Background/Question/Methods Research shows that students tend to reduce complexity to simpler patterns and that this results in difficulty achieving deep understanding of fundamental ecosystems concepts (e.g. Grotzer & Basca, 2003). The Understandings of Consequence Project, funded by the National Science Foundation (NSF), has investigated students’ reasoning and mined the extant literature on students’ reasoning about patterns of causality (domino, cyclic, mutual, relational), obvious vs. non-obvious causes, and time delays and spatial gaps between causes and appearance of effects. Helping students learn to broaden their understanding of the nature of causality through explicit reflection on causal dimensions during the course of subject specific learning, results in significantly deeper understanding of ecosystems concepts (Grotzer & Basca, 2003; Mittlefehldt & Grotzer, 2006)  The current work outlines findings from an accompanying qualitative investigation that considered tacit assumptions that students brought to this learning. The methodologies included open-ended interviews conducted before, during, and after learning, students’ in class responses, drawings, and written explanations. The work was carried out in third through eighth grades in schools with diverse populations of students.
Results/Conclusions A set of tacit assumptions were identified that interact with how students interpreted ecosystems and food web concepts including balance and flux, passive energy transfer, opportunistic feeding relationships, caloric storage, competition and availability. For instance, students who viewed the organisms in a food web as being similar to a menu were more likely to miss understandings related to opportunistic feeding or the need for caloric storage than students who did not use this analogy.  This in turn appeared to impact their ability to understand the potential impact of direct and indirect effects in the ecosystem on particular organisms. The assumptions are analyzed in terms of students’ experiences as defined by exposure to certain kinds of information and to the cultural value of certain sets of ideas. The presentation will outline tacit assumptions that we identified and consider how these interact with students’ causal and ecosystems concepts. The results are analyzed in terms of particular cohorts of students at a particular interval in time during which they had a number of shared, relevant experiences both in school and out of school.