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Box 1 The utilitarian redundancy model and the utilitarian equivalence model

From: Taste and chemical composition as drives for utilitarian redundancy and equivalence: a case study in local medical systems in Northeastern Brazil

Utilitarian redundancy model (URM): the URM derives from the ecological redundancy [18] by adopting a functional perspective in the analysis of natural resource use by human populations, and it evaluates the role of functional overlap within a socioecological system (see Albuquerque and Oliveira [19]). This analytical perspective arose from the observation that several species within a local medical system are used for the same therapeutic function—for example, mint and lemon can be both used to treat colds—they are culturally redundant regarding local use indications [20]. The model can be applied to any function of a socio-ecological system, not only medical ones. The URM model is based on the following assumptions: (a) species have different functions within social-ecological systems, but a level of overlap in function (i.e., redundancy) occurs; (b) increased redundancy promotes resilience in social-ecological systems, and (c) redundancy depends on the knowledge characteristics and practices of a given human community [20]. For this study, we propose that redundant species in a medical system were selected for the same therapeutic function because they have a similar taste or chemical components
Utilitarian equivalence model (UEM): The UEM is an operational concept based on Odum's [21] ecological equivalence model, which aims to understand the cases of overlapping of useful species in different socio-ecological systems [17]. Utilitarian equivalence, thus, indicates species that are used for the same purposes or similar purposes (not only medicinal ones) in different socioecological systems. Especially for medicinal use, equivalent species provides the ideal scenario to seek common selection criteria, in order to identify the shared characteristics among the equivalent pairs and consequently the main types of perceptions or stimuli, which led to the inclusion of such species in different local medical systems [17]. The model assumes that: (a) utilitarian equivalence, understood as the high overlap of use between two species in distinct socio-ecological systems, is relative and not absolute, since, in the absence of intrinsically identical plant species or culturally equal peoples, the medicinal uses are not necessarily identical, but rather similar; (b) equivalence is due to two groups of complementary variables, cultural traits and environmental factors; (c) the evolutionary events that led to utilitarian equivalence may be associated with the similarity between intrinsic characteristics of useful species. Thus, in this study we rely on one of the predictions of the UEM, that plants of distinct medical systems tend to share certain traits in common, such as taste and chemical compounds