Species that have been introduced into areas outside their natural range, through human actions, are known as non-native, or alien, species.

Some have been introduced intentionally for use in agriculture, forestry etc, and others as the accidental result of human activity, e.g. in ships’ ballast water or via the transport of goods. This is a global issue. In Britain alone, there are over 3,000 non-native species. Many non-native species are harmless, causing no disruption to the environment or native wildlife in their new locations, but occasionally a species will establish and thrive in a way that can pose a threat to native biodiversity. These species are referred to as invasive non-natives.

How are they spreading?

A huge number of living species – perhaps as much as half of global biodiversity – owe their existence to the separation of regions of the world by physical barriers that stop different ecosystems and ecological communities from mixing. These barriers are the oceans, mountain ranges, deserts, rain-shadows and currents. They have forced evolution to operate independently in different parts of the world, producing diverse and regionally characteristic floras and faunas – giving us, for example, tigers as forest cats in Asia, but jaguars as forest cats in South America.

People, however, have a long history of moving animals and plants around the globe and – either deliberately or inadvertently – allowing them to establish in new areas. This effectively breaks down these geographical barriers to species movements. Free from native predators, pathogens and competitors, the new non-native arrivals often flourish and sometimes create severe problems for native wildlife.

What’s the problem?

The introduction of invasive non-native species is the second biggest threat to global biodiversity after habitat loss. Islands and freshwater habitats are particularly vulnerable, and bird species across the world have experienced severe impacts: invasive non-native species have been involved in the extinction of 68 out of the 135 bird species lost in the wild globally over the last 500 years.

Even moving species outside their range within the same country can cause problems. There are several examples where mammals native to one part of a country have been introduced to offshore islands where they do not occur naturally and have caused problems for biodiversity.

Invasive non-native species can cause problems for native wildlife in several ways:

·         Predation - introducing new predators into an area can have devastating effects on the native wildlife and ecosystems. Biodiversity on islands and lakes is extremely susceptible to introduced predators, as native species have often evolved in the absence of predators and are not able to adapt quickly enough when they are introduced.

·         Competition for resources - introduced species can out-compete native wildlife for resources like food, breeding sites, space etc. Japanese knotweed, an introduced plant to the UK, forms dense stands that can prevent native plants from growing. This can change the habitat structure of an area, making it unsuitable for the other organisms that live there.

·         Introducing new diseases - introducing new diseases can have serious consequences, as native species will not have developed immunity. Signal crayfish, an introduced species to the UK, is a carrier and host of the crayfish plague, which can kill our native crayfish.

·         Hybridisation - some species are capable of breeding with another related but distinct species, creating hybrids. Over time, the unique genetic diversity of one species can be lost and the species can become extinct. See our page on white-headed ducks and ruddy ducks.

Invasive non-native species are not just a threat to biodiversity; they can also damage economic interests such as agriculture, forestry and fisheries. The cost of dealing with invasive non-natives in Britain is estimated to be several billion pounds annually.

What can be done?

It can be hard to identify which non-native species are likely to cause a problem. It may appear that the introduction of a new species is harmless, only for problems to arise once the species has become established. There are concerns that climate change may make it easier for non-native species to become established in new areas as environmental conditions change.

Preventing invasive non-native species from being introduced in the first place, or, if that fails, acting quickly to prevent them becoming established, are key to tackling this issue. In places where invasive non-natives are already present and considered a threat, control or eradication of the population may be considered. There have been notable successes, especially on islands, where removing invasive non-native species has enabled the recovery of native wildlife populations.

The RSPB has been closely involved with constructing policy and legislation on non-native species issues, both nationally and internationally. We do not advocate the ‘demonisation’ of non-native species, or the eradication of every plant or animal brought to the UK by people; some non-native species do not have a direct detectable effect on native wildlife, and some species are too well established for any realistic practical response. However, where native wildlife is threatened, and where a practical response is possible, we do promote effective and humane measures to protect native species and habitats from damage or extinction.

We will continue to help develop and implement the government’s Invasive Non-native Species Framework Strategy for Great Britain, and we are pressing for effective EU legislation on non-native species. Currently national legislation is patchy and inconsistent across the EU, and we need to bring all Member States to a minimum standard in this regard: if any one country allows the preventable establishment of damaging non-native species, wildlife across the whole European Community is then put at risk.