There are more than 3700 non-native animal and plant species currently in the UK. Many of these only turn up occasionally and don’t breed here. Some are harmless and are unlikely to cause any problems, such as the three species of stick insects that can be found in South-West England.
In fact, only fewer than 300 of the UK’s non-native species are thought to have a negative impact on our environment or health. But these ‘invasive’ species are rampant and difficult to control. An ecosystem is a delicate balance of species which have lived with each other for millions of years. Carnivores keep herbivores in check, herbivores chomp down on the vegetation within a landscape. Add a new species and sometimes that balance can be turned upside down, as we have seen with the introduction of non-native crayfish.
Control is often difficult. Many aquatic organisms are transported on the hulls of ships or in ballast water, which is released to coastal waters without treatment. Others are moved accidentally during fish and shellfish farming or are sold as aquarium specimens. Terrestrial invertebrates are imported with young trees and pot plants. Bumblebees are brought in for pollination in greenhouses and poly-tunnels, but then run the risk of escaping and spreading diseases to native bees.