We studied the initial conceptions of 73 pre-service primary teachers regarding the concept of ecosystem and examined how their understanding evolved as a result of participation in an inquiry-based learning exercise. The inquiry process involved identifying students' initial conceptions, making them aware of these, comparison of their ideas with scientific knowledge and knowledge building through activities in which they analysed points of agreement, discrepancies and conclusions. The activities were performed in groups and centred on the production of posters, which participants were required to compare in both the first and final sessions. This comparison, together with the qualitative analysis of the content of the posters, was carried out using a rubric designed on the basis of a literature review. The results showed that students progressed in their understanding of key aspects related to the concept of ecosystem. In particular, they became more aware of the role that humans play within such systems, although they continued to have difficulties with aspects such as identifying species in the aquatic ecosystem and discriminating between biotic and abiotic components.