Students have difficulty understanding ecosystem concepts. This article argues that the difficulty stems partly from not grasping the underlying causality that structures the concepts. We report on an intervention study designed to teach eight- and nine-year-olds to reason about domino, cyclic, and mutual causality by infusing causally focused activities and explicit discussion about the nature of each type of causality into a teacher-taught unit on ecosystems. The teacher-taught unit was typical of ecosystems units taught in many elementary schools. The students were third graders from a suburban middle class community and ranged from low to high achieving students. Three conditions were contrasted: 1) activities with discussion; 2) activities only; and 3) no infused activities. Students who participated in both the activities designed to reveal the underlying causal structure and the discussion of the nature of causality showed significantly deeper understanding of the connectedness within ecosystems and demonstrated a significantly better grasp of the process of decomposition and the mechanisms that cause it. The results suggest that it is important to teach students how to structure ecosystems concepts in addition to teaching ecosystems information. (Contains 4 tables and 2 figures.)