Background/Question/Methods Conservation of rare species is frequently complicated by their narrow niches and poor competitive abilities. These features limit their distribution to few small (potentially unviable) populations inhabiting specialized environments. Understanding both environmental and population factors that affect long-term persistence of rare endangered species is required in order to plan effective conservation actions before the threatened populations decline beyond recovery levels. We studied how the population size and structure of the rare cycad Dioon sonorense, an ancient endemic gymnosperm that occurs only in northwestern Mexico, responds to its environment and how, in turn, the population structure can affect its long term persistence. We surveyed all known populations of D. sonorense in the Sierra de Alamos-Rio Cuchujaqui Biosphere Reserve in northwestern Mexico, characterized the environmental drivers of the population structure, and estimated population persistence using life tables and matrix population models.

Results/Conclusions   The abundance of D. sonorense adults was negatively related to slope (r²= 0.59, p= 0.03) and positively to soil exchangeable potassium (r²= 0.63, p= 0.02), while seedling abundance was related even more negatively to slope (r²= 0.83, p= 0.003) and positively to adult abundance (r²= 0.70, p= 0.01). Strong non-linear relationships of seedling abundance to slope and adult abundance suggest threshold effects that may limit D. sonorense regeneration to less suitable environments leading to smaller populations (i.e., on steep slopes). The population models suggest that the studied populations are not likely to be viable in the long-term, except for populations having many individuals in all life-stages (population growth rate, λ = 1.06). Modelling large increments in seedling survival produced modest gains in population growth, and greater seed production did not have any effect on population increase. In addition to plant extraction by humans, long-term persistence of D. sonorense populations is threatened by (a) small size of adult populations, (b) low-quality habitat in which most populations occur, and potentially (c) low seedling survival rates. We urgently recommend immediate strict conservation of all remaining larger populations of this rare cycad.