Biological invasions

Biological invasions can be defined as the introduction of a species into a habitat where it has not previously occurred. Such invasions may introduce thousands of new and foreign genes into a different habitat. The effects of this change are completely unknown and potentially catastrophic. Genetic modification in comparison to biological invasions is happening on a relatively small scale and introduces very few new genes into an environment.

In terms of global biodiversity, the World Conservation Union (IUCN) have recently added 2,000 more names to the endangered animals and plants list increasing the numbers of endangered species to 12,259. Latest predictions of the world species extinction rate are between 1,000 and 10,000 times greater than the natural rate of extinction. Invasive species are estimated to be a major contributor to this increased rate making biological invasions the second greatest threat to global biodiversity after habitat destruction. This is because when an invading species establishes itself it can often outcompete native species which have similar roles within the environment and will also often have no natural enemies that prey on it and restrict its population.

A classic example of this is in the United Kingdom and other areas of Europe where the native red squirrel has been displaced by the larger North American grey squirrel.


The latter was initially introduced to the UK by humans during the 1870s. Since then the population of the grey squirrel has spread restricting the red squirrel to residing in small populations, mainly in areas of coniferous forest in the UK.

A new species within an environment can cause major changes to populations of native species sometimes resulting in populations declines, extinctions and modifications of the whole structure of that community. In addition there are huge economic costs associated with the introduced species. Therefore biological invasions carry an immense threat to the global species diversity, so it is important to understand how they occur and try to find ways in which to reduce their occurrence in the future. Invasions can be both natural, for example by dispersal, or facilitated by humans.

Natural invasions

Natural invasions can effect ecosystems just as much as those caused by humans. However in natural invasions the rate of the invasion is set by the dispersal ability of the invader. These types of invasion are often referred to as the natural spread of plants or animals into new areas rather than invasions. However they result in the same effect as human-facilitated invasions, albeit over a far greater timescale.

When invasive species are spoken about in a conservation context it is usually in relation to human-facilitated invasions and they are usually referred to as introductions.

Introductions by humans

There are three distinct differences between human introductions and natural invasions. Firstly, human introductions cause a rapid change in the community composition obtaining far quicker results than through the natural movements of organisms. Secondly, introductions artificially place species into environments that would probably have not be chosen by the species if left to their own devices. Thirdly, natural communities have been shaped throughout evolutionary time and to some extent a balance is established within them. Introduction of these new species can affect this balance much more rapidly than natural movements.

Damage caused by biological invasions

So how do biological invasions damage the environments which they invade? They can affect the environment both directly by competition or predation, or indirectly by spreading disease or by hybridisation with native species. In general predation causes the most damage to native diversity.

New Zealand has more introduced mammalian predators than any other archipelago. The introduced mammals are thought to be the biggest threat to New Zealand's native fauna and flora. Since the first human occupation more than 40% of the original bird fauna has become extinct and although some extinctions have been attributed to the decline in lowland forest and wetland areas, the mammalian predators pose the greatest threat to the birds.

Hybridisation is another way in which introduced species can pose a threat to native wildlife. Hybridisation involves the cross-breeding between parents of different species. For example the European White-headed Duck, a globally endangered species, readily hybridises with the introduced North American Ruddy Duck and this is seen as the most serious threat to the survival of the White-headed Duck.

Success of invasions

Considering the often severely detrimental affects of invasions on biodiversity, it is fortunate that most invasions are actually unsuccessful. Scientists use a rough approximation to estimate how many invading species will actually establish themselves, known as the ‘tens rule’. This states that only 10% of invading species will form casually occurring colonies, 10% of these will become naturalized in their new environment and only 10% of these will actually become pests. This rule works for a great number of invading species.

In order to understand why certain species can become pests we need to know what it is that makes them successful invaders? Scientists believe that successful invasions depend on two main factors, the characteristics of the invading species and also the characteristics of the receiving area.

Specific characteristics of invading species that make them successful varies between plants and animals but in general they require similar properties such as growing/developing quickly, a short time between each generation of adults or being able to compete well with other plants and animals for resources. It has been identified that plants need some of the following characteristics to be successful invasive species:

• Perenniality
• Quick growth
• Early flowering
• Self-compatibility
• Germination in wide range of conditions
• Vegetative reproduction
• Good seed dispersal
• Good competitive ability

Therefore it is important that the movement of plants and animals which fit these characteristics is specifically restricted to reduce the chances of successful biological introductions.

The second factor affecting the success rate entails the characteristics of the receiving area. Certain communities are more vulnerable to the introduction of foreign species. For example, islands and habitats that have been disturbed by humans or or by natural phenomena are more susceptible to invasions. Many islands are also often isolated and have greater numbers of endemic species and thus invasive species that are taxonomically isolated or ecologically distinct from the native habitants, may have an advantage. Therefore increasing the stringency of importation regulations of plants and animals to island areas provides another possible way to reduce the chances of introduced species causing damage.

Future outlooks

The invasion of species is set to become an increasing problem in the future because of two main reasons. Principally, the world’s ecosystems have increased susceptibility to invasion because humans have increased the disturbance of these environments. Therefore environments already under threat by human disturbance are at even greater risk because they are less able to cope with increased biological introduction.

The second reason for the bleak outlook revolves around the globalisation of economics that has caused a worldwide increase in the movement of people and materials. It is now easier and quicker to move around the globe and so a greater number of species are being introduced into new areas and the invasions continue.

Countries which had previously not been under such threats from biological invasions, are now experiencing greater problems. For example Australia, once a relatively isolated country, is now much more accessible and thus is experiencing increasing problems of biological introductions. In recent times Australia has seen a 40% increase in reported introduced plant incursions. In the 1970s figures lay under 300 but plant incursions in the 1990s have now risen to over 420.

Australia has also undertaken research regarding the likelihood of instinction of the unique Australian marsupials. The results indicate that competition from non-native species alongside climatic change are the main factors that will determine the possibility of extinction. In order to fight back against the threat to their unparalleled biodiversity, Australia enforces some of the world’s strictest customs regulations on importing plant and animal material.

Biological invasions are a global problem threatening the biodiversity of the world’s habitats on a local scale. Due to the success of the human race we are causing more and more disturbance to previously untouched habitats and not only directly affecting them but also making them more susceptible to the establishment of foreign species. We as humans need to ensure that we try to reduce the introduction of new species to areas of the world where they have not previously been in order to protect the amazing and wonderful diversity that exists on our planet.

Sources/Further Information

DEFRA-Department for Environment, Food and Rural Affairs
The World Conservation Union
Begon, M., Harper, J.L., Townsend C.R. (1996). Ecology: individuals, populations and communities. Blackwell Science Ltd.
Groves, R.H. and Burdon, J.J. (1986). Ecology of biological invasions. Cambridge university press, Cambridge
Hamblin, A. (2001). Land Theme Report, Australia State of the Environment Report 2001, CSIRO Publishing on behalf of the Department of the Environment and Heritage, © Commonwealth of Australia 2001
Walker & Steffen, (1997). The terrestrial biosphere and global change: implications for natural and managed ecosystems: a synthesis of GCTE and related research
Williams J. (2001). Biodiversity Theme Report, Australia State of the Environment Report 2001, CSIRO Publishing on behalf of the Department of the Environment and Heritage, © Commonwealth of Australia 2001
The Wildfowl and Wetlands Trust:
Williamson, M.H. (1996) Biological invasions. Chapman & Hall.

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