Environmental science is a broad, interdisciplinary field integrating aspects of biology, chemistry, earth science, geology, and social sciences. Mastery of basic science concepts and reasoning are therefore necessary for students to understand the interactions of different components in an environmental system. How do teachers identify and assess the learning that occurs in introductory environmental science courses? In this article the authors introduce a pre-test and post-test in which students draw and label an ecosystem, showing interactions, terms, and concepts. The assignment is open-ended. The drawings can be a useful diagnostic tool for both the student and the teacher. They may also give insight into geographical, cultural, or social biases and provides a different way of assimilating and processing information. Ecosystem concepts seem to be a powerful way of capturing and reflecting student thinking about environmental settings as dynamic systems.