在重疊區域，灰松鼠(Sciurus carolinensis)引起紅色松鼠的消失(請參閱Sciurus vulgaris在 IUCN 紅皮書 )經過競爭的排除。啃食樹皮的行為也可能造成森林地區的損害，對西克莫無花果( Acer pseudoplantanus)與山毛櫸( Fagus sylvatica)影響特別嚴重。(Bertolino 與 Genovesi,2003)灰松鼠也是一個痘病毒的載體，紅松鼠會受其感染。有灰松鼠的地方，紅松鼠趨向的原因，可能在此； 此現象稱為病原體居間競爭。(Gurnell et al.2006)松鼠會挖出球莖、吃裝飾性植物的樹皮，而成為一個對花園有害的生物。
United Kingdom (UK)
傳播疾病: The incidence of squirrel poxvirus (SQPV) in red squirrels appears to be related to the level of presence of the grey squirrels (Sciurus carolinensis) in an area, suggesting they are involved in transmission of the disease. The presence of the disease significantly increases the rate of replacement of the native red by the grey squirrel by as much as 20 fold. Greys can be infected by SQPV, but it appears to be benign (Mayle, B. and Smith, L., in press).
捕食: Grey squirrels (Sciurus carolinensis) predate the nests of a wide range of bird species both in North America and Britain. Species most at risk are those with open nests in the
canopy, although birds nesting on the ground and in the understorey are also vulnerable (Mayle, B. and Smith, L., in press).
棲地改變: Grey squirrels (Sciurus carolinensis) effect woodland structure and composition and sustainable woodland/forest management through damage to tree stems and branches, by feeding on
seeds and plant bulbs (e.g. the English bluebell, Hyacinthoides nonscripta) thus preventing regeneration (Mayle, B. and Smith, L., in press).
物理干擾: Grey squirrels cause serious damage by stripping the bark from trees. This exposes the timber to fungal and insect attack, disrupts the flow of nutrients up
the tree, and weakens the stem (Mayle, B. and Smith, L., in press).
競爭: Grey squirrels (Sciurus carolinensis) compete for resources with with native small mammals including mice, voles and in particular, the red squirrel (see Sciurus vulgaris in IUCN Red List of Threatened Species) and common dormouse (see Muscardinus avellanarius in IUCN Red List of Threatened Species) (Mayle, B. and Smith, L., in press).
農業: Damage by grey squirrel (Sciurus carolinensis) is a major concern to the forestry industry, and the current national policy of increasing the level of broadleaf planting is
being seriously undermined (Mayle, B. and Smith, L., in press).