How important are columnar cacti as sources of water and nutrients for desert consumers? A review

Succulent CAM plants, such as columnar cacti, are important physiognomic elements of many arid lands. Although, these plants are often ecologically important because they provide abundant resources in the form of nectar and fruit, their contribution to the energy, nutrient and water budgets of consumers has not been quantified. We describe an isotopic approach that allows quantifying the ecological importance of CAM succulents. We first briefly review our work on the interaction between saguaros, an archetypical CAM succulent, and the desert doves that feed on its fruit. We then describe the potential importance of saguaro fruit as a function of its abundance, macronutrient composition, and seasonal availability. We argue that the resources provided by saguaros do much to satisfy the energy and water requirements of the birds that reside in hot subtropical deserts during the summer. We then describe the carbon isotope composition of saguaros and of the plant community in which they are imbedded and use two species of desert doves to illustrate how stable isotopes can reveal the importance of a single plant as a source of carbon and water for consumers. The second section of this review presents new data on the importance of saguaros for the entire community of birds that inhabit the Sonoran Desert during the summer. We show how the resources of saguaro reach across dietary guilds and account for a large proportion of the diet of many insectivorous species as well as that of granivorous and frugivorous species. We demonstrate that many of these species probably obtain significant water as well as nutrients from saguaro fruit. Finally, we point out the current limitations of using deuterium as a water tracer in animal systems.