Mae’r wiwer lwyd yn rhywogaeth estron oresgynnol o bwys sydd wedi lledaenu’n helaeth yng Nghymru. Mae'n effeithio'n andwyol ar boblogaethau'r wiwer goch drwy gystadlu am fwyd a chynefinoedd, a thrwy ledaenu feirws brech y wiwer sy’n lladd gwiwerod coch. Mae'r wiwer lwyd hefyd yn effeithio ar gynhyrchiant pren ac ar gynaliadwyedd a chyflwr cynefinoedd coetir ehangach drwy dynnu rhisgl oddi ar y coed. Gall hynny bydru a diraddio ansawdd y pren, a pheri i goeden gyfan neu ran o goeden farw.
Grey squirrels are a significant and widely spread invasive non-native species (INNS) in Wales. They adversely affect red squirrel populations through direct competition for food and habitat, by spreading the squirrel pox virus which is lethal to red squirrels. Grey squirrels also affect timber production and the sustainability and condition of wider woodland habitat through bark stripping which can lead to rot, down grading of timber and partial or whole tree death.