Comparison of population structures and ecology of a congeneric pair of common and rare neotropical tree species

We compared the general ecology and population profiles of Jatropha standleyi and J. chamelensis, two ecologically similar, dioecious forest trees in order to illuminate why they differ in range size, an important measure of relative commonness and rarity. Jatropha standleyi is widespread throughout the tropical deciduous forest in Mexico and the endangered J. chamelensis is restricted to a small region on the Pacific coast of Mexico. Using data collected over the same time period from a site central to the distributions of both species, we found significant differences in population size distributions and seedling mortality, but no discernible differences in seedling or adult growth rates, local abundance or local distribution. The observed difference in relative rarity could be explained by a number of hypotheses depending on whether the populations are at a stable equilibrium or not. The high ratio of pre-reproductive individuals may indicate a growing, rather than stable, population for J. chamelensis. Jatropha standleyi is more widespread and has a significantly lower ratio of pre-reproductive to mature individuals. Alternatively, both populations may be at equilibrium with J. chamelensis restricted to its limited range by large-scale environmental constraints or niche differences not elucidated in this study. In either case however, the greater and less variable seed output of the J. standleyi population will also mean a greater number of rare long-distance dispersal events and a better chance of establishing a persistent remote population for J. standleyi than for J. chamelensis. The dioecy to be found in both these species would exacerbate the limiting effects of low seed production on the ability of a species to ‘travel’ successfully. A more extensive field investigation of mortality and factors contributing to mortality in sub-canopy and pre-reproductive individuals would allow more light to be shed on relative population processes in these species.