Defining a biodiversity damage metric and threshold using Habitat Directive criteria

• Changes in the environmental suitability of a site for particular plant species in response to nitrogen (N) load can be predicted fairly objectively using model chains such as MAGICGBMOVE. • Lists of positive and negative indicator plant species, such as those in UK Common Standards Monitoring Guidance, provide operational definitions of habitat quality and damage. • Whilst N-sensitive species can provide early warnings of change, they may not be representative of the desirable features of the habitat. • A metric of habitat quality is proposed, based on predicted environmental suitability for positive and negative indicator species. • This metric allows assessment of the impact of N pollution on habitat quality as defined independently of the N effects research community.