Grey squirrels have two main adverse impacts on UK biodiversity.
Damage to trees and woodland
The first of these is the enormous damage they cause to trees, stripping bark and in many cases causing the death of the tree. The squirrels strip the bark to access the ploem tissues below the bark, which contain sweet tasting sap. This phloem is responsible for the movement of sugars around the plant and so removal of this layer results in the death of the tree if large amounts are removed, like in the photograph below. Removal of even small amounts of bark and associated tissue will check or restrict the growth of trees, and lead to damage or fungal infection, which in turn can lead to the death of the tree.
This bark stripping usually occurs between late April and the end of July. Very young trees or saplings (with a stem diameter less than 5 cm) are generally not attacked as they cannot support the weight of a grey squirrel, the main stem of older trees (40 years or older) are usually safe as the bark is too thick for the squirrels to strip. The most vulnerable trees are sycamore, beech, oak, sweet chestnut, pine, larch and Norway spruce, aged between 10 and 40 years old; though almost any broadleaved species of tree can be attacked. One of the reasons Trewithen was so keen to have their captive breeding pen of reds is that in 2003 they lost an entire 3.8 acre newly planted native broadleaved woodland to grey squirrel damage.
The economic cost of this damage to the UK is estimated at over £50 million per year for commercial forestry alone. The real cost of damage is many times this figure, as the majority of forestry in the UK is not commercial, and damage to gardens, private woodlands and public parks is usually not quantified.
An example of this is at Tehidy Park near Camborne, where the high winds and wet weather of May and June this year resulted in many of the Sycamores having their tops blown out, having been too badly damaged by grey squirrels to withstand the weather.
Egg and chick predation
The second major impact on UK biodiversity by the grey squirrel is the predation of eggs and even chicks by grey squirrels. Studies in the past have shown significant decreases in fledging succes rates of some woodland birds where grey squirrels are found. The Game and Conservancy Wildlife Trust is in its final year of a three year study this year, which shows similar findings, and will be published in early 2013.
This picture, taken by Nigel Housden, shows a grey squirrel with two spotted flycatcher eggs in its mouth. This is a red listed bird, and so its numbers are already extremely low. A past British Trust for Ornithology study has also shown a significant negative impact on blackbird, Eurasian collared dove, green woodpecker, long-tailed tit and Eurasian jay from grey squirrel predation.