The demographic variability and life history differentiation of two closely related shrubs (Atriplex canescens and A. acanthocarpa) were investigated in the Chihuahuan Desert, and the results were interpreted in the context of theories of coexistence in fluctuating environments. Demographic information was recorded during three annual intervals and analyzed employing matrix projection models. A. canescens had lower lambda (finite rate of population increase), higher longevity and generation time and slower convergence to a stable population structure than A. acanthocarpa. In favorable years for recruitment (those when, for both species, lambda > 1), the lambda of A. acanthocarpa was higher than that of A. canescens; in unfavorable years (when lambda < 1), the reverse was true. Regardless of conditions (year), A. acanthocarpa had a type 2 survivorship curve (constant rate of mortality with age), while A. canescens had a type 3 survivorship curve (declining mortality with age). Elasticity analyses highlighted the larger influence that fecundity and growth would have in modifying the lambda of A. acanthocarpa in comparison to that of A. canescens. In contrast, survival would have a larger influence on the lambda of A. canescens. Atriplex acanthocarpa behaved as an opportunistic species that benefitted from sporadic favorable conditions and declined rapidly when conditions deteriorated. In contrast, A. canescens behaved as a tolerant species able to withstand years when conditions were poor, but which could not gain any advantage over A. acanthocarpa when conditions improved. By each having a relative advantage over the other on opposite ends of the contrasting climatic conditions experienced in the Chihuahuan Desert, they are able to coexist. Their contrasting life histories agreed with the theoretical predictions for the operation of the two mechanisms of species coexistence in fluctuating environments: the storage effect and the relative non-linearity of competition. Based on these results, we conclude by speculating on the nature of succession in arid communities.

Our aim was to assess the conservation status of the cactus Ariocarpus scaphirostris by describing its population dynamics and spatial distribution in the only known large population left. Population dynamics were analyzed with the use of matrix projection models from a census period that encompassed 2 yr (2005 and 2006). Density of the individuals (0.25 individuals/m 2) changed when compared with previous studies, and similar to other dense globose cacti, the pattern of spatial distribution was aggregated. Our results indicate that the original population has decreased considerably during a 20-yr period (r ¼ À0:08 individuals/individual/yr). However, on the basis of annual projections, the population is stable at equilibrium and could even show moderate growth (l ¼ 1:07 6 0:17). When we simulated changes in survival, we found no significant decreases in l, basically because of the equal distribution of elasticities for survival. When growth was changed, l decreased by 10%. Ariocarpus scaphirostris needs to be revaluated because the demographic evaluation assigns a higher risk (in risk of extinction) than that to which it is currently assigned (vulnerable); the actual size of the population compared with historical data shows the species has reduced its population by more than 80% in 20 yr.

No abstract