (most likely Opuntia monacantha) arrived in Australia in 1788 followed by more species, and by 1840 there was a thriving plantation in Parramatta, NSW, which had spread to Chinchilla in Queensland by 1843.
By 1920 Opuntia stricta had infested 23,000,000 hectares in NSW and QLD. Prickly pear was becoming so dense that farming was impossible in many areas, and leasehold land was being abandoned.
Image credit: WikiCommons, Peripitus
Opuntia’s seeds are dispersed mainly by animals and birds that feed on its fruit. The tough, coated seeds pass undamaged through the digestive system of these vectors, and once the seed is out of the body, it germinates and a new infestation takes root. Plant segments are also easily detached from the parent plant by animals, wind or flood water and transported elsewhere. It is believed that the flood of 1893 spread seed and plant parts to many new areas. Settlers also took plants to their properties across QLD and NSW to be used as hedges and fodder during droughts, causing the weed to spread throughout the country destroying millions of hectares of agricultural land.
Prickly pear forest 1935. Credit: QLD Gov State Archives.
A property in Chinchilla, Queensland, Australia, infested with prickly pear in 1928. Photo: State Library of Queensland/Flickr
It wasn’t until 1920 that the Commonwealth, Queensland and New South Wales Governments established the Commonwealth Prickly-Pear Board (CPPB) which investigated recommendations for controlling Opuntia strictathrough biological agents.
Almost immediately, the CPPB sent a group of entomologists to America to acquire the previously identified biological agents. They simultaneously established a breeding centre for Cactoblastis cactorum moths in Queensland. Cactoblastis cactorum moths are indigenous to a small area of Argentina.
Female moths lay their eggs on the prickly pear plants. Working as a team, the hatched larvae then eat through the tough outer layer of the cactus pads to get at the edible interior. There they feast on the soft tissue until they reach around 25mm in length. In a matter of weeks, the larvae can destroy an entire plant.
In Chinchilla, QLD, the moths underwent strict breeding and feeding assessments to ensure they would not attack other plants. In 1926 the first moth was released.
The first release of Cactoblastis cactorum moths into the Australian environment in 1926 is regarded as one of the world’s most spectacular examples of biological weed control.
By 1933 it was estimated that 80 per cent of the infested land in Queensland, and 50–60 per cent in New South Wales, had been cleared. Although not on the same scale as the 1920s crisis, prickly pear continues to be a problem in NSW and QLD, where new varieties that do not act as hosts for Cactoblastis moths have become established.
Image credit: WikiCommons, Mokkie
Lantana camara is a heavily branched shrub that can grow in compact clumps, dense thickets or as a climbing vine. The stems are square in cross section, with small, recurved prickles. Most leaves are about 6 cm long and are covered in fine hairs. They are bright green above, paler beneath and have round-toothed edges. Leaves grow opposite one another along the stem. When crushed the leaves produce a distinctive odour. Flowers appear throughout most of the year in clustered, compact heads about 2.5 cm in diameter. Flower colours vary from pale cream to yellow, white, pink, orange and red. Lantana produces round, berry-like fruit that turn from glossy green to purplish-black when ripe.
Lantana currently covers more than 5 million hectares of subcoastal New South Wales to Far North Queensland. Small infestations of lantana have also been found in central west Queensland, the Northern Territory, Western Australia, South Australia and Victoria. It has taken over native bushland and pastures.
Lantana is mainly spread by fruit-eating birds and mammals. It forms dense thickets that can smother and destroy native vegetation and are impenetrable to animals, people and vehicles.
Lantana covers 5 million hectares throughout most coastal and hinterland areas of Australia, from north Queensland to southern New South Wales and including the Northern Territory and Western Australia. Lantana could also spread to Victoria.
Research indicates more than 1400 native species are negatively affected by lantana invasion, including many endangered and threatened species (it has been reported that lantana impacts 95 threatened species in Australia). As lantana is a woody shrub that has thin, combustible canes, its presence can also create hotter bushfires, altering native vegetation communities and pastures.
We have touched on only a few of the more well-known invasive species above. Invasive species have a major impact on Australia’s environment, threatening our unique biodiversity and reducing overall species abundance and diversity.
Invasive species include: diseases, fungi and parasites, feral animals, insects and other invertebrates, introduced marine pests, and weeds.
There are a number of excellent online resources in each State for identifying species and what researchers, governments and individuals can and are doing to reduce the impacts of invasive species.