This study describes how we developed a learning progression (LP) for systems thinking in ecosystems and collected preliminary validity evidence for the LP. In particular, the LP focuses on how middle and high school students use discipline-specific systems thinking concepts (e.g. feedback loops and energy pyramid) to analyze and explain the interdependent relationships in ecosystems and humans' impact on those relationships. We administrated written assessments with 596 secondary students. Based on the data, we developed and validated an LP for systems thinking in ecosystems. The LP contains four levels that describe increasingly sophisticated reasoning patterns that students commonly use to explain phenomena about interdependent relationships in ecosystems. We used a Wright Map based on the Rasch model for polyotmous data to evaluate the validity of the LP. We also used the LP to compare the performance of students from different subgroups in terms of socioeconomic status (SES), gender, and school settings. The data suggests performance gaps for students with low SES and students from urban schools, but not for other traditionally under-served or under-represented groups such as female students and students from rural schools.