Are protected populations of two globular cactus species facing a demographic explosion or just a “bonanza” year?

Abstract

Despite decades of research, controversy has emerged as to whether positive and negative interactions predictably shift with increasing environmental stress. We used projection matrices, to determine growth rate and elasticity and a LTR experiment to quantify the contribution of each matrix elements to the differences in λ observed between sympatric and syntopic sub-populations of globular cactus species in Brazil (Discocactus placentiformis and D. pseudoinsignis). We addressed the following questions: What is the growth rate of the two species when in sympatry and syntopy? What are the relative contributions of survival, growth, and fecundity to population growth? Which life cycle stages have the highest mortality? Do demographic differences between the two species account for observed differences in sub-population sizes? Both species exhibited higher densities when in syntopy than in sympatry. Seedling establishment and survival were high, and fecundity increased as plant diameter increased. Asymptotic population growth rates were significantly greater than one. The highest elasticity values were for stasis for the later ontogenetic stage and for the growths. It seems that the protection provided by the conservation area is efficient, and that the data were collected during a “bonanza” year, with high rainfall, when all or most reproduction and/or recruitment occur.