Can niche use in red and grey squirrels offer clues for their apparent coexistence?
Introduced species are, worldwide, one of the most serious threats to biodiversity. Grey squirrels Sciurus carolinensis are one of many introduced species to have threatened a native congener; they are thought to have replaced red squirrels S. vulgaris throughout much of the UK as a result of competition. The rate of competitive replacement may be influenced by habitat composition, with some red squirrel populations persisting for prolonged periods in the presence of greys in predominantly coniferous forest. Here the similarity of red and grey squirrels' pattern of habitat use was investigated in Craigvinean forest in Scotland, UK, a site that has experienced apparent coexistence for up to 30 years. Overlap was examined in several dimensions: spatial overlap of home ranges, dynamic association and niche overlap. Habitat selection was examined at three levels: selection of core home range areas, selection of tree species within the home range, and the characteristics of patches used intensively by each squirrel species in comparison with random locations within their home range. Although there was overlap between red and grey squirrel ranges, there were clear differences in the macrohabitats utilized, with red squirrels selecting areas of Norway spruce Picea abies and grey squirrels selecting riparian corridors of mixed woodland for their home ranges. Within their home ranges, habitat selection by individual red and grey squirrels was similar, but again with reds selecting Norway spruce and greys selecting patches of mixed conifers and broadleaved trees. As no habitat variables consistently affected the microdistribution of red and grey squirrels within blocks or 'stands' of trees, stands that were used were thought to constitute good and relatively homogeneous habitats for squirrels of either species. There was no evidence to suggest that red and grey squirrels avoided using the same areas at the same time, and potential niche overlap was considerable (0.77). However, partitioning of habitats may have reduced competition between red and grey squirrels and hence have contributed to red squirrel persistence at this site. This work: (i) reinforces earlier proposals that forest management offers a promising tool to assist the conservation of red squirrels; (ii) raises the issue of determining the spatial scale at which coexistence operates; and (iii) offers an illustration of how the management of invasive species can be mediated through the manipulation of niche availability.