Comparative demographic analysis of three Neobuxbaumia species (Cactaceae) with differing degree of rarity

In this study we use a demographic approach to analyse the differing abundance of three congeneric columnar cacti: Neobuxbaumia macrocephala (the rarest), Neobuxbaumia tetetzo (intermediate), and Neobuxbaumia mezcalaensis (the most common). Populations of these species were studied in the Tehuacan Valley (Central Mexico) over a 3-year period. We employed traditional models and life table response experiments (LTRE) to explore the association between particular demographic traits and the degree of rarity of each species. Most matrices showed population growth rate (λ) values close to unity; the only exception was N. mezcalaensis in 2001–2002 (λ=1.091±0.088). In the three species the highest elasticity values corresponded to entries referring to the stasis of pre-reproductive plant and small adults. However, the LTRE indicated that most important differences in the λ values between years and species were associated with variation in fruit production and seedling recruitment. N. mezcalaensis yielded the highest mean λ due to its high fecundity in 2001–2002. This result suggests that the variation in demographic behaviour observed between species is sufficient to explain the higher abundance of N. mezcalaensis compared to the rare N. macrocephala. The high fecundity values and recruitment success observed in the former, even if they occur only seldom, may result in higher average λ values accounting for its high population densities.