Contemporary and historical impacts of megaherbivores on the population structure of tree euphorbias in South African subtropical thicket

African elephant and black rhinoceros - both megaherbivores - impact negatively on the abundance of succulent plants, including tree succulents, in South Africa's subtropical thicket. We sampled 35 sites in subtropical thicket to assess historical and contemporary impacts of African elephant and black rhinoceros on the population structure of three species of succulent tree euphorbia. Population age structures were highly variable and showed no species-specific effects. Almost half the sites had growing populations dominated by young individuals. Sites having contemporary impacts had significantly fewer individuals in the 30-75 years age range, but this effect declined with increasing terrain slope. Eighty-one percent of sites assumed to have been impacted by megaherbivores historically had individuals that predated rhino and elephant extirpation in the region. This and other population age structure data indicate that megaherbivores and tree euphorbias coexisted historically on terrain of low relief. However, in areas not subject to contemporary impacts, euphorbia populations are likely to be much higher now than historically, owing to population relaxation after the extirpation of megaherbivores in the mid 1800s. Given the sensitivity of tree euphorbias to megaherbivore impacts, managers should consider monitoring the population structure of these species in order to identify thresholds of potential concern regarding megaherbivore impacts.