There is growing recognition of the importance of helping children to develop an ability to think about biological and environmental issues in terms of systems interactions and impacts. Several progressions have been published that suggest how their conceptual understandings may develop over time. However these are not necessarily as informative for teachers as for researchers or specialist resource developers, nor do they take account of "moment in time" interactions between an individual's contextual and conceptual knowledge. This research aimed to develop examples to support assessment for learning by helping teachers recognise students' next learning steps in relation to interactions between the components of an ecosystem (both conceptual and contextual) with which the children had varying degrees of familiarity. (Contains 3 figures and 3 tables.)