Distribution, population structure and microhabitat profile of Euphorbia bupleurifolia

Abstract

Many South African succulent Euphorbia spp. are threatened by anthropogenic activities such as habitat loss and illegal harvesting. However, only a few species have been studied in sufficient detail to aid their conservation through the development of effective management plans. This study documents the distribution, abundance, and population biology of the formerly widespread Euphorbia bupleurifolia, a species endemic to the Eastern Cape and KwaZulu-Natal provinces of South Africa. Until recently, the status of wild populations of this species was largely unknown. Searches for plants in each of the 31 previously recorded localities revealed that only nine populations (29 %) still had plants in 2018, indicating a contraction of the distribution, with the species apparently now ranging from Makhanda to Port Edward. Population size and structure, flowering, microhabitat requirements and various other factors, including potential threats and disturbances, were quantified for the nine populations. Complete counts were conducted, and the diameters of plants were measured and canopy areas calculated. Population sizes varied from one (effectively extinct) to 675 plants (with a total of 1724 plants recorded across all populations). Measurements of cultivated plants at the Walter Sisulu National Botanical Garden established that the onset of reproduction can be expected for a plant with a canopy area of 2.63 cm2, and annual growth rates of 0.55±8.37 cm2 are possible. Bell-shaped size class distribution (SCD) patterns were dominant, which is attributed to an adult persistence population survival strategy. Unsustainable harvesting, inappropriate fire frequencies as well as habitat loss and degradation remain a threat to the species. Euphorbia bupleurifolia persists mostly in sites with low vegetation biomass, high bare ground cover and minimal overstorey shade. This study highlights the drastic decline of a Least Concern species, formally considered widespread (i.e. common), and provides baseline demographic data, which may be employed in future monitoring and management of the remaining populations, to improve the conservation of E. bupleurifolia. Intensified field surveys involving citizen science programmes (e.g. CREW) and subsequent uplisting of the species to a higher Red List threat category is recommended.