Overgrazing and poaching of plants are also factors leading to the decline of the rare Prince Albert snipe. (SAEON)
isDrought and warming temperatures may be causing declines in dwarf succulents such as the endangered Prince Albert Vigie, which is also being affected by overgrazing and unscrupulous succulent collectors.
Researchers from the Southern African Environmental Observation Network (Saeon), a long-term environmental observation and research facility of the National Research Foundation, have found that populations of endemic species in the Prince Albert region of the Western Cape have declined over the past two decades.
Prince Albert Vigie (Vigilia dilatata) is listed as Threatened by the South African National Biodiversity Institute's Threatened Species Programme's Red List of South African Plants, which describes it as “a restricted-range species occurring in three to five locations and declining due to ongoing habitat loss and degradation.”
Details of these locations “are particularly hazy in the Succulent Karoo region due to the widespread risk of plant poaching,” wrote Scion's Drylands Node Researcher Sue J. Milton and Drylands Node Scientist Helga van der Merwe, who are based at the Wolwekraal Conservation and Research Institute in Prince Albert.
The researchers said their findings support predictions that global warming, or a combination of drought and warming, could cause population declines in dwarf succulent plants, including: B. Diratata.
“Although this species only lives at an altitude of 300 metres, higher altitudes may offer a safer refuge from rising temperatures associated with climate change and should be included in protected area networks,” the researchers concluded.
Different populations of this species were surveyed across an elevation gradient similar to a hillslope in 2002, the researchers said. Subsequent surveys were conducted during periods of severe drought in 2020 and 2021, and after drought-breaking rains in 2023, to assess the status of the populations.
Vigilia dilatata Population trends in different locations were examined in terms of precipitation, elevation, small sites, and land use. The researchers noted that a measure of drought severity can be obtained by using the Standardized Precipitation-Evapotranspiration Index, which takes into account both precipitation and potential evapotranspiration as a function of temperature.
“These data indicate that the recent drought in the Prince Albert region was particularly severe,” Milton and Vandermerwe wrote. B. Diratata During the 20-year study period, population density at each site decreased.”
According to the researchers, these variations could be explained by altitude, with higher elevations being cooler than lower elevations and micro-shaded areas maintaining higher relative humidity for longer after rain.
But other factors that affect microsite quality may also influence a plant's ability to survive exceptionally hot and dry periods, the researchers noted.
“Grazing intensity [no grazing, grazing or heavy grazing] No significant effects on population size were found, but there was a negative effect on abundance. Microsite differences are influenced by grazing intensity by reducing the shade produced by nurturing plants and thereby increasing air temperature.”
Grazing can also have a direct effect on plants, as parts of the plant are eaten or trampled by animals. At all study sites, young individuals were predominant in 2002, but over time, medium-sized individuals became more prevalent.
Moreover, most of the dead plants were found in the smallest size classes, whereas larger individuals were more likely to survive. A 2023 survey, conducted after sufficient rains to break the drought, did not find any recently recruited individuals. “Hence, recruitment failures may also have influenced the observed population changes.”
Recruitment refers to the process by which new plants find a population or are added to an existing population.
Possible reasons for this failure to recruit include poor flowering and seed set prior to the survey in 2023, leading to depletion of the seed bank. More than half of the population lived in shady micro-sites associated with rocks and living or dead nurse shrubs (nurse shrubs are large shrubs that depend on survival and reproduction).
These individuals were larger than those found at the open microsites.
“Nurse shrubs were severely affected by the last drought, with 42% of these shrubs dying as of the 2023 survey,” the researchers wrote, adding that other surveys conducted in the region had also reported mass mortality of the shrubs.
Intensive grazing also removes shrubs that buffer the effects of heat and moisture.
“Declines in specialist succulent populations are further influenced by unscrupulous succulent collectors removing them from their natural habitats,” the researchers said.
