The use of scientific models to explain phenomena: Modelling and external representations

Natural and physical sciences have as a goal to explain the material world. To do so, scientists use theories and models. Models can be defined as representations of a phenomenon, simplifications of the phenomenon to be used in inquiries to develop explanations of it (Gilbert et al., 2000). These models are not immutable but are continually reviewed by the scientific community using empirical and conceptual criteria. In science it is important for students to learn the content knowledge of the discipline, but it is also crucial that they understand and participate in the processes by which knowledge is originated. However, in most secondary school classrooms, students use models as objects of communication, they rarely construct them and almost never reflect on their meanings (Grosslight et al., 1991).

The role of external representations in learning is being reappraised, by an approach conceiving them as forms of knowing. According to Pérez Echeverría and Scheuer (2009), besides contributing to make knowledge content more visible to learners, external representations expand their consciousness about how do they relate to the (represented) content and how do they change as they learn. This metacognitive dimension is relevant for our study, as we are interested in the metaknowledge about epistemic practices, that is in the learners' knowledge about the nature and role of models (Schwarz et al., 2009), be them theoretical, curricular or teaching models, and in this case, also about the nature and role of representations. As Schwarz et al. point out such knowledge helps to make the practice purposeful for learners. Learners need to understand how models (and, we add, representations) are used, why they are used, and their limitations.

In ecology, diagrams of trophic pyramids constitute a relevant type of representations, which are present in textbooks or electronic documents about ecosystems. We think that this presence implicitly assumes that students do not experience difficulties for understanding the meaning of these images. Perhaps in connection with this assumption, students are not usually asked to produce the representations themselves. Although there is a body of research about ecology learning, only one study (Adeniyi, 1985) was located addressing students' understanding of trophic pyramids.

This study makes part of a research project about students' engagement in the epistemic practices of modelling and argumentation in a teaching sequence about trophic pyramids and energy flow in ecosystems. We start from the assumption that producing external representations of the trophic pyramids is a necessary dimension in students' appropriation and use of the ecosystem model. The research objective is to analyze the process of production by 10th grade students of representations of trophic pyramids, which is split in two dimensions:

1) To analyze the production of external representations of trophic pyramids in terms of students' discursive moves across different worlds of knowledge.

2) To analyze students' appropriation of meanings for the representations of trophic pyramids explaining their shape, syntactic relationships and patterns, in terms of students' discursive moves. 

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