Harrisia portoricensis is a threatened columnar cactus whose distribution range is currently restricted to three small islands of the Puerto Rican Bank: Mona, Monito, and Desecheo. In this work we present a brief summary of results on natural history and population size estimates of H. portoricensis populations, extracted from two years of population and community-level intensive studies performed at Mona and Monito Islands. Analyses of the demographic profiles of H. portoricensis populations demonstrated that for Mona Island populations included plants in the adult, juvenile, and seedling stages, whereas population of this species on Monito Island only included adult plants. For both islands, we detected a high percentage of H. portoricensis plants showing vegetative lesions. Additionally, we found that the presence and abundance of these vegetative lesions in H. portoricensis were significantly associated with increases in total plant length. Our combined results suggested that active management in situ is necessary for the recovery of this threatened species and also highlight the need for long-term monitoring to determine the effects of changing biotic and abiotic interactions on the future of H. portoricensis populations.