We investigated what students learnt about biodiversity, a broad and multi-dimensional concept, challenging to understand, following a visit to a nature reserve. Acknowledging the individual nature of informal learning, we explore Personal Meaning Maps (PMMs) coupled with semi-structured interviews to investigate learning among 13 scouts aged 13-15 years as they participated in a single guided tour to a nature reserve in Mauritius. PMMs were analysed both qualitatively and quantitatively. Adopting a constructivist approach, biodiversity learning was framed along three lenses, namely, 'ecological literacy', 'biodiversity and society' and 'nature and self', recognising that biodiversity is a socio-scientific concept. Following the visit, there was an increase in ecological literacy through an understanding of the importance of biodiversity, conservation measures and endemic species. Participants retained a species-centred view of biodiversity. Students could better critique biodiversity and society issues and displayed a higher appreciation for nature and the intrinsic value of biodiversity. The visit contributed to an enrichment in biodiversity-related vocabulary especially regarding local endemic and extinct species. Prior knowledge determined vocabulary enrichment but not conceptual additions among individual students. We suggest a wider exploration of PMMs to investigate prior knowledge and subsequent learning during nature reserve visits which can powerfully enhance biodiversity education.