RARITY AND COMMONNESS AMONG COLUMNAR CACTI: AN EXPLORATION OF THE STORAGE EFFECT FROM A DEMOGRAPHIC VIEWPOINT
Authors: Teresa Valverde and Ligia Esparza-Olguín
Abstract: In this study we explore the idea that the storage effect (i.e. the presence of certain life-history or demographic features that allow persistence while environmental conditions are harsh, but trigger growth and regeneration during favorable periods) may be an important mechanism affecting the success of certain species over others, thus determining their relative commonness or rarity in variable environments, such as semideserts. To address this important topic of the ecology of arid environments, we used a system of three congeneric cacti differing in their degree of rarity: Neobuxbaumia mezcalensis (the most common), N. tetetzo (intermediate) and N. macrocephala (the rarest). We studied the demography of these three species in the Tehuacán Valley (Central Mexico) over a three-year period. We used previously published population projection matrices for these species to carry out stochastic simulations to explore the association between particular demographic variability patterns and the degree of rarity of each species. In most cases λ values were close to unity, with the only exception of N. mezcalaensis in 2001-02 (λ = 1.091 ± 0.088). Stochastic population growth rates (λs) were significantly above unity for all species, but Neobuxbaumia mezcalaensis yielded the highest λs due to its high fecundity in 2001-02. Our results contradict the emergent view of conservation biology which states that high variance in demographic rates may increase the risk of population extinction, since in this system, variance in reproductive performance appears to favour population persistence. The interaction between certain life history traits (low adult mortality) and demographic variability may allow N. mezcalaensis to be positively influenced by good years but only marginally affected by bad ones. Demographic variability results in higher persistence probabilities compared to demographic constancy, supporting the idea that the effectiveness of the storage effect accounts for the high population numbers in this species.