Feedback loop reasoning is an important stepping-stone toward systems thinking. To date, studies on feedback loop reasoning in ecology have focused on college students, while only a limited effort has been made to investigate a more general systems thinking of students at the lower elementary level. The goal of this study is to investigate how elementary students reason about feedback loop relationships among interdependent organisms (predator-prey), and address the knowledge sources of students as part of context of reasoning about this topic. This study was conducted in three countries, with 128 first through fourth grader participants from Japan, Lebanon, and the United States. These students participated in semi-structured clinical interviews focusing on the predator-and-prey relationships. We found that the majority of students used one-way causal reasoning, and only a few students reasoned using two-way causality with multiple feedback loops in the predator-prey relationship.