Identifying uncertainties in judging the significance of human impacts on Natura 2000 sites

In European nature conservation law, Natura 2000 sites are protected towards ensuring biodiversity through the conservation of natural habitat types and of wild fauna and flora. Anyone planning a potentially harmful activity needs to assess significant effects on a site's conservation objectives. While EU case law currently demands certainty provided by science, we will show that science can never rule out uncertainty. We distinguish three sources of uncertainty: ignorance (inadequate understanding), unpredictability of ecological system behaviour and ambiguity in the science–policy interface. Only ignorance can be solved by science alone. We will specify sources of uncertainty encountered in the significance decision procedure as part of the assessment of article 6 Habitats Directive. We will explore how they affect the use of knowledge during the three steps of the assessment process, i.e. identification of site conservation objectives, predicting the impact of the planned activity and assessing the significance of any effects on the Natura 2000 site. The claim that certainty has to be provided by science is unrealistic, because policy causes a good deal of uncertainty affecting how science can operate. This is discussed in the light of a common learning process by science and society. The European precautionary principle should not be limited to ignorance alone. Within the precautionary principle risk reduction measures can be allowed and thus uncertainties could be accepted, including those uncertainties caused by unpredictability and ambiguity. Finally we propose strategies to manage uncertainty in nature conservation and law planning.