As human activities threaten biodiversity on a global scale, preventing further biodiversity loss requires scientifically literate and environmentally responsible citizens. Biodiversity education (BE) as part of education for sustainable development (ESD) may lay the foundation of fostering a sense of responsibility for biodiversity and its preservation among future generations. Meaningful learning depends on the integration of new knowledge into already existing student conceptions (i.e., students’ pre-instructional mental concepts and ideas of teaching subjects). Thus, assessment and consideration of student conceptions on biodiversity are of high relevance for the development of successful BE and ESD lessons. In the present qualitative study, we applied three open-ended questions in a paper-and-pencil format to assess adolescents’ pre-instructional conceptions on biodiversity, its benefits, and protection. Almost our total sample of 275 German 10th graders equated biodiversity with species diversity, revealing an only fragmented understanding of the scientific concept of biodiversity. Conceptions of the benefits of biodiversity were almost equally balanced between environmental and economic benefits. Notions of environmental preservation considerably outnumbered notions of environmental utilization in student conceptions of biodiversity protection.