Fears over grey squirrels in Europe
Scientists are in a race against time to stop the grey squirrel launching an invasion across mainland Europe.
The grey squirrel, which originated in the US, is common in the UK but on the continent currently only has a foothold in Italy.
Until recently the slowly expanding small populations of Italian greys have posed little threat.
But new research shows that genetically distinct groups of the squirrels are now at the point of merging. Scientists fear this would greatly accelerate their ability to thrive in new environments.
If the greys manage to cross the Alps, it could prove disastrous for native red squirrels in other parts of Europe.
In the UK, red squirrel numbers collapsed as a result of grey squirrels out-competing them and spreading the deadly squirrel pox virus.
Grey squirrels have also changed the composition of British forests, by debarking trees such as Garry oaks.
Dr Lisa Signorile, from the Department of Life Sciences at Imperial College London, said: "Italian grey squirrels are edging closer to the northern border and are perilously close to crossing the Alps.
"If the Italian populations interbreed, they will increase in genetic diversity, which will increase their chances of invading the rest of Europe. To stop the spread, we need to understand what makes some populations such successful invaders.
"Our new study, which is the first to specifically examine grey squirrel population genetics at a large scale, helps us uncover some of those reasons."
The international team compared 12 DNA markers from grey squirrels in Piedmont, northern Italy, with the same markers from greys in Northern Ireland, Northumberland and East Anglia.
The research showed that if numbers of a founding group of squirrels are small, genetic diversity is lowered which in turn reduces their ability to adapt to new environments.
Bigger grey squirrel groups led to greater genetic diversity and faster expansion.
Several of introductions of grey squirrels have been documented, making it possible to track their spread.
One of today's Italian populations springs from a deliberate introduction in 1948 by diplomat Giuseppe Casimiro Simonis Vallario. He took a shine to the 'exotic' animals while visiting Washington DC for meetings after the end of the Second World War.
Mr Vallario brought just four squirrels back to Italy, which he released in the park near his Turin villa.
"The fact that today's grey squirrel populations depend on the size of their founding population has important management implications," said Dr Signorile: "We need to find ways of helping Italian managers prevent breeding between genetically distinct populations."
The research is published in the journal Diversity and Distributions.