Are tree species mixtures too good for grey squirrels?

Abstract

The introduced North American grey squirrel causes problems for foresters by stripping bark from young trees, and for wildlife conservation by displacing the native British red squirrel. Trapping surveys at thirty damage-prone beech and sycamore sites showed that squirrel density was highest where tree diversity was greatest. Surveys of damage accumulated over 5-9 years at fifty-three sites showed that bark-stripping increased with the number of seed-bearing tree species near the vulnerable crop, confirming that vulnerable trees should not be planted close to mature seed-bearing trees, or with a nurse crop of those conifer species which may provide food for squirrels in winter. Young sycamores were more prone to damage than beech, especially in plantations and where pheasants were fed in winter, but damage was reduced where there was a high percentage of ground cover and where Warfarin was used to kill squirrels. Conceptual models are presented to show how factors interact to increase the risk of bark-stripping, and of red squirrel replacement by grey squirrels through the presence of oaks in tree mixtures. Further work is needed (i) to determine how small or isolated plantations should be to minimize risk of damage, (ii) to define why damage reduction is associated with extensive ground cover, (iii) to discover whether tree growth can be managed economically to reduce damage, and (iv) to determine whether the species content of woodland can be managed to ensure the survival of red squirrels in mainland Britain.