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Why are invasive non-native species a threat?

The problems caused by invasive non-native species

Invasive non-native species are one of the biggest threats to native plants and animals. They threaten some of Scotland's most iconic species such as the red squirrel and sensitive habitats such as Scotland's Atlantic oakwoods.

It can also be very expensive both to control and to mitigate their negative impacts such as flooding or the damage they can cause to forestry or crops. In fact, it has been estimated that invasive non-native species may cost up to £2 billion pounds each year in Great Britain - that might be as much as £200 million for Scotland alone.

Invasive non-native species can also have a negative impact on the way we live. For example, some species such as giant hogweed can directly affect people's health. Others can prevent people doing things they enjoy, an example being North American signal crayfish or aquatic plants which can interfere with angling and boating.

How do invasive non-native species cause problems?

As travel, trade, and tourism have increased, humans have facilitated the movement of plants and animals around the world, beyond natural barriers (such as oceans, mountain ranges and deserts) that they would not otherwise have been able to cross.

If these plants and animals are native to areas which have similar environmental conditions to the country they are introduced to, they have the potential to become established. As introduced species, they often arrive without their usual predators, pests and diseases; they may be advantaged over native species and can become invasive.

Importance of prevention

It is not possible to predict with certainty what species will become invasive.

There is a rule for invasive non-native plants called the 'Tens rule' which states that of imported non-native plant species, 10 per cent establish, and of these, 10 per cent become invasive.

However, matters are complicated for invasive non-native plants because they commonly display a lag effect, where they can be present for many years before 'taking off' and causing problems. This 'lag phase' can last for decades and sometimes over a 100 years.

For animals, the percentage that have success at each stage of establishment and invasiveness is likely to be over 50 per cent. For mammals introduced to islands, invasiveness is thought to be closer to 100 per cent, they almost always have invasive impacts of some kind.

Because of these issues, and the practical difficulties and costs associated with controlling species once they are causing problems, prevention is given the highest priority. This is the approach advocated though the Convention on Biological Diversity and promoted through the Non-Native Species Framework Strategy for Great Britain.

How will climate change impact on non-native species?

Whilst the impact is uncertain, climate change is likely to increase the range of some invasive non-native species and result in more non-native species that are currently benign becoming invasive.