A natural experiment on the effects of high deer densities on the native flora of coastal temperate rain forests

The introduction of Sitka black-tailed deer (Odocoileus hemionus sitkensis Merriam) to Haida Gwaii (Queen Charlotte Islands, BC, Canada) in the late 19th century, provided an opportunity to understand the long-term effects of deer populations on the vegetation of temperate rain forests in the absence of their natural predators wolves (Canis lupus L.), and cougars (Puma concolor L.). Using seven small islands with different browsing histories (no deer, deer for <20 years, deer for >50 years), we tested the long-term effects of high deer densities on plant cover and species richness in the understorey of forest interior and forest edge habitats. Overall vegetation cover exceeded 80% in the lower vegetation layers on islands without deer and was less than 10% on the islands with deer for more than 50 years. Although overall plant species richness was similar on islands with or without deer, plant species richness at the plot scale (314 m2) was reduced by 20-50% on islands with deer for >50 years. The differences were most pronounced for the species-rich edge communities and among herb and shrub species. These results suggest that in the absence of predators, deer have the potential to greatly simplify the forest ecosystem.