Declining populations are commonly taken as signs of environmental degradation. However, changes affecting organisms with long lifespans must be evaluated in a long-term perspective. Theories of remnant population dynamics, the persistence niche and storage effect suggest that rare recruitment events are normal and when eventually occurring, they can restore seemingly degraded populations.
Dracaena ombet Heuglin x Kotschy and Peyr. is a long-lived and endangered tree native to the mountains surrounding the Red Sea. In the mist oasis of Erkowit in Sudan, a historical key site, it was thought to have disappeared completely after a mass death event in 1958–1961. A nearby population was later found, but also that was severely affected by more recent mass death events. Other species in the Dragon tree group are also in decline, with a noticeable lack of recruitment. Nevertheless, a recent study from Erkowit found signs of a recruitment event. Here we investigate the status and realised niche of the dracaena population in nearby Berreakey.
We surveyed a wadi and surrounding hillsides along transects. Visible dracaenas were mapped (#152). An equal number of absence points were distributed at random (>20m away from dracaenas) within a 100m buffer surrounding the transects. In lack of meteorological data, a set of environmental variables, including indicators of moisture conditions, were extracted from an SRTM digital elevation model for presence and absence (PA) points. These variables were used to fit a GLM model to describe current realised niche of dracaenas compared to absence points. We used a similar approach to investigate a potential shift in the distribution among saplings versus adult/dead individuals. Both models were validated using 100 repeated – 5 fold cross-validation (CV).
Results/Conclusions
Of the recorded observations (#152), 19 were of dead individuals. Of the living (#133), 90 individuals have only one leaf rosette (90), of which 67 are defined as saplings (height < 0.5m). The dracaenas grow at higher and steeper locations than absence points (Spatial CV AUC 0.74). Furthermore, saplings grow closer to the coast/water divide, i.e. at higher elevations, than older dracaenas (CV 0.703).
Our result confirms that a rare recruitment event has occurred recently both in Erkowit and the nearby Berreakey. However, an altitudinal shift might be ongoing. Any signs of a beginning recovery can therefore be negatively affected by a possible tapering of the life-giving mist belt that is a requirement for this outpost of Abyssinian vegetation.