Protected areas are one of the main policy instruments used by policymakers to tackle the current biodiversity crisis. While numerous studies highlight the inability of such areas to protect the full range of biodiversity, the procedures by which protected areas are created nevertheless remain understudied. A better understanding of the related policy processes is necessary to overcome the “research-implementation gap” and, hopefully, decrease biodiversity loss. This article seeks to fill this blind spot in conservation by conducting interdisciplinary research at the crossroads of ecology and policy studies. We applied mixed methods (i.e. quantitative and qualitative analysis) to the historical archives of national nature reserve (NNR) projects to identify the weight of scientific statements and other factors involved in the decision-making process. Our results reveal a two-step process. Scientific opinion about NNR projects operates as the primary filter. Then, another triage is made under social, political and economic interests. Such situation challenges the idea that more evidence would lead to better conservation. In our opinion, the key issue is to determine the ways to improve the success of NNR projects rather than improving data and algorithms. In this sense, we call for the implementation of an “informed opportunism” approach and suggest some leads to favor its practical application. This research highlights the importance of interdisciplinary research to reach conservation goals.