There are three principle causes of the decline and retreat of the red squirrel in Wales:
Competition from grey squirrels
The grey squirrel (Sciurus carolinensis) is not native to Britain. They were intentionally introduced to the UK in the 1870s. Since the original introduction of a small number of animals, grey squirrel numbers have climbed to more than 2.5 million and they are now thought to outnumber red squirrels by approximately 66-1.
Greys can colonise woodlands at a rate of six miles per year and reds can disappear within 15 years of the arrival of greys to an area. The greys don't actually drive reds out or attack them. However, the presence of greys often means reds do not have enough to eat.
Although we now think of red squirrels as a species of coniferous forest, they would originally (and preferentially) have inhabited our broadleaved woodlands, using acorns as a major food resource.
Grey squirrels, unlike reds, having originated from hickory and oak woodlands of eastern North America, are better able to digest acorns, coping better with high concentrations of toxic tannins found in them. In broadleaf woodlands therefore, greys are more efficient at digesting acorns and end up dominating food resources. This competitive exclusion from food sources causes weight loss, reducing their chances of surviving the winter and breeding successfully.
The grey squirrel is also larger than the red. Their extra body weight means they can store three to four times more fat than red squirrels and so have a better chance of surviving the winter. Greys also produce more young than reds and live at higher densities (more squirrels per hectare) making them a stronger contender in the survival stakes.
However, although the grey squirrel has the advantage over reds in broadleaf woodlands, they are less able to dominate in coniferous woods. Conifers produce much smaller seeds and are therefore a less efficient food source for the large grey squirrels, who are also less able to feed in the very tips of the conifer limbs. As a result in such habitats reds can still persist, by eating the small seeds which do not provide enough nutrition for the larger greys.
However, care still needs to be taken to avoid planting large-seeded broadleaves nearby, or dominance of larger-seeded conifers such as pines, to prevent the woodland becoming too favourable to grey squirrels again.
Reds favour particular species within conifer forests : Norway Spruce, Scots Pine, Japanese Larch and Lodgepole Pine are all used. The proportion of species in the Tywi forest represents both an opportunity and a threat depending on how it is managed into the future.
Habitat destruction and fragmentation.
A significant decline in broadleaved tree cover, increased fragmentation and overgrazing, causing a reduction in regeneration, contributed to the decline in 19th century of red squirrels. This however cannot fully explain the current decline.
Ironically, the more recently planted conifer woods do, in fact, provide some new suitable food resources for reds and may now also help their future conservation, for the reasons described above. Grey squirrels are also better able to move through a fragmented habitat than reds.
Red squirrels are at risk from the deadly Squirrel pox virus (parapox), capable of devastating red squirrel populations. The Squirrel pox virus can be carried by grey squirrels without causing them harm, but red squirrels have no immunity.
Once infected, reds will die within a matter of weeks, or even days, as there is no cure. Squirrels may be lethargic and shivering with scabs or lesions around the eyes and nose.