Once one of the most widespread and common woodland creatures in the country, red squirrels are now seldom seen in many parts of the UK as numbers have declined significantly over the past century. Still commonly found in isolated regions and throughout the European continent, one of our most historically iconic native species is becoming increasingly threatened in the UK due to a number of factors but most notably with the introduction of grey squirrels from America.
Along with increasing traffic on roads, habitat loss and being affected by growing numbers of domestic predators such as dogs and cats, the introduction of grey squirrels has had by far the biggest impact on red squirrels throughout the UK. According to the Woodland Trust, the three main reasons why grey squirrels are such a threat to reds are:
Grey squirrels are known to carry the squirrel parapox virus which is a disease that is not thought to affect their health but often kills red squirrel individuals.
The larger size of grey squirrels means that they require more food in order to gain the nutrients that they need to survive, and can quickly consume food sources such as green acorns before red squirrels have even found them. This is a particular problem as red squirrels cannot digest mature acorns and therefore rely heavily on the green ones to survive.
Another big reason that red squirrels have been so affected by the introduction of greys, is that the pressure from habitat and food loss means that they do not breed as often so populations have not remained stable.
There are currently numerous projects throughout the UK that help to encourage the conservation of the remaining red squirrel populations, and protect the few places that they can still be found.
Link
https://a-z-animals.com/blog/why-are-red-squirrels-so-threatened-in-the-uk/