The reproduction, distribution, and socio-ecological impacts of an aggressive plant invader, Calotropis procera (Aiton) Dryand

Abstract

In South Africa, as in other countries worldwide, invasive alien plants (IAPs) are among the drivers of biodiversity and ecosystem service loss. The IAPs in the Vhembe Biosphere Reserve (VBR), South Africa, are transforming habitats, threatening native species, and the region's livelihoods. One of these IAPs is the aggressive invader Calotropis procera. This species is listed as a Category 1b invasive alien plant under the National Environmental Management: Biodiversity Act (NEM: BA) Alien Invasive Species regulation, meaning its control is mandatory. However, there is a gap in the understanding of C. procera reproduction, spatial distribution, and socio-ecological impacts in the VBR. This study aimed to determine the reproduction, distribution, and socio-ecological impacts of Calotropis procera. Objectives of the study included determining: (1) seed production and the effect of temperatures on seed germination, (2) the potential distribution and invasion risk of C. procera under current and future climatic scenarios, and (3) the socio-ecological impacts of C. procera on livelihood in the VBR. The study was conducted across three sites in the VBR (Musina, Muswodi, and Sanari). To determine seed production, approximately ten C. procera seed capsules were harvested from plants growing in riparian zones and upland areas at each site. Two-way ANOVA and Tukey's HSD test revealed significant differences in seed production across sites (P < 0.001), but no significant difference between riparian zones and upland areas. Musina produced the highest overall mean seeds per capsule (435 ± 15.3 in riparian zones; 411 ± 11.6 in upland areas), followed by Muswodi (394 ± 9.5 in riparian zones; 416 ± 15.5 in upland areas) and Sanari (354 ± 21.3 in riparian zones; 353 ± 15.2 in upland areas), demonstrating that C. procera produces abundant seeds regardless of habitat type. To determine the effect of temperature on seed germination, seeds were germinated at 20 °C, 25 °C, 30 °C, and 35 °C, with germination percentage and MGT calculated using the GerminaQuant package in R and analysed using two-way ANOVA. The mean germination percentage of C. procera was significantly affected by both sites (P < 0.001) and temperature (P < 0.001), with a significant interaction between the two factors (P < 0.001). Germination of C. procera seeds was observed across a wide temperature range of 20–30 °C. Across all sites, 25 °C yielded the highest germination percentage (90–99%), while seeds from Musina and Muswodi failed to germinate at 35 °C, with only 2% germination recorded for Sanari at this temperature. The MGT was similarly significantly influenced by both site (P < 0.001) and temperature (P < 0.001), with a significant interaction between the two factors (P = 0.01). Overall, C. procera exhibited rapid germination across all sites and temperatures, with MGT ranging from 1 to 4.5 days. Musina seeds germinated fastest at 20 °C (MGT = 2.5 days), with germination slowing as temperatures increased. Muswodi seeds showed the slowest overall response, with the fastest germination at 25 °C (approximately 4 days). In contrast, Sanari seeds showed a linear acceleration with increasing temperature, reaching an MGT of only 1 day at 35 °C, the only seeds to germinate at this temperature. These results indicate that while 25 °C is the optimum germination temperature across all sites, the rapid germination across a wide temperature range, combined with population-level differences in germination speed, underscores the high invasive potential of C. procera. The current and future potential distribution of C. procera in South Africa was predicted using species distribution modelling under two climate change scenarios (SSP2-45 and SSP5-85) through to the 2050s and 2070s. Limpopo Province was identified as the invasion hotspot under current climate conditions, with total suitable areas covering ~1.95% of South Africa's landmass. These suitable areas are projected to expand to 7.84% and 8.67% under SSP2-4.5 and SSP5-8.5, respectively, by the 2050s, and 8.89% and 15.44% by the 2070s, respectively, predominantly driven by increases in low and moderate suitability classes. Range change analysis further revealed a substantial gain in distribution of 413.85% and 524.72% by the 2050s, and 531.32% and 866.57% by the 2070s under SSP2-4.5 and SSP5-8.5, respectively. This reflects a broad southward expansion into the provinces of Gauteng, North West, Northern Cape, Mpumalanga, and KwaZulu-Natal as climate change progresses. Moreover, to determine the socio-ecological impacts of C. procera on livelihoods in the VBR, 80 questionnaires were administered across three areas: Musina, Muswodi, and Sanari. The results revealed that the respondents were aware of the presence of C. procera and believed it was brought with gravel during road construction. Awareness of its impacts on crops was universal (100%), with respondents attributing this to C. procera outcompeting crops (54.47%) and hosting crop pests (45.53%), thereby negatively affecting crop productivity. Negative effects on livestock were also reported by 34.4% of respondents. Respondents highlighted that C. procera is difficult to manage due to its high regenerative capacity, herbicide resistance, environmental tolerance, and abundant wind-dispersed seeds, with cutting being the most preferred management method, though it provided only temporary control when combined with other mechanical and chemical approaches. The study concludes that the production of abundant wind-dispersed seeds across both riparian zones and upland areas, coupled with rapid germination across a wide temperature range and population-level variation in germination speed, drives C. procera invasion, calling for coordinated, inter-country, and large-scale management interventions.