The glacial–interglacial climate cycles of the Pleistocene played a significant role in dramatically altering species distributions across the globe. However, the climate of the Greater Cape Floristic Region is thought to have been decoupled from global fluctuations and the current Mediterranean climate remained relatively buffered during this period. Here we explore the roles of climate stability and the topographic complexity of the region on the range history of an endemic Little Karoo plant, Berkheya cuneata, using ensemble species distribution modelling and multi-locus phylogeography. The species distribution models projected onto downscaled climate simulation of the Last Glacial Maximum demonstrated a considerable range contraction and fragmentation into the western and eastern Little Karoo, separated by the Rooiberg inselberg. This population fragmentation is mirrored in the phylogeographic structuring of both chloroplast and nuclear DNA. These results suggest that sufficient climatic buffering coupled with regionally complex topography ensured the localised population persistence during Pleistocene climate cycles but these features have also promoted population vicariance in this, and likely other, Little Karoo lowland species
The Greater Cape Floristic Region (GCFR, Born et al., 2007) consists of the Fynbos and Succulent Karoo biomes, both of which are globally recognised for their unusually high levels of plant diversity and endemism (Goldblatt and Manning, 2002; Linder, 2003) and are considered biodi versity hotspots (Myers et al., 2000). This diversity is suggested to have arisen, in part, due to exceptional climatic stability during the Pleistocene (Cowling, 1992; Cowling and Lombard, 2002; Cowling et al., 2009; Dynesius and Jansson, 2000; Hopper, 2009). Pleistocene climate cycled between glacial and interglacial states and this had global impacts on the size, distribution, and local extinctions of species’ ranges (Hewitt, 2000; Jansson and Dynesius, 2002). However, the climate of the south western Cape was decoupled from the global Pleistocene climate fluctuations, and the current rainfall regime (winter-wet and summer-arid) is suggested to have remained relatively buffered during this period (Chase, 2010; Chase and Meadows, 2007; Dynesius and Jansson, 2000; Meadows and Sugden, 1993). This localised climatic stability would weaken the effects of orbital-forcing on species’ range dynamics and thus reduced species and population extinction rates (Dynesius and Jansson, 2000).
In this study we investigate the role of Pleistocene climate on the range dynamics within the GCFR using species distribution modelling (SDM) and phylogeography (sensu Hugall et al., 2002; Svenning et al., 2011). Species distribution modelling can predict ranges under current and altered climate conditions (Franklin, 2010). We project the SDM onto modelled and downscaled climatic conditions of the Last Glacial Maximum; this period represents a glacial extreme of the Pleistocene cycles where global ice volumes were at their maximum. We then compare these predictions with phylogeographic patterns as range dy namics leave genetic imprints on species (Avise, 2000; Waltari et al., 2007). Species distribution modelling and phylogeographic analyses may therefore help to understand the role of climatic factors in promoting differentiation (e.g. Hugall et al., 2002) and, ultimately, speciation within the GCFR flora.
Our focus is on Berkheya cuneata (Thunb.) Willd. (Fabaceae), a plant species that is endemic to the Little Karoo sub-region of the eastern GCFR. The Little Karoo is a dry intermontane valley within the Cape Fold Belt of the Western Cape, South Africa (Fig. 1). The region is complex, both in terms of topography and climate (Vlok and Schutte-Vlok, 2010; further details below). Our results suggest that sufficient climatic buffering coupled with regionally complex topography ensured localised population persistence during Pleistocene climate cycles and that these have played key roles in promoting population vicariance in B. cuneata. in a number of localities due to overgrazing by livestock, and the extinction of at least three local populations has been observed (JHJ Vlok, unpublished observations). The ploidy level within this species and across populations is unknown.