Ecological niche models of Welwitschia mirabilis and its subspecies in the Namib desert

Abstract

Welwitschia mirabilis, an endemic gymnosperm of the Namib Desert, has two subspecies: ssp. namibiana, found in Namibia, and the typical ssp. mirabilis occurring in Angola and Namibia. Despite decades of research little is known about the distribution and ecological preferences of this species. In this paper, we study the ecological niche and the geographical and environmental niche overlaps of the subspecies. We applied ecological niche models using Maxent to an extensive occurrence dataset of W. mirabilis encompassing all known localities of the species. We prepared sets of environmental predictors for climate, vegetation biomass, geology, and topography. Subspecies models were best explained by climate and geology and revealed clearly disjunct ranges. Furthermore, subspecies models were superior to the species model in terms of realism and model quality. This supports the view that infraspecific variability in ecological preferences for Welwitschia mirabilis is high. This conclusion is further supported by the finding that subspecies showed little overlap in geographical and environmental niche space, confirming their unique environmental preferences. Rainfall above 200 mm was a clear threshold for both subspecies although they were separated at about 22°C mean annual temperature. In sum, our study identified the important large-scale climatic drivers and thresholds that define the disjunct ranges of the subspecies and represent the first realistic distribution map of Welwitschia mirabilis.