Engaging in systemic reasoning about ecological issues is critical for early elementary students to develop future understanding of critical environmental issues such as global warming and loss of biodiversity. However, ecological issues are rarely taught in ways to highlight systemic reasoning in elementary schools. In this study, we conducted semi‐structured interviews with 44 students from the first through fourth grades. Using an iterative process, we developed an empirically grounded learning progression that captures how elementary students use systemic reasoning to explain interactions in ecosystems. This learning progression contains five reasoning patterns: anthropomorphic reasoning, concrete practical reasoning, simple causal reasoning, semi‐complex causal reasoning, and complex causal reasoning. The results also show that many students exhibited mixed‐level reasoning, meaning that they used reasoning patterns at multiple levels to construct a single response. We discuss the implications of the study for learning progression research and teaching ecosystems at early elementary grades. © 2016 Wiley Periodicals, Inc. J Res Sci Teach 53: 1524–1545, 2016