How much is too much? Simulated population responses of a South African dwarf succulent to wild harvesting under climate change

Natural populations often face multiple threats that jeopardise their viability, yet most population forecasts consider only single threats, such as shifts in precipitation or temperature. To better understand ecological and conservation challenges, incorporating multiple threats is essential. South African endemic dwarf succulents, for example, are under dual threats from climate change and wild harvesting for the ornamental plant trade.

We developed a stochastic Integral Projection Model (IPM) using demographic data from 1999 to 2003, for 4776 individuals of the dwarf succulent, Argyroderma pearsonii , to evaluate the impacts of climate change and harvesting on its short‐term (<40 years) extinction risk and long‐term viability. We built simulations to explore 270 scenarios combining climate conditions (historical, mild climate change [SSP126], and moderate climate change [SSP245]), harvesting methods (fruit harvesting, non‐selective plant harvesting, or size‐selective plant harvesting), harvesting intensities (20%, 50%, or 90% of mature individuals/fruits) and harvest frequencies (annual to once every decade). We supplemented our approach with perturbation analyses to identify critical demographic components influencing population persistence.

We show that historically A. pearsonii populations have been stable or growing. Without harvesting, populations are projected to remain viable under SSP126, but SSP245 results in long‐term declines for approximately 50% of simulated populations. Moderate intensities and frequencies of plant harvesting, especially of adults, significantly increased extinction risk by disrupting demographic components critical to population growth. In contrast, the most severe fruit harvesting scenarios had relatively minor impacts on population viability. In general, the effects of harvesting became more severe with increasing severity of climatic changes.

Synthesis and applications . Wild harvesting poses an important threat to the persistence of South African dwarf succulent populations. Here, we identify sustainable harvesting thresholds for dwarf succulents and suggest that plant harvesting may be unsustainable under moderate to severe harvesting pressure, particularly with projected climatic pressures. Our study highlights the importance of incorporating multiple threats into population forecasts, providing valuable tools for ecologists and conservation managers.