Population trends in an endemic dwarf succulent over two decades: rainfall, elevation, microsite and landuse effects

Abstract

Bijlia dilatata (Prince Albert Vygie) is a red-listed succulent (Aizoaceae) endemic to a single valley in the Western Cape and restricted to disjunct sites in quartzite pavements. Populations of this species were surveyed in 2002 during above-average rainfall and again in 2020 and 2021 during drought, and in June 2023 following drought-breaking rains. We found that populations declined over the two low-rainfall decades, that declines differed significantly among sites, that mortality was greater among smaller plants and that recruitment had failed. Our hypothesis that elevation provides a refuge from heat-induced mortality was supported by data that showed decreasing population decline along a 300 m elevation gradient. Factors significantly influencing population structure included landuse (grazing intensity), microsites and initial population density. Shaded microsites beneath living shrubs and adjacent to large rocks were significantly cooler and stayed moister longer after rain and B. dilatata plants in such microsites were larger than those in open sites or beneath dead shrubs. Grazing and climate change, including reduced winter rainfall, and increased ambient and soil surface temperatures, appear to be contributing to declines in endemic, habitat specialist dwarf succulents already threatened by land transformation and poaching in the biodiverse, Succulent Karoo region of South Africa.