Habitat Loss
Forests are no longer simply a habitat for animals and birds. Many are managed by timber companies, which plant trees most suited to their business needs, irrespective of the effect this has on wild fauna populations. Recent forestry plantations tend to favour grey squirrels. If forests were composed of Norway spruce, larch and Douglas fir, red squirrels would be well catered for, and grey squirrels would be discouraged.
Displacement
There are areas of Britain where greys and reds have co-existed for a number of years but greys are hardier than reds and able to live in a wider variety of habitats. The competitive advantage they have is greater in deciduous woodlands but in coniferous woodland, the two can coexist for longer. Inevitably, species that can best adapt to their environment survive.
Disease
Red squirrels are particularly susceptible to disease, including a lethal pox virus. Whether grey squirrels were harbouring this virus when they were brought to the UK, or whether it was already endemic in the native reds by then is unclear but it is known that grey squirrels carry the disease without developing it whilst reds usually succumb. When the reds die out, greys take over their niche. However, a 2008 study found that some red squirrels had developed an immunity to this virus – an indication that some red squirrels, at least, can survive it.24