Demography of an endangered endemic rupicolous cactus

Abstract

We compared the demography of two populations of Mammillaria huitzilopochtli, an endemic and threatened rupicolous cacti species with a narrow distribution in the semiarid Tehuacan-Cuicatlan region of central Mexico. Censuses were conducted over a 5-year period in two populations: a disturbed site (S1) and a well-preserved site (S2). Five annual size-based matrices and a mean transition matrix of each population were constructed to estimate demographic trends. Prospective time-invariant analyses were performed to calculate population growth rate and elasticities, whereas prospective stochastic analyses were performed to assess quasi-extinction probabilities and how simulated changes in recruitment, stasis and growth affected the population growth rate. Retrospective perturbation analyses (life table response experiments, LTREs) were used to explore the contributions of demographic processes, plant sizes, and temporal variability (years) to the observed variations in population growth rate. The species showed decreasing population growth rates for almost all years and sites, S1 showed lower population growth rates than S2. Quasi-extinction probabilities were 1 after 9 years for S1 and 17 years for S2. Elasticity values were highest for matrix entries corresponding to plants remaining in the same category (stasis) followed by growth and fecundity. LTREs showed that fecundity had negative contributions to population growth rates for all years in S1 population, while it had positive contributions in 4 out of 5 years in population S2. Prospective stochastic analyses showed that increasing recruitment by 50% could give population growth rates >1 in S2 while none of the simulations give this value in S1. Increasing survival also raises population growth rates but always below one. These results indicate that populations under the influence of human disturbance will eventually be lost. According to the models the most promising management strategy to conserve this species is to increase recruitment rates and to give special care to reproductive adults.