How does nurse plant facilitation influence seed predation and recruitment in the living rock cactus Ariocarpus retusus?

The most vulnerable stage in the plant life cycle is recruitment (seed germination and seedling survival), influenced by seed predation, herbivory, competition, and the availability of water, light, and nutrients. Nurse plants facilitate the recruitment of various species in arid environments. Therefore, we evaluated their effect on seed predation, germination, and seedling survival of Ariocarpus retusus through both field and controlled experiments. Seeds were attached to aphid mesh, and we monitored seed predation and germination at the study site under two factors: (1) microhabitat (presence or absence of a nurse plant) and (2) predator exclusion (with or without protection). Under controlled greenhouse conditions, we assessed germination and seedling survival using jiffy pots covered with shade mesh to simulate the presence of a nurse plant (artificial nurse). Seedlings obtained under these conditions were reintroduced into the field 15 months later to evaluate their survival. Natural nurse plants reduced seed predation, possibly by concealing seeds from predators; however, they did not offer the same protection to seedlings, which experienced high mortality due to herbivory. Although the artificial nurse favored both germination and seedling survival, natural nurse-plant facilitation is facultative, with A. retusus not strictly dependent on it for establishment. The differences observed in germination and survival between nurse types could be attributed to predator exclusion and constant watering in the artificial nurse treatment, highlighting the roles of water availability and herbivory in the recruitment of A. retusus.