How important is clonal recruitment for population maintenance in rare plant species?: the case of the narrow endemic cactus, Stenocereus eruca, in Baja California, México

Stenocereus eruca is a postrate columnar cactus whose regeneration seems to occur mainly through clonal propagation. It is a narrow endemic species of the Sonoran desert in Baja California Sur and currently considered as threatened under Mexican legislation. In this paper we describe the demography of ramets in four populations along its distribution range and the demography of genets in one population during a 3-year-study period in order to evaluate its conservation status. We also analyze the relative contribution of sexual reproduction and clonal propagation to population maintenance and provide guidelines for the formulation of conservation programs. Elasticity analyses were used to explore the relative contribution of sexual and clonal recruitment to projected population growth rate (λ). During the three years of study, regeneration occurred only through clonal propagation while sexually derived seedlings were not detected within or outside the permanent plots. Our demographic data showed that the four population of S. eruca are in equilibrium (λ ≈ 1), and elasticity analyses showed that the relative contribution to λ of clonal recruitment was larger than sexual recruitment, at least during the analyzed ecological time scale. Simulations showed that removing sexual recruitment had a minor impact on λ, but the absence of clonal propagation alone was sufficient to keep below unity. We propose the establishment of at least one reserve with adequate protection from human disturbance to conserve S. eruca.