Aloidendron pillansii

The population has three distinct subpopulations (northern, central and southern) each with differing climatic and habitat characteristics. The total population number was estimated to be around 5,935 with 1,891 of those individuals subject to an ongoing monitoring effort (Swart and Hoffman 2013). Discovery of a few additional stands of plants since 2013 means that the population can be confirmed to be more than 5,935 individuals but less than 9,000 individuals.

The northern subpopulation in southern Namibia is senescent, it has the highest density of individuals with 46% of the overall population occurring here. It receives the highest concentration of winter rainfall and fog. This subpopulation occurs predominantly on east and southwest facing slopes and receives the coolest average annual temperature. There is a 4.6:1 ratio of dead to live individuals (77% of known individuals are dead). The majority of the live individuals are adults between 3 and 7 m tall (Swart and Hoffman 2013). There are no seedlings and very few juveniles. While dead skeletons persist for long periods of time, and the rate of decay is not known, this species has a very long generation length of over 150 years, the proportion of dead individuals are therefore assumed to represent those that have died in the past two generations (300 years). This subpopulation has therefore declined by at least 70% over the past two generations.

The central subpopulation constitutes an estimated 16% of the population and occurs mainly in and on the outskirts of the Richtersveld Transfrontier Park. This subpopulation has the lowest density of individuals in comparison to the other two subpopulations. Plants occur on all slope aspects and receive both winter and summer rainfall. For every two live individuals there is one dead skeleton, this subpopulation has a higher concentration of juvenile individuals than the northern population (Swart and Hoffman 2013). It is subject to significant recent threat from stock overgrazing and illegal collection for the horticultural trade. There has been extensive monitoring at one of the stands within this subpopulation that occurs at Cornellskop which constitutes around 5% of the individuals from this central subpopulation. A study in 2006 noted that the number of mature individuals at Cornellskop had halved between 1937 and 2004 (Duncan et al. 2006). This same study showed there to be an annual mortality of 1.4% for individuals above 3 m leading to the conclusion that within 60 years there will be no more adults on Cornellskop. While this stand of individuals does have recruitment taking place (juveniles and seedlings were observed to be present), there has also been a significant loss of small individuals to illegal poaching with 10–15% of juveniles stolen in one year. Cornellskop has since 2018 also been losing habitat to large movement of sand, with the western slopes now covered in sand. This is a new form of habitat change that started in 2015 and occurs as a result of loss of vegetation cover from drought and overgrazing, and changes in wind direction and frequency linked to ongoing climatic change within the Richtersveld region. In other parts of the central subpopulation between the years 2017 and 2020 a number of mature individuals were observed to have died as a result of the severe and prolonged drought that took place between 2015 and 2019. At Five Sisters, another stand in the central subpopulation, 70% of individuals that were alive in 2007 had died 2020. Given these observations of recent declines and decline of 70% of this central subpopulation over the past two generations (300 years) is also inferred.

The southern subpopulation, that constitutes an estimated 38% of the population, is blocked off from winter precipitation due to its location east of the Stinkfontein mountains. This subpopulation receives the highest average annual temperature. Plants grow predominantly on east and west aspects. There were almost four live individuals for every dead skeleton here and the population was considered to be growing prior to 2015 due to a high concentration, almost 50% of the subpopulation, consisting of seedlings and juveniles (Swart and Hoffman 2013). Unfortunately, there has been high mortality between 2015–2020 which has been caused by predation from baboons. Monitoring of individuals present in 2006 indicated that by 2014, 6.4% had died while by 2020, 48.8% of the 2006 individuals had died. This reflects that the drought that took place from 2016–2020 has caused increases in mortality.

Given the above data the population is inferred to have declined by at least 61% over the past 300 years (two generations). Future declines to the population are extremely likely since modelled climate envelopes for the time period 2061–2080, have very low overlap with the current species occupied habitat (supporting information). Climate models for the likely emission scenarios where emissions stay at present day levels (RCP 2.6) (Hausfather and Peters 2020) and worst-case scenarios where emissions continue to increase during the 21st century (RCP 8.5) indicate that there will be a loss of suitable bioclimatic envelope of between 92% and 100% between 2061 and 2080 (see attached Supporting Information document).