Identification and evaluation of vulnerability factors affecting the Caribbean cactus species Harrisia portoricensis

Conservation and adaptive management of rare and endangered species is best addressed with holistic approaches to the evaluation of their population vulnerabilities. During their life, columnar cacti are exposed to diverse mortality factors associated with high solar radiation, high temperatures, water deficit, predation, and competition. Early stages of life-cycle of columnar cacti, such as seeds, seedlings, and juveniles appear to be critical stages whose alteration may produce dramatic changes in the dynamics of entire populations. In this work, I evaluated the role of different biotic and abiotic factors in the dynamics of a natural population of the endangered Caribbean cactus Harrisia portoricensis on Mona Island. Direct observations and videotaping on flowers of H. portoricensis showed that visits to flowers by animals were uncommon events. Using controlled pollinations I detected that H. portoricensis is partially self-compatible, but it is not autogamous, therefore requiring the participation of an external agent to deposit the pollen onto the stigma. I also found that the progeny resulting from natural and self-pollination treatments revealed signs of inbreeding depression compared with progeny resulting from cross-pollination. After two years of field and laboratory experiments, the combined results for this species demonstrated that reproductive phenology was not restricted to any particular season and a pattern of continual reproductive activity throughout the year was observed. Field experiments with seeds and seedlings demonstrated that suitable conditions for germination and establishment of H. portoricensis seedlings occur in shaded areas beneath the canopy of shrubs, but experiments also demonstrated that the identity of the shade providers have a significant effect on these processes. Specifically, growth and survival were depressed when seedlings were growing under the exotic grass Megathrysus maximus and thus the presence of this exotic grass has the potential to alter the demographic profile of this species on Mona Island. Analysis of the spatial distribution of H. portoricensis on Mona Island suggests that this species has strong environmental affinities which are likely to influence plant distribution in space. In general, H. portoricensis plants were more abundant in areas with the highest percentage of ground cover by rock, lowest percentage of ground cover by litter, and lowest canopy cover at the highest vegetation layer. Population dynamics of H. portoricensis was assessed using matrix models. Matrix analyses showed that the population of this species studied on Mona Island is not decreasing and exhibits population growth rates that were not different from unity. However, even when a population is not decreasing, simulations performed evaluating environmental changes and alterations in matrix entries clearly indicated that changes in mortality, seedlings establishment, and fecundity values as well as changes in climate and environmental conditions have an important effect of the fate of populations and may lead to the extinction of this species. On the whole, the data suggest that management strategies for the conservation of H. portoricensis should consider the following components: (1) active in situ management of the exotic grass, (2) the protection of native shrub species as a way to enhance germination and establishment of seedlings, and (3) the performance of long-term studies considering the impact of climate change on plant responses are necessary to achieve the permanence of natural populations of this species.