What Are Invasive and Introduced Species?

An introduced species is an animal, plant or other organism that has been moved beyond its natural range through human activity, either deliberately or accidentally. Introduced species are now part of many Tasmanian landscapes, but being introduced does not necessarily make a species invasive.

A species is generally considered invasive when it establishes a breeding population, spreads beyond where it was introduced and causes, or has the potential to cause, significant harm. For wildlife, that can include predation, competition for food or nesting sites, habitat degradation, disease transmission or disruption of ecological relationships. The distinction is based on the impacts a species causes, rather than simply whether it is native to Tasmania.

Tasmania’s island geography makes this distinction particularly important. Many pests, weeds and diseases found on mainland Australia are not established here, while others have already become widespread. Preventing new invasive species from establishing remains an important part of protecting Tasmania’s native wildlife and ecosystems.

How Invasive Species Affect Native Wildlife

Invasive species can affect wildlife directly through predation and competition, but their impacts can extend through an ecosystem. They may reduce the availability of food, occupy nesting or shelter sites, damage native vegetation or change the habitats that other species depend on.

Native wildlife can be particularly vulnerable when it has evolved without certain types of predators or competitors. Ground dwelling animals, nesting birds, reptiles, frogs and small mammals may have few effective defences against a newly introduced predator. Species with restricted ranges or small populations can also be affected by relatively small changes to their habitat or food supply.

These pressures do not occur in isolation. Habitat loss, disease, road mortality, climate change and invasive species can affect the same wildlife populations at the same time, making existing pressures more difficult for some species to withstand.

Competition for Food, Shelter and Habitat

Introduced species can compete with native wildlife for many of the same resources. Food, water, shelter, nesting hollows and breeding sites can all become more difficult to access when another species becomes abundant or relies on the same resources.

Competition can be difficult to see. Native animals may have to spend more time searching for food, use poorer quality habitat or lose access to important nesting and shelter sites. Over time, this can affect breeding success and survival.

The effects can be particularly important where suitable habitat is already limited. A hollow occupied by an introduced species, for example, is one less nesting or shelter site available to native wildlife in landscapes where old trees and natural hollows have become scarce.

Sugar Gliders provide a particularly Tasmanian example. Introduced to Tasmania in the nineteenth century, they now occur across much of the state’s north and east and use tree hollows also relied upon by native wildlife. Their impact on the critically endangered Swift Parrot has received particular attention, with Sugar Gliders documented preying on nesting females, eggs and nestlings. The relationship between the two species, and continuing research into the extent of that predation, is explored further in The Debate Over Sugar Gliders and Swift Parrots.

Feral and Domestic Cats

Cats are among the most significant introduced predators affecting Australian wildlife, and Tasmania is no exception. Feral cats occur throughout mainland Tasmania, from agricultural areas and urban fringes to remote wilderness, as well as on several offshore islands. They prey on mammals, birds, reptiles, frogs and other animals, including species already facing pressure from habitat loss, disease and other threats.

Research has estimated that feral cats kill hundreds of millions of mammals, birds and reptiles across Australia each year. In Tasmania, cat densities can become particularly high where food is concentrated. Studies around some seabird rookeries have recorded approximately 45 to 55 feral cats per square kilometre, compared with an estimated national average of around 0.2 to 0.7 feral cats per square kilometre.

Domestic cats retain the same ability to hunt. Australian research found that a roaming pet cat kills an average of about 186 mammals, birds and reptiles each year, including approximately 110 native animals. Most of those kills are never seen by the owner, with studies indicating that pet cats bring home only about 15 per cent of the animals they kill. Domestic cats can also occur at much greater densities around towns, suburbs and rural properties than feral cats do across natural landscapes, and a well fed cat can still hunt.

Cats can also affect wildlife through disease. They are the definitive host of the parasite that causes toxoplasmosis, which can infect native mammals and birds as well as livestock and people.

Keeping pet cats contained prevents them from hunting wildlife, while desexing and preventing unwanted litters can reduce the number of cats entering stray and feral populations. You can learn more about containment, desexing and other practical steps in our guide to Responsible Cat Ownership in Tasmania.

Introduced Herbivores and Habitat Change

Introduced herbivores can alter wildlife habitat by grazing, browsing, trampling vegetation and preventing native plants from regenerating. Where their numbers become high, they can reduce ground cover and change the vegetation that native animals rely on for food, shelter and breeding.

Wild fallow deer and wild European rabbits occur in parts of Tasmania, although their distribution and impacts vary considerably. Fallow deer are particularly widespread across parts of the Midlands and eastern Tasmania and have expanded beyond the areas where they were historically concentrated. Browsing, grazing and trampling can damage native vegetation and disturb sensitive areas.

Rabbits can also place substantial pressure on native vegetation, particularly on offshore islands and in dry environments where plant growth may already be limited. By consuming seedlings and young plants, they can interfere with regeneration and gradually change the composition of plant communities.

These impacts can become more pronounced after drought, fire or other disturbance, when vegetation is recovering and new growth is particularly vulnerable to browsing. In areas supporting threatened species or sensitive ecological communities, controlling introduced herbivores can form part of broader habitat protection and restoration.

Invasive Plants and Weeds

Introduced plants can spread through native vegetation, roadsides, waterways and disturbed areas, sometimes displacing native plants and changing wildlife habitat. For native animals, this can reduce the plants they rely on for food, shelter and breeding, as well as the native vegetation that supports insects and other food sources.

Tasmania has hundreds of introduced plant species, but only some become serious environmental weeds. Gorse, Spanish heath, blackberry, sea spurge and common foxglove are among those established in parts of the state, affecting environments ranging from forests and grasslands to wetlands and coastal areas.

Around Mount Roland in north west Tasmania, foxglove provides a local example of the work involved in limiting the spread of an invasive plant. Since 2011, Mount Roland Land Care has coordinated weed control in and around the Mount Roland Regional Reserve, with around 70 weed sites mapped in or close to the reserve. Foxglove is considered one of the major current threats, and community working bees regularly remove plants around the mountain to limit its further spread. You can learn more about this work through the Mount Roland Weeds Project.

Disturbance can create further opportunities for weeds. Some invasive plants spread rapidly after fire, clearing or soil disturbance, establishing while native vegetation is still recovering. Dense infestations can then compete with native plants for space, light, water and nutrients and interfere with regeneration.

Protecting native vegetation from invasive plants also protects the food, shelter and other habitat features that Tasmania’s wildlife depends on.

Freshwater and Marine Invaders

Tasmania’s rivers, lakes, estuaries and coastal waters are also affected by introduced species. Aquatic invaders can prey on native wildlife, compete for food and habitat, damage vegetation and alter the ecosystems in which they become established.

Eastern gambusia, a small and aggressive introduced fish, has established populations in the Tamar estuary and can compete with and attack native fish and frogs. European carp have been the focus of a long running containment and eradication effort after being discovered in lakes Crescent and Sorell in 1995. Carp can damage aquatic vegetation, increase nutrients in waterways and threaten native galaxiids.

Introduced marine species include the northern Pacific seastar and wakame. The northern Pacific seastar, first recorded in Tasmania in the 1980s, is now established in parts of the state and is particularly abundant in the Derwent Estuary. It preys on shellfish and other marine animals and can affect both native biodiversity and commercial species.

Wakame, an introduced kelp first reported from eastern Tasmania in the 1980s, can grow rapidly and compete with native algae for space and light. Other established marine pests can alter sediments, occupy habitat and compete with native species, changing the communities found on reefs, shorelines and the seafloor.

How Invasive Species Reach Tasmania

Bass Strait provides Tasmania with an important natural barrier against some invasive species, but people, freight, animals, plants and goods move into the state every day. These movements can also provide pathways for unwanted organisms to arrive.

Some species are introduced deliberately, while others arrive unnoticed. Seeds can travel in soil or on machinery, insects can be transported among plants and produce, and aquatic organisms can be carried on boats, fishing equipment and other wet gear. Eggs, larvae, seeds or small fragments of plants may be enough for some species to establish in a new environment.

Invasive species can also spread within Tasmania. Once an invasive species has established in one area, vehicles, boats, equipment, soil and plant material can help spread it further. Offshore islands, waterways and areas of relatively intact native habitat can be particularly vulnerable to these secondary introductions.

Preventing New Species From Establishing

The best opportunity to limit the damage caused by an invasive species often comes before it becomes widespread. Preventing its arrival is preferable, but when a new species does reach Tasmania, finding it early can make the difference between eradication and decades of ongoing control.

Border controls, surveillance, monitoring and reporting are used to reduce the risk of new pests, weeds and other invasive organisms becoming established in Tasmania. Ports and airports are important, but monitoring also occurs in forests, waterways, agricultural areas and other environments where a new species may first be detected.

Invasive populations can be much easier to remove while they are still small and confined to a limited area. Once a species begins reproducing and spreading through suitable habitat, eradication can become increasingly difficult, expensive or no longer practical. Management may then shift towards containment or reducing its impacts.

Members of the community can also help identify new arrivals. An unfamiliar animal, plant or marine organism may first be noticed by someone walking a track, working on a property, fishing, boating or simply spending time outdoors. Photographs, an accurate location and a prompt report can help determine whether further investigation is needed.

Managing Invasive Species in Tasmania

Managing invasive species is rarely as simple as removing an unwanted animal or plant. The appropriate response depends on how widely a species has spread, whether eradication is still possible, the environment being protected and the potential impacts of the control methods themselves.

Management can involve government agencies, land managers, researchers, farmers, conservation organisations and community groups. Approaches range from surveillance and weed removal to trapping, exclusion, biological control and targeted population reduction. Programs may also focus on protecting particular islands, waterways, breeding colonies or threatened species rather than attempting to control an invasive species everywhere it occurs.

Monitoring after control helps determine whether a program is achieving its intended result and whether management needs to change.

When Invasive Species Control Can Affect Native Wildlife

Control methods intended to protect wildlife and habitat can also affect non target animals, making the choice of method an important part of invasive species management.

In Tasmania, 1080 poison has been used in baiting programs to control browsing animals, including through poisoned carrot bait. Its use has raised concerns about animal welfare and the potential exposure of non target wildlife. The history, regulation and wildlife impacts of its use in Tasmania are explored further in 1080 Poison in Tasmania.

Wildlife control in Tasmania is also not limited to introduced species. Native animals can be subject to population control where they are considered to be causing damage to crops, pasture or property. The tension between protecting native wildlife and permitting its control is explored further in Licensed to Care, Licensed to Kill.

Different control methods carry different risks, making careful planning, monitoring and consideration of non target wildlife important. The conservation benefit of a control program needs to be considered alongside the potential harm it may cause elsewhere.

What Community Members Can Do

Everyday choices can help prevent invasive species from spreading and reduce their impact on native wildlife. Keeping pet cats contained, never releasing unwanted pets, aquarium fish or aquatic plants into the environment, and preventing invasive garden plants from escaping beyond property boundaries are practical steps people can take at home. Environmental weeds can also be removed before they flower or set seed.

People travelling between natural areas can avoid carrying invasive species with them by cleaning soil, seeds and plant material from footwear, vehicles and equipment. Boats, fishing gear and other equipment used in waterways should also be cleaned and allowed to dry where appropriate before being moved to another location.

Unusual animals, plants or aquatic organisms should be photographed where possible, with the location recorded and the sighting reported to the appropriate authority. Early reports can help determine whether a new species has arrived while there is still an opportunity to investigate or control it.

Community weed removal, habitat restoration and local Landcare projects also contribute directly to invasive species management. Removing weeds before they spread and restoring native vegetation can help protect wildlife habitat at a local level.

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Cover Image by Dhx1 / Wikimedia Commons / CC0 1.0.