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We tested the species-energy and environmental-stability hypotheses to evaluate if they explain the species richness patterns of terrestrial vertebrates in Caatinga, an arid biome in northeast Brazil. Furthermore, we determined which climate conditions constrain richness patterns in different taxonomic groups, taking into account their ecological and life-history characteristics. Amphibian richness and reptile richness were best explained by environmental instability and were constrained by the driest environmental conditions, whereas bird richness and mammal richness were driven by environmental energy and constrained by the hottest environmental conditions, highlighting their limited physiological budgets for handling high temperatures. Thus, our results revealed the primary climatic factors driving the species richness of vertebrates in this arid environment and determined, for each vertebrate group, which climate variable constrains its distribution. These findings can also be used to help improve conservation strategies in Caatinga by recognizing that the alteration of natural habitats for human dwellings, as well as changes in the global climate, could enhance desertification processes in this region and threaten the region’s biodiversity. (C) 2010 Elsevier Ltd. All rights reserved.