Population ecology of tree succulents (Aloe and Pachypodium) in the arid western Cape: decline of keystone species

Abstract

Arborescent succulents are conspicuous elements of the arid flora of the far north-western Cape Province. In the Richtersveld and two adjacent more southerly areas (Springbok and Nieuwoudtville), we surveyed the population structure of the following three species, Pachypodium namaquanum (Apocynaceae), Aloe dichotoma and A. pillansii (a rare endemic) (both Liliaceae). Only in the Richtersveld, did both Aloe species have a high proportion of dead individuals and all three species had few seedlings there. Populations in adjacent areas had lower levels of mortality and greater recruitment. Mortality appears to be due to damage by baboons and porcupines. This apparent decline is potentially serious because all three species appear to be keystone species. They supply perches to raptors, nesting sites for other birds, nectar for birds, and food, habitat and moisture to other animals.