Trade-off dynamics in a rare cactus: What are the demographic consequences of temporal variation in fitness?

Cacti usually grow in areas with severe resource limitations, making them vulnerable to local extinction. To ensure their survival, cacti have evolved a differential allocation of resources that is reflected in trade-offs among vital rates. In this study, we assess the effect of interannual climatic variation on resource allocation to vital rates in a cactus with biogeographical rarity (Thelocactus leucacanthus ssp. schmollii). Eight-year observations were used to relate vital rates to climatic data using population projection matrix models and Spearman correlations. The finite rate of population growth (λ) indicates, on average, a population in equilibrium. The transition from seeds to seedlings and the growth of seedlings, juveniles, and adults are the stages of the lifecycle with the highest selection pressure, with adults and juveniles being the categories with the greatest contribution to λ. This species allocates more resources to survival than to growth or reproduction during its lifecycle. The trade-off between the survival of adult individuals and fertility seems more marked in years with higher rainfall and minimum temperature turned out to be a key factor for population maintenance. This research provides new insights into the strategies used by rare globose cacti to avoid extinction in these limiting environments.