Phylogeny explains why less therapeutically redundant plant species are not necessarily facing greater use pressure.
Abstract
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 fulfil 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 fulfil 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. Furthermore, 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. Furthermore, 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.