Phylogeny explains why less therapeutically redundant plant species are not necessarily facing greater use pressure

Understanding which factors influence medicinal plant species selection and harvest or use pressure can provide valuable insights for sustainable management of natural resources and conservation efforts. The utilitarian redundancy model, a theoretical framework in ethnobotany, suggests that species that are therapeutically redundant or fulfill similar therapeutic functions within traditional ethnomedicine are less likely to be under greater use pressure. However, species’ evolutionary relatedness and the preference of certain species over others to treat a given illness can directly affect how use pressure is diffused across several groups of species. These factors may alter the strength of the therapeutic redundancy-use pressure relationship.

Medicinal plant species that fulfill the same therapeutic functions may experience greater use pressure despite their level of therapeutic redundancy because they are preferred—where most people select these species preferably over other species that are equally available for a given treatment. Further, species that are closely related evolutionarily may be more likely to be harvested not because they are therapeutically unique but because they share evolutionary traits such as secondary chemistry with other medicinally important species which may make them more prone to being harvested.

We investigate the effects of species therapeutic redundancy, use value, preference, and evolutionary relatedness on species use pressure in the Shipibo-Konibo community of Paoyhan in the Peruvian Amazon region. We used phylogenetic generalized least squares models to identify significant predictors of species use pressure for 62 medicinal plant species cited by 30 participants and fulfilling 31 therapeutic functions in Shipibo-Konibo ethnomedicine.

Our model controlling for species’ shared evolutionary history indicated that therapeutically redundant medicinal plants experienced greater levels of use pressure. However, as preference increased, the effect of therapeutic redundancy on species use-pressure became less positive. Contrary to predictions, species preference by local people alone did not predict use pressure. Further, when we control for species’ shared evolutionary history, the effect of preference on species use pressure was dependent on therapeutic redundancy.

Our study illustrates the importance of controlling for evolutionary relatedness between species in studying plant-human interactions and the complexity involved in employing the utilitarian redundancy model to inform management and conservation efforts.

Methods
We used semi-structured interviews and free listing, an elucidation technique commonly used in the social sciences that seeks to identify specific information on a given cultural domain of the investigated community (Albuquerque et al., 2014).  As such, each participant was asked to list the medicinal plants they know and their uses for treating illnesses in ethnomedicinal contexts according to the participants’ emic perspective.  Semi-structured interviews were used because they allow for greater flexibility during the interview process compared to structured interviews and they allow for guided or specific data collection on an given cultural domain compared to unstructured interviews.  Further, this approach allowed for more precise data collection as we aimed to get an in-depth understanding of medicinal plant use by knowledgeable participants without having to interview the same participant more than once, which, is not always possible (Albuquerque et al., 2014; Bernard, 2017).  These approaches were coupled with focus group discussions supplemented by participant observations and walk in the woods (Albuquerque et al., 2014) to collect data that were used to estimate species therapeutic redundancy, use values and preference and to test their effects on species use pressure of medicinal plants used by the Shipibo-Konibo for healing.  The use of such triangulation of methods recommended in ethnobiology seeks to ensure the reliability of data collected where one method is used to verify or cross-reference the responses obtained from another elicitation technique (Albuquerque et al., 2014).