The local Biodiversity Action Plan recommend land management measures to control the spread of ragwort and bracken and identify four non @-@ native , invasive species as threatening native biodiversity : Japanese knotweed , rhododendron , New Zealand flatworm and mink . It also identify problems of over @-@ grazing resulting in the impoverishment of moorland and upland habitats and a loss of native woodland , caused by the large numbers of red deer and sheep .