Background: Technologies may successfully be used to mediate, aid, and diversify learning, but several studies indicate that technologies cannot substitute outdoor learning and its experiential nature. To halt human-induced biodiversity loss, teachers should acquire a deep understanding of the principles of ecology, including the challenging concept of biodiversity. Only few studies have investigated learning about biodiversity outdoors guided by technology. Objectives: The aim of this study was to examine student teachers' current knowledge of the concept of biodiversity, to test how well technology-mediated mobile learning in outdoor settings affects their understanding of the concept, and to determine the students' ability to estimate biodiversity in situ. Methods: Student teachers (N = 121) participated in a technology-mediated nature trail. Research data was gathered through open-ended questions in pre- and post-test questionnaires and an assignment during the trail. Data were analysed using both qualitative and quantitative methods. Results and Conclusions: Results showed that significantly, more students included species and ecosystem diversity in their definitions of biodiversity after the trail, but the inclusion of genetic diversity remained low. Student's estimations of biodiversity in situ and related arguments indicate that they had difficulties in connecting their observations with their assessments of biodiversity. Implications: The study indicates that biodiversity is still a challenging concept, but also that outdoor learning mediated by technology has potential to untangle the concept. Understanding the importance of genetic diversity for species survival should be given special attention.