Are native bluebells (Hyacinthoides non-scripta) at risk from alien congenerics? Evidence from distributions and co-occurrence in Scotland

The UK has the highest density of the worldwide distribution of its native bluebell, Hyacinthoides non-scripta (Liliaceae), and the prevalence of alien bluebells (hybrids or ‘Spanish’) has been interpreted as an urgent threat. To assess the potential for competitive and hybridising interactions between natives and alien taxa in the UK, we quantified abundance and co-occurrence in south-central Scotland in relation to physical variables, land cover, and habitat types. To do this we tested the influence of explanatory variables on incidence rates, densities and group sizes at three spatial scales (10 km, 1 km, and records) in selected 10-km squares. We found that (1) natives were nearly 99% of all bluebells recorded, (2) aliens were encountered more frequently than natives though in much smaller maximum numbers per group, (3) increasing rainfall was associated with increasing native and decreasing alien densities, (4) the presence of aliens related to variables correlated with human density, and (5) there was little evidence for habitat exclusivity. Mixed groups accounted for 10% of natives recorded, and over 40% of natives grew within about one kilometre of aliens. These distributions suggest that a high proportion of natives lie within range of potential gene flow via insect pollinators.