Poaching exacerbates the effects of climate change on the long-term viability of an endemic South African succulent plant species

1. 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 illegal harvesting for the ornamental plant trade. 2. We developed a stochastic integral projection model (IPM) using demographic data from 1999-2003, 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 90 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), and harvest frequencies (annual to once every decade). We supplemented our approach with perturbation analyses to identify critical demographic components influencing population persistence. 3. We show that historically A. pearsonii populations have been stable or growing. Populations are predicted to remain viable under mild climate change, but moderate climate change results in declines in approximately 50% of simulated populations, though extinctions are not projected within 40 years without harvesting. In contrast, plant harvesting significantly increases extinction risk, especially size-selective harvesting of mature individuals, which disrupts demographic components critical to population growth. Even harvesting as infrequently as once per decade under historical climates leads to declines, with climate change exacerbating these effects. 4. Synthesis: Illegal harvesting poses a critical threat to biodiversity, as shown in this case study of a South African endemic succulent. Our findings indicate that plant harvesting is unsustainable under any scenario, particularly with projected climatic pressures. However, fruit collection emerges as a potentially sustainable alternative to meet the ornamental plant trade's demands. Our study highlights the importance of incorporating multiple threats into population forecasts, providing valuable tools for ecologists and conservation managers.