With a high percentage of endemics along the west coast of South Africa, especially in the family Aizoaceae, the region is considered one of the earth’s biodiversity hot spots. It has been suggested that the diversity and radiation of the Aizoaceae are coincident with low but predictable rainfall and lack of competition between species. In this study we examine the relationship between water source and the efficiency of PSII photochemistry for representative Aizoaceae and non-Aizoaceae. We do this to determine the extent to which the different genera are adapted to the frequent, low volume, precipitation characteristic of the region and to ascertain the extent to which there is competition for water. Our water isotope results show that the Aizoaceae use shallow surface water while the non-Aizoaceae use a deeper water source. We are however not able to show the extent to which the Aizoaceae utilize fog or dew. Our chlorophyll fluorescence results show that there are no differences in efficiency of PSII photochemistry between the species in the wet season. The decline from wet to dry season for the Aizoaceae is, however, more dramatic than that of the non-Aizoaceae reflecting the differences in rooting depth between the different families. These results suggest that, during the dry season, there is no competition for resources between families but there is competition between species. We conclude that the adaptation to using shallow water, coupled with susceptibility to drought of adult short lived Aizoaceae may be a, mechanism for the diversification of this family. Keywords: δ18O, δ2H, Water source, Photosynthesis, Community structure.