This study examines to what extent elementary students use feedback loop reasoning, a key component of systems thinking, to reason about interactions among organisms in ecosystems. We conducted clinical interviews with 44 elementary students (1st through 4th grades). We asked students to explain how populations change in two contexts: a sustainable ecosystem and an ecosystem that is missing predators. We used an iterative process to develop a learning progression for feedback loop reasoning, and used the learning progression to code interview episodes. The study produces three findings. First, very few students recognised the cyclical relationships among populations in a sustainable ecosystem (Level 7). Second, very few students identified both reproduction and food as the factors affecting population in a context missing predators (Level 4). Finally, students' reasoning was inconsistent across the two contexts. We also discuss the implication of these findings for teaching and learning of food webs at elementary school.