Demographic consequences of chronic anthropogenic disturbance on three populations of the endangered globose cactus Coryphantha werdermannii

Cacti are thought to be highly vulnerable to chronic anthropogenic disturbance (CAD). To assess its effects, we modeled the dynamics of three populations of the endangered cactus Coryphantha werdermannii exposed to different intensities of disturbance. Because no seedlings germinated and survived in the less-disturbed site, we analyzed two different scenarios: If the same, average germination probability was used for all sites, the population growth rates (λ) decreased with disturbance, but λ remained unchanged if a very low germination probability was assigned to the less-disturbed site and the observed probabilities were used for the remaining populations. Retrospective analysis showed that different demographic processes affected λ as a result of increasing disturbance: The unsuccessful establishment of seedlings in the preserved site diminished λ. The damage caused by livestock to reproductive structures in the site with intermediate disturbance was responsible for a reduction in λ. In the most disturbed site, the reduced individual growth rates that seemingly resulted from land degradation caused a decrease in λ. No simple management strategy can be proposed because different threats require attention depending on disturbance intensity. CAD had both positive and negative effects on λ, conferring this species with a certain degree of tolerance.

Research highlights
► We assess the impact of chronic anthropogenic disturbance in three threatened cactus populations.
► Different demographic processes are affected over the disturbance gradient.
► We find that disturbance has a minor, or even no effect, on population growth.
► This results from some demographic processes balancing the negative effects of others.
► A greater recruitment in disturbed sites causes disturbance tolerance in this cactus.