Teaching and learning approaches in ecology very often follow linear conceptions of ecosystems. Empirical studies with an ecological focus consistent with existing syllabi and focusing on cognitive achievement are scarce. Consequently, we concentrated on a classroom unit that offers learning materials and highlights the existing complexity rather than following linear conceptions. We developed an educational unit consisting of 14 individual lessons ("ecosystem lake") with the aim of increasing pupils' cognitive achievement in 8th and 9th grade secondary schools. To evaluate our approach, we chose a quasi-experimental design and five teachers with ten classes participated in our study. A matched-pair tandem-design was provided by five German teachers using first his/her conventional approach in one class (instruction-1; control group) and second, our modern ecological approach in their second class (instruction-2). Pupils subjected to the latter performed better, i.e., achieved higher scores, especially in the delayed post-test. Further, we found a teacher effect and an interaction between treatment (i.e., whether treatment or control class) and the teacher regarding the class test. Teachers had a significant influence on the learning outcome irrespective of the educational unit, i.e., some teachers generally produced higher achievement than others. Further, the interaction showed that some teachers coped better with their traditional teaching. Nevertheless, in retention (delayed post-test) all classes of the modern approach scored higher than the traditionally taught ones. Boys and girls benefited equally from our program and pupils from the modern approach rated the educational unit better. (Contains 1 figure.)