Demography of the columnar cactus Neobuxbaumia macrocephala: a comparative approach using population projection matrices

Neobuxbaumia macrocephala is a long-lived columnar cactus endemic to the Tehuacán-Cuicatlán Valley in south-central Mexico. This plant has a very restricted distribution and few recruitment events have been detected in its populations. In this study, we analyze the N. macrocephala demographic pattern using a projection matrix in order to determine the main limiting factors of this species. To accomplish this goal, we compare our results with those obtained for another species of the same genus, N. tetetzo. Considering that both species inhabit the same valley, we believe that this comparative study will offer insights into the main demographic limitations of N. macrocephala. Results showed that these species of columnar cacti have similar demographic patterns in which survival is the process with the highest relative contribution to λ, followed by growth and reproduction. Of all the life cycle stages, seeds and seedlings have the lowest survival probabilities due to a high mortality caused by seed predation and effects of direct solar radiation on germinated seeds. The estimated growth rates indicate that populations of these species of Neobuxbaumia are in a numerical equilibrium. With respect to reproduction, N. macrocephala produce a lower number of seeds per plant than N. tetetzo. This low level of sexual reproduction may decrease the probability of establishment of new individuals in N. macrocephala populations. It is suggested that pollen limitation and pre-dispersal seed predation could be some factors that limit the distribution and abundance of this columnar cactus. This revised version was published online in June 2006 with corrections to the Cover Date.