Understanding the patterns and processes surrounding plant use has been at the forefront of ethnobotanical research since its inception. Several theories and hypotheses in ethnobotany have been proposed recently to facilitate a greater understanding of the roles culturally important plants play among human societies in addition to the factors that influence plant selection, harvest and use-pressure. Cultural keystone species are plant and animal species considered irreplaceable to cultural communities and expected to play fundamental roles in maintaining cultural community structure and cultural stability. Although this theoretical framework in ethnobotany has been proposed to help inform biological and cultural conservation strategies, it is unclear if quantitative methodologies often employed to measure or infer cultural keystone designation are adequate. Further, culturally important plant species that fulfil unique or nonredundant therapeutic functions, that are preferred and used for multiple purposes in ethnomedicinal contexts are expected to experience greater use-pressure while plant species that fulfill redundant therapeutic functions are expected to experience reduced impact or harvest pressure. Though, the major predictions surrounding species use-pressure and species functional redundancy in ethnomedicine are expected to aid defining conservation priority, our understanding of the factors that predict species use-pressure and of the effect of harvest on culturally important plants are still limited. This dissertation tested if the fundamental components of species cultural keystone designation were predicted by cultural importance indices, which factors are strong predictors of medicinal plant species use-pressure, and if the current rate of harvest of ayahuasca (Banisteriopsis caapi) is sustainable in a localized area of the Peruvian Amazon Rainforest. The dissertation is divided into four chapters including (1) an in-depth literature review of the cultural keystone species theory to assess how the theory has been tested over time and geographic ranges, (2) a critical assessment of the use of cultural importance indices to predict species cultural keystone designation of medicinal plant species used by the Shipibo-Konibo community of Paoyhan, (3) a test of the utilitarian redundancy model to evaluate which factors predict medicinal species use-pressure while controlling for evolutionary relatedness among plant species used by the Shipibo-Konibo community of Paoyhan, and (4) an assessment of the effect of harvest on ayahuasca (Banisteriopsis caapi) in a localized area of the Peruvian Amazon region. Results have indicated most studies on cultural keystone species have occurred in North America and applied cultural keystone designation to species without a direct measure of cultural keystone status, most cultural importance indices are correlated and are limited in terms of a direct measure of species cultural keystone status, and the elasticity patters of the population growth rate to perturbation of vital rates of ayahuasca (B. caapi) population are driven by survival of long-lived individuals in both the short- and long-term. These findings help to further our understanding of the use of cultural keystone species theory and the most common methods employed to predict species cultural status, patterns surrounding medicinal plant use with respect to the utilitarian redundancy model and the factors that predict species use-pressure, and the population dynamics of ayahuasca, a culturally and economically important plant species.