To make the change from being considered ‘introduced’ to ‘invader’, a species has to be able to produce offspring, to spread, and to cause problems for humans or other species.
Establishment
Many imported species can survive in their new environment, although conditions are not suitable for them to reproduce. If these single animals or plants escape into the wild, its impact on the environment will usually be small and localised. However, a small proportion of exotic species are likely not only to survive, but also to produce offspring. At this stage the species is considered to be ‘established’ (sometimes called naturalisation). This means they can start to increase in numbers and is a critical step in becoming invasive.
Spreading
Once a species is able to survive and reproduce, the population can start to radiate out from the site (or sites) of introduction. The rate of this spread can vary enormously between species from a few metres to many kilometres per year on land, and hundreds of kilometres for marine species helped to spread by ocean currents. The spread of a species is unlikely to be even, as not everywhere will provide the right conditions for survival. For example, Oxford Ragwort escaped from Oxford Botanic Garden and was first recorded in the wild in 1794, and is now common on wasteland, railways and verges in south and central England. Favouring rocky areas, it spread rapidly along railways, as the stony ground around railways lines provided similar conditions to its native habitat, Mount Etna in Sicily, and the trains going past helped to disperse its feathery seeds.
Understanding why species spread to some areas and not others is important if we are to work out where they are likely to go next. This knowledge can also provide us with information that could help to create barriers to stop them spreading further.
One in a hundred
Roughly one in every 100 introduced species is likely to become an invader. This is known as ‘the tens rule’; approximately 10% of introduced species become established, and of those established species, around 10% are likely to become pests. Although there are some exceptions to the tens rule, it demonstrates that generally only a small proportion of exotics will become invasive.
One in a hundred may not sound that much, but there are large numbers of species imported and the cost of dealing with each invader is high, so that one in a hundred is something to be concerned about.
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