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Factors affecting red squirrel populations

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.

© I. Macho, Llyn Brianne

Competition from grey squirrels.

Red squirrels are under increasing pressure from non-native grey squirrels. The grey squirrel was introduced to the UK in the 1870s. Since the introduction of a handful of animals, grey squirrel numbers have climbed to more than 2.5 million and they now outnumber reds by 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. Acorns are one of the main ways reds fatten themselves up to get through the lean winter months. Greys, 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. Here reds can eat the small seeds of the coniferous trees, which do not provide enough nutrition for the larger greys and so enable their survival in this habitat.

Reds favour particular species within conifer forests : Norway Spruce, Scot’s Pine, Japanese Larch and Lodgepole Pine are all used but these are at low density within the Sitka-dominated plantations and may not be coning. This means that even though the conifer habitat area in Mid Wales is large only a very small area will be optimal for reds.

Disease.

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.