Identifying the impacts of chronic anthropogenic disturbance on two threatened cacti to provide guidelines for population-dynamics restoration

Abstract

Species frequently have to be preserved in disturbed areas. Restoring their predisturbance population dynamics may increase their population growth rate (lambda) while preventing the development of vulnerable genetic- or age-structures in the population. We assessed how population dynamics have been affected by anthropogenic disturbance in the threatened cacti Mammillaria dixanthocentron and Mammillaria Hernándezii. We worked in areas exposed to different intensities of disturbance, and applied a retrospective analysis to detect which of the vital rates affected by disturbance were responsible for reductions in lambda. M. dixanthocentron had lower lambda s in more disturbed conditions as a consequence of the reduced survival of adults because of livestock trampling and increased predation by hares. M. Hernándezii had significantly higher lambda s with intermediate disturbance. The reduction in lambda under intense disturbance resulted from reduced reproduction and adult plant mortality. The death of these individuals seemingly was associated to the change in soil conditions due to overgrazing. However, the factor with the greatest influence over both species was temporal variability, which interacted with disturbance sometimes increasing its negative effects. The identification of hypothetical mechanisms reducing lambda was facilitated by the retrospective analysis, which is then a valuable heuristic tool guiding research. As long as they are not properly tested, these hypotheses should be considered carefully in management plans, but they may provide timely information. Retrospective analysis allows taking initial conservation measures, while suggesting guidelines for future research that may improve such measures.