The researchers, Dr Bellinda Gallardo and Dr David Aldridge from the University of Cambridge, focussed on the 'dirty dozen' – a group of high-risk invasive aquatic plants and animals. Some, like the killer shrimp (Dikerogammarus villosus) and the bloody red mysid (Hemimysis anomala) are already in UK but have yet to spread. Others, such as the Asian clam (Corbicula fluminalis) and the marmokrebs, a crayfish (Procambarus fallax) may not yet have arrived.
Working with Species Distribution Models, which are routinely used to predict which regions most suit invasive species, the Cambridge pair made the models more accurate by including human factors such as population density, land-use and proximity to ports. Traditionally, the models are based on environmental conditions such as temperature and rainfall.
According to Dr Gallardo: "Invasive species need to be in the right place at the right time; they need the right environmental conditions, but they also need a helping hand from humans. This can happen intentionally, for example through introduction of commercial fish, or accidentally via hulls of boats, fishing equipment, or ballast water."
The role of humans, plus invasive species' great adaptability, make predicting their spread challenging. By including both human and environmental factors in the models, the researchers found the risk of invasion in the UK was increased by 20% in coastal, densely populated areas and places near transport routes, with the Thames, Anglian and Humber river basins at highest risk.
"These river basins already host many aquatic invaders. This is particularly worrying because invasive species often modify their habitat, making it more favourable to other invaders. This can eventually lead to a process known as invasional meltdown," she says.
Tackling the problem is costly. Invasive species are the second biggest threat to biodiversity after habitat loss. Around 12,000 invasive species have already arrived in Europe, where their combined impact on native biodiversity, agriculture, health and the economy costs at least €12 billion a year.
By producing more accurate maps, the study should allow environmental managers and policy makers to target resources at the most invasive species and the areas most under threat. "Effective management of invasive species depends on rapid detection and control. Our maps are fundamental to direct biomonitoring efforts towards areas most suitable for the 'dirty dozen', so they can be detected as soon as possible," Dr Gallardo explains.
"At present, environmental managers and policy makers have few tools to make informed decisions about the risk posed by existing and future invaders. Our study gives them basic information to prioritise management and control decisions regarding 12 of the most worrisome freshwater invasive species."
The study is also timely because of climate change, which might further favour invasive species, she adds: "Invasive species might better adapt to climate change than natives because of their wide environmental tolerance and highly competitive biological traits. And because they usually reproduce rapidly, invasive species may be better than native species at resisting and recovering from extreme events."