In this study, I utilized a learning progression framework to investigate lower elementary students (G1-4) systemic reasoning in ecology and I related students reasoning to their sources of knowledge. I used semi-structured interviews with 44 students from first through fourth grade, four teachers, and eight parents. The results revealed that a hypothetical learning progression begins with anthropomorphic reasoning as the lower anchor and ends with complex causal reasoning as the upper anchor for students in this age. However the results showed that many students revealed mixed-level reasoning--meaning that they can reason at different levels in the same context. Very few students were able to use scientific terms and even those who did use the terms were not able to capture the scientific meaning of those terms. The results also revealed that students' accounts about scenarios in the various categories of systemic reasoning were inconsistent. Finally, the results concerning sources of knowledge revealed that students acquire their ideas from various sources, the media being the most frequently mentioned source, followed by books, personal experiences and parents. Those results have implications for defining the learning progression in general, for the validation of the hypothetical learning progression and for practical development of curriculum and instruction. With regard to defining the learning progression, the presence of mixed-level reasoning opens the discussion whether learning progression levels should be strictly pure levels or should include combinations of various levels to identify students' reasoning. With regard to validation of the hypothetical learning progression, the inconsistencies of students' answers and low correlations across various categories of systemic reasoning suggest that the categories used in this study were distinct. Finally the results of students' sources of knowledge has implications for designing a curriculum that capitalizes on students interests such as media and general books and scaffold students to move them up to scientific reasoning. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]