French environmental public policies on biodiversity are designed drawing upon scientific expertise. The framework of this expertise has been deeply remodeled in the past decade through a series of reforms and the creation of new institutions. This exploratory study was based on interviews with stakeholders working at the interface between scientific knowledge and political action. The concept of “expertise” describes the difficult and complicated relationship between science and action, which is not specific to biodiversity policies but which lead to a feeling of uneasiness for scientists and policymakers. The reforms engaged in the last decade are then often justified by this uneasiness described by interviewees when thinking about expertise processes. However, our results indicate that the recent institutional proliferation is perceived as unsettling by both scientists and policymakers, comforting the existing difficulties. To go beyond these difficulties and meet the demand for expertise, experts adopt what can be described as three archetypal postures – ensuring sound science as “guarantors”, defending conservation principles as “guardians”, or working closely with policymakers as “agents”. "The influence of "guarantors" and "agents" has grown" over time at the expense of traditional “guardians”. Results are analyzed with regard to the strategic stakes of each stakeholder and to the recent changes in academic and political actions. Finally, new research perspectives are discussed to enlarge our results.