The Prince Albert Vygie: declining populations over two decades

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#03 2024

The Prince Albert Vygie: declining populations over two decades

By Sue J. Milton, SAEON Arid Lands Node Research Associate* and Helga van der Merwe, Arid Lands Node Scientist

The Prince Albert Vygie, Bijlia dilatata, is an endemic species found in the Prince Albert region of the Western Cape, South Africa. This species is listed as Endangered on the Red List of South African Plants of the South African National Biodiversity Institute’s Threatened Species Programme.

According to the website, B. dilatata is a “range-restricted species occurring at three to five locations and declining due to ongoing habitat loss and degradation”. The populations are restricted to sites with a particular set of edaphic conditions and are therefore edaphic specialists. Details of the sites are specifically kept vague due to the risk of plant poaching that is rife in the Succulent Karoo region (Succulent poaching).

Flowering Bijlia dilatata, an edaphic specialist and endemic succulent to the Succulent Karoo (Photo: C. Hundermark)

Bijlia dilatata seed capsules triggered by raindrops to open and disperse seeds over time and space (Photo: S.J. Milton)

Dead nurse plant under which Bijlia dilatata individuals are growing (Photo: S.J. Milton)

In 2002, various populations of this species were surveyed across an elevation gradient. Subsequent surveys were conducted in 2020 and 2021, during an intense drought period, and again in 2023 following 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 (Milton et al., 2024).

An indication of the severity of the drought can be gained by using the Standardised Precipitation Evapotranspiration Index (SPEI). The SPEI takes into account both rainfall and potential evapotranspiration determined by temperature (Vicente-Serrano et al., 2010). These data show that the recent drought in the Prince Albert area was particularly severe (Figure 1).

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.

However, other factors influencing the quality of a microsite may also influence an individual’s ability to survive an exceptionally hot and dry period. Grazing intensity (no grazing, grazing, 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. Also, grazing can directly influence plants by removing plant parts (being eaten by an animal) or the trampling of individuals.

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 individuals were found in the smallest size class while larger individuals were more likely to survive.

Figure 1. Standardised Precipitation Evapotranspiration Index (SPEI) for the Prince Albert area (33.25S, 22.25E) from 1902 to 2022. Ten-year running mean shown as thin black line. Hatched box indicates period covered by this study. SPEI data downloaded from https://spei.csic.es/spei_database/#map_name=spei12#map_position=1463 on 2023.11.27 (Begueria et al., 2023). SPEI uses the global CRU TS 4.03 dataset of precipitation and potential evaporation with a spatial resolution of 0.5 degrees [available at: https://spei] (Milton et al., 2024).

Failure to recruit

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. Possible reasons for this failure to recruit could include a depleted seed bank because of poor flowering and/or seed set prior to the 2023 surveys.

More than half the population occurred in shaded microsites associated with rocks, or live or dead nurse shrubs. These B. dilatata 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. This large die-off of shrubs was also reported by another study conducted in the region by Milton et al., 2023.

Our findings support predictions that warming, or the combination of drought and warming, may cause decline in populations of dwarf succulents such as B. dilatata. Although this species spans an altitudinal range of only 300 m, 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.

Intense grazing is also a threat to the population due to the removal of 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.

Acknowledgements

The authors thank Nicola Wagner, Ulrike Müller, Callum Clark, Courtney Hundermark and Collette Hurt for conducting the fieldwork and capturing data. The authors acknowledge the Gouritz Cluster Biosphere Reserve and SAEON for funding.

Furthermore, we would like to thank the various landowners for permission to access their land to deploy data loggers, to document B. dilatata populations and/or for providing weather data.

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