Drought and warming a threat to endangered Western Cape vygie

Drought and warming could cause a decline in dwarf succulents such as the endangered Prince Albert vygie, which is further affected by intense grazing and unscrupulous succulent collectors.

Populations of the endemic species found in the Prince Albert region of the Western Cape have declined over the past 20 years, researchers at the South African Environmental Observation Network (Saeon), have found. Saeon is a long-term environmental observation and research facility of the National Research Foundation. 

The Prince Albert vygie (Bijlia dilatata) is listed as endangered on the Red List of South African Plants of the South African National Biodiversity Institute’s Threatened Species Programme. It is described it as a “range-restricted species occurring at three to five locations and declining due to ongoing habitat loss and degradation”. 

Details of these sites are “specifically kept vague because of the risk of plant poaching that is rife” in the Succulent Karoo region, wrote researchers Sue J Milton, an arid lands node research associate at Saeon who is based at the Wolwekraal Conservation and Research Organisation in Prince Albert, and Helga van der Merwe, an arid lands node scientist.

The researchers said their findings support predictions that warming, or the combination of drought and warming, could cause a decline in the populations of dwarf succulents such as B. dilatata.

“Although the species spans an altitudinal range of only 300m, it is likely that the higher elevation would provide a safe refuge from temperature increases associated with climate change and higher-elevation sites should be included in protected area networks,” the researchers found.

In 2002, various populations of this species were surveyed across an elevation gradient — similar to a slope — they said. Subsequent surveys were conducted in 2020 and 2021, during an intense drought period, and again in 2023 after drought-breaking rains, to assess the condition of the populations.

Bijlia dilatata population trends across various sites were investigated in terms of rainfall, elevation, microsite and land use. The researchers noted that an indication of the severity of the drought could be gained by using the Standardised Precipitation Evapotranspiration Index, which takes into account both rainfall and potential evapotranspiration determined by temperature.

“These data show that the recent drought in the Prince Albert area was particularly severe,” wrote Milton and Van der Merwe. “We found a fourfold difference in B. dilatata population density declines among sites over the two-decade study period.”

These changes were explained by elevation, with higher-elevation sites cooler than low-elevation sites and maintaining a higher relative humidity for longer after rain in shaded microsites, the researchers said. 

But other factors influencing the quality of a microsite can also influence a plant’s ability to survive an exceptionally hot and dry period, they noted. 

“Grazing intensity [no grazing, grazing or heavy grazing] was not found to significantly affect population size, however, there was a negative effect on populations. Microsite differences are affected by grazing intensity by reducing shade produced by nurse plants and therefore increasing temperature.” 

Grazing, too, can directly influence plants when parts of them are eaten by an animal or they are trampled. At all the sites, young individuals were dominant in 2002 but changed to a dominance of medium-sized individuals over time. 

Additionally, most of the dead plants were found in the smallest size class while larger individuals were more likely to survive. No recently recruiting individuals were encountered during the 2023 surveys that followed good drought-breaking rains, “therefore, recruitment failure may have also played a role in the observed population changes”. 

Recruitment refers to the process by which new plants found a population or are added to an existing population.

Possible reasons for this failure to recruit could include a depleted seed bank because of poor flowering and/or seed set before the 2023 surveys. More than half the population occurred in shaded microsites associated with rocks or live or dead nurse shrubs (nurse shrubs are larger shrubs which they rely on to survive and reproduce).

These individuals were also larger than those found in open microsites. 

“Nurse shrubs were severely impacted by the preceding drought, with 42% of these shrubs having died by the time of the 2023 survey,” they wrote, adding that this large die-off of shrubs was also reported by another study conducted in the region.

Intense grazing also removes the shrub cover that dampens heat and humidity effects.

“Declining populations of specialist succulents are further impacted by unscrupulous succulent collectors that remove them from their natural habitat,” the researchers said.