The presence of the grey squirrel in Italy constitutes a serious threat to the survival of the red squirrel in our country and throughout Europe. The two species are, in fact, in competition and in areas of introduction of the gray squirrel extinction of red squirrel occurs.
The causes of this ecosystem alteration have been investigated in researches conducted in Italy and Great Britain. The hypothesis of direct interference, through aggression between animals or linked to reproductive behavior, was considered invalid. The two species also do not hybridize.
The coexistence of similar species in the same area is possible if there is a difference of diet, use of space and activity rhythms between the two species. The researchers, investigating the characteristics of the two species, found large differences in how they use space and move. Diet is very similar, but some very important difference make a difference for the possibility of cohexistence between the two species.
In deciduous forests, American squirrels feed on acorns (the seeds of oak trees) more intensely than red squirrels, since they can better digest certain substances, such as tannins. In addition, there is evidence that grey squirrels are able to find and consume part of the seed reserves (up to 50%) collected by common squirrels for the winter season. Considering that the body weight, survival and reproduction of the females after the winter increase with the consumption of seeds stored (cached tree seeds), a smaller amount of seeds recovered from the red squirrels will affect their chances of survival and reproduction. The competition for food resources due to a lower growth in young red squirrels and thus a lower survival in the first year of life, result in population decline and local extinction in a few years. In coniferous forests grey squirrels occupy the forest areas with a greater availability of food, pushing the red squirrels in the poorest areas. The results of the competition in these habitats will be further investigated, however, for the time being the American species found mainly in deciduous and mixed forests.
In Britain, the competitive exclusion between these two species is also mediated by the presence of a squirrel Poxvirus: grey squirrels are resistant to the virus and act as carriers, while the red squirrels that come into contact with it die in a very short period of time. In this case the process of replacing the red squirrel by the grey squirrel can be accelerated by twenty times. The Poxvirus is widespread in England and Wales, but some squirrels infected with the virus have been found recently also in Scotland, where the virus was not recorded before. Squirrels living with HIV have been found in Ireland, but there are no confirmed cases of the disease in red squirrels. So far the virus has not been found in Italy, but up till now there have been no specific studies. A study is underway in the frame of the LIFE project.