Scientific evidence will be better incorporated into conservation action and environmental policy if it is deemed credible by decision-makers. However, huge uncertainties are inherent in large-scale ecosystem statistics. Two wetland case studies relating to mangrove areal extent in Peninsular Malaysia and saltmarsh loss in the UK indicate the major causes of information uncertainty, relating to poor methodological description, traceability and assumptions associated with the use of grey literature. Furthermore, potentially inaccurate information can be propagated throughout the research community and gain ‘proof by assertion’, especially if the information originates from an authoritative source. Researchers must better consider implicit and explicit uncertainty and be critical of secondary information. Future information production requires the use of rigorous peer-reviewed methodologies in order to decrease and quantify error. Such steps will increase the credibility of scientific evidence, so researchers can better contribute to a two-way science and policy deliberation approach.