Causes of red squirrel decline

The numbers of native red squirrels are in decline. In this section we look at some of the causes of this decline, both man made and natural.

Grey squirrels

This introduced species from North America remains the greatest single threat to current red squirrel populations. Competition from the grey squirrels generally result in the displacement of red squirrels from broadleaved habitat within 15 years. In Ireland, the current population originates from a few individuals that were introduced into Castleforbes Estate, Co Longford in 1911.
Fortunately, dispersal of grey squirrels in Ireland has been slower than that experienced in England and Wales. This may be attributed to the existence of much smaller areas of broadleaved habitat, and fewer mature hedgerows which act as corridors along which the grey squirrels can travel. Both squirrels compete largely for the same food in a broadleaved woodland. Grey squirrels hold an advantage where food is limited, due to their ability to consume unripe food such as hazelnuts in October. The red squirrel, however, can only ingest ripened nuts, and therefore it is more likely to suffer from food shortages over the winter months. Red squirrel densities tend to be lower than greys, particularly where food shortages exist. This may be a direct result of lower breeding rates when the prevailing conditions are unfavourable. Where food supplies are plentiful, red squirrels appear to breed at similar densities to greys.

Fragmentation of favourable habitat

It is widely believed that the red squirrel became extinct in Ireland in the early 1700's. Tree cover in this period had dwindled from 80% of the land area which occurred after the last ice age 10,000 years ago, to below 2% of the land area. Fragmentation of the remaining broadleaved habitat was probably one of the main reasons for the red squirrel's disappearance. The red squirrel was re-established at ten sites throughout Ireland, between 1815 and 1856, and these were derived from squirrel populations in England,. Although tree cover has increased to 8% in Northern Ireland, it is still considerably lower than the European Union average of 35%. Populations of red squirrels have proved difficult to sustain in wooded areas smaller than 200 ha. Therefore, current long term conservation efforts focus on wooded areas in excess of 2000 ha.

Poxvirus

This is a potentially fatal virus for the red squirrel, but the grey squirrel appears unaffected, and is thought to be a carrier of the disease. The origins of poxvirus are unknown, but antibodies of the virus are present in some of our grey squirrel populations in Northern Ireland. Visible external signs on a red squirrel harbouring the virus include wet, discharging lesions or scabs around the eyes, mouth, feet and genitalia. The first reported cases of poxvirus in Northern Ireland occurred during 2011 at Tollymore and Glenarm forests. Red squirrel populations are now recovering from both outbreaks. The first reported case of poxvirus in the south of Ireland was reported in Hollywood, West Wicklow in December 2011.

Predators

Pine martens, raptors such as sparrow hawks and buzzards, rats, cats and foxes are predators of both red and grey squirrels. However, the effects of predation on Northern Ireland squirrel populations are considered negligible. Research is underway in the National Unoiversity of Ireland, Galway, to determine whether the pine marten has contributed to the sharp decrease in the grey squirrel population in counties Laois and Offaly.