A team from Italy, Germany and the United Kingdom looked at the case of the eastern grey squirrel (Sciurus carolinensis), originally from North America, which has virtually wiped out the native red squirrel (Sciurus vulgaris) in large parts of the British Isles and northern Italy where it spread after being introduced more than a century ago.
Using records on the occurrence of the grey squirrel both in its native and introduced ranges, accessed through GBIF, online collections and field observations, the researchers compared three different models to see which best matched the actual spread of the invasive squirrels.
One model used only data from the squirrel’s home in eastern North America to calibrate the mammal’s ‘niche’ – the range of climate conditions such as maximum and minimum temperatures and rainfall in locations where it has been recorded. When this niche was projected onto the UK, the model excluded large parts of eastern Britain where the squirrel has thrived in wetter and colder conditions than in its native habitats.
A second model using only records from the invasive range predicted occurrence in large areas of central and western North America where the squirrel has never been found. Only a third model using records from both ranges accurately predicted the areas where the squirrel has spread.
The authors conclude that this supports the hypothesis that the species shifted its climatic niche after it was introduced to new European environments – in other words it has colonized areas very different from those in its native range. They suggest that risk assessment based only on native occurrences is likely to underestimate the areas an alien species might invade – and recommend caution in using such models to construct ‘white lists’ of species assumed to pose a low risk of invasion.