Native dominants in British woodland – a potential cause of reduced species-richness?

The invasion of native habitats by alien species has received considerable attention. However, in Britain high levels of dominance by a small number of aggressive native plant species may have an equal, or greater, impact on the richness of native woodlands. Here, we examine this hypothesis by modelling the realized niche of native-dominant species along the principal coenocline of British woodlands, and examined niche overlaps with 78 woodland specialist species and two alien species. Four native species had a much greater cover than all other field-layer species, and between them they entirely covered the response range of all other field-layer species, replacing one another along the coenocline. These findings, combined with autecological information suggest that Hedera helix, Mercurialis perennis, Pteridium aquilinum and Rubus fruticosus have the potential to become ‘over-dominant’ and perhaps may impinge on other field-layer species. Our results also identified which field-layer species are likely to be impacted by a change in abundance of each of these dominant-species, and as such, provide a novel quantitative method of risk assessment to aid conservation policy. Understanding how woodland communities remain diverse, even in the presence of aggressive native species, may provide insights into how the impact of exotic invasive species can be managed.