Ok so it's a rhetorical question, but it raises an important point: there are undoubtedly some alien invasives here that we don't know about yet. Non-native invasive species are here, and here to stay; so we need to be pragmatic about how to deal with them.
Not all non-natives are invasive, and not all invasives are calamitous to our native wildlife and ecosystems. But some of them are, and knowing which ones are dangerous is crucial for designing a sensible response. Knowledge is key; knowledge about occurrence here, but also about impacts in other countries.
It is widely held that invasive species pose one of the largest threats to wildlife. Habitat loss and degradation are still the biggest overall culprits for declines in biodiversity, species abundance, and outright extinctions; but novel competitors are a serious and growing problem, globally and locally.
International trade and people movements have never been so extensive as they are today, and the number of invasive species hitching a ride is growing rapidly. The movement of species beyond their natural range can pose a threat to indigenous species that are unable to compete. It can also bring novel pathogens to which indigenous species have no immunity, and it can entirely change delicately-balanced ecosystems.
Within the UK there has been a long history of artificial species introductions. Probably for as long as humans have colonised these Isles, we have brought alien species with us; from skin parasites to economic crop plants and commercially-farmed animals.
The trend is unfortunately likely to accelerate. More alien species arrive each year - some by accident, some delibarately - and we often fail to detect them for many years afterwards.
One of the latest discoveries is the freshwater shrimp Dikerogammarus haemobaphes - a close relation of the 'killer shrimp' D. villosus. Both originated in the Ponto-Caspian region of western Asia, and both are considered a serious threat by Defra's Non-Native Species Secretariat (NNSS), the Environment Agency, and NGOs such as Buglife.
So how long has D. haemobaphes been here? Ironically it seems to have been here quite some time - perhaps a lot longer than¬†its sister species D. villosus which was discovered here in 2010.
Surveys in 2012 show that D. haemobaphes is now widely distributed in several river basins, including large stretches of the Thames, Severn and Trent.
For further information on this, see the NNSS advisory note: https://secure.fera.defra.gov.uk/nonnativespecies/news/index.cfm?id=97