Biology distinguishes 29 species of squirrels. Here in Central Europe, however, we mainly see the Eurasian squirrel, which usually has a reddish fur; in winter, however, it can also be dark brown to black. If you want to distinguish it from the North American gray squirrel, you should better orient yourself to the ears, because the gray squirrels lack the brush-shaped hairs there.
The fact that you have to distinguish the animals that are actually native to different continents from each other at all is due to British travelers from the 19th century. They thought it was a good idea to bring grey squirrels from America and leave them in the UK. Since then, they have been spreading rapidly there, displacing the native Eurasian squirrels. Andy White from the University of Edinburgh has spent years working on exactly how this works with mathematical models based, for example, on such formulas:
These five equations describe the time development of the hearing populations, whereby the domestic animals are marked with the index R and gray horns with G. The other variables describe parameters such as mortality and reproductive rate. Anyone who has a little idea of theoretical biology will recognize a SIR model in the equations: S refers to individuals who can infect with a certain disease (susceptible), with i those who are already infected and with r those who have recovered from the disease (recovered). The fact that you have to deal with a model to spread contamination diseases when dealing with squirrels is due to the Squirrelpox virus that triggers the squirrelpox.
In the 1980s, it was found in Great Britain that the grey squirrels displaced the native ones, although no aggressive behaviour was actually observed. The animals originating from America were better at finding their food supplies in winter and also tended to plunder the hiding places of the Eurasian squirrels. Nevertheless, the mathematical models of the time showed that they should not spread as fast as one observed. The reason for this was discovered later: the American squirrels transmit a smallpox virus to which they themselves are largely immune; European animals, on the other hand, have no defences and die quickly if they become infected.
Squirrels in quarantine
Only when you introduce epidemiology into the population models do you get results that describe reality properly. Which is also urgently needed if you want to protect the local species. Andy White's work has shown that the prevailing strategy - the mass killing of the gray hearing - is not very useful. It makes more sense to ensure that the animals do not get in the way. Where large populations exist on Eurasian squirrels, they have to be protected, almost a "lockdown". Among other things, this has an impact on forestry, which, for example, can influence the spread of animals and their mobility behavior by falling trees (or reforestation).
The animals brought to the UK have not yet spread to Central Europe. However, after the Second World War, grey squirrels were also abandoned in several parks in Italy. They have not yet spread further north and the smallpox virus has not yet appeared. But sooner or later they will probably come to us too – at least that's what the computer models predict.
