Abstract
The monumental work of Olov Hedberg provided deep insights into the spectacular and fragmented tropical alpine flora of the African sky islands. Here we review recent molecular and niche modelling studies and re-examine Hedberg’s hypotheses and conclusions. Colonisation started when mountain uplift established the harsh diurnal climate with nightly frosts, accelerated throughout the last 5 Myr (Plio-Pleistocene), and resulted in a flora rich in local endemics. Recruitment was dominated by long-distance dispersals (LDDs) from seasonally cold, remote areas, mainly in Eurasia. Colonisation was only rarely followed by substantial diversification. Instead, most of the larger genera and even species colonised the afroalpine habitat multiple times independently. Conspicuous parallel evolution occurred among mountains, e.g., of gigantism in Lobelia and Dendrosenecio and dwarf shrubs in Alchemilla. Although the alpine habitat was ~ 8 times larger and the treeline was ~ 1000 m lower than today during the Last Glacial Maximum, genetic data suggest that the flora was shaped by strong intermountain isolation interrupted by rare LDDs rather than ecological connectivity. The new evidence points to a much younger and more dynamic island scenario than envisioned by Hedberg: the afroalpine flora is unsaturated and fragile, it was repeatedly disrupted by the Pleistocene climate oscillations, and it harbours taxonomic and genetic diversity that is unique but severely depauperated by frequent bottlenecks and cycles of colonisation, extinction, and recolonisation. The level of intrapopulation genetic variation is alarmingly low, and many afroalpine species may be vulnerable to extinction because of climate warming and increasing human impact.
Highlights
The spectacular flora occurring on the tops of the tropical African sky islands has attracted the attention of generations of biogeographers because of its extreme fragmentation and iconic examples of gigantism and convergent evolution: the impressive plants of Lobelia and Dendrosenecio can rise up to 10 m above the otherwise low-growing alpine vegetation
Whereas the meta-analysis of Vellend et al (2014) found that correlations between species diversity and genetic diversity are generally positive, we find no correlation in these afroalpine habitats, probably reflecting long-distance dispersals (LDDs) stochasticity combined with frequent habitat disturbance caused by the Pleistocene climate oscillations
We have shown that several of Hedberg’s insightful inferences and hypotheses on the origin and evolution of the afroalpine flora have been tested with abundant molecular data and niche modelling tools, in many cases providing evidence fully consistent with his views
Summary
The spectacular flora occurring on the tops of the tropical African sky islands has attracted the attention of generations of biogeographers because of its extreme fragmentation and iconic examples of gigantism and convergent evolution: the impressive plants of Lobelia and Dendrosenecio can rise up to 10 m above the otherwise low-growing alpine vegetation. Quite a few of the afroalpine species that Hedberg (1986) attributed to the endemic afromontane element seem rather to have originated after recent dispersal from temperate regions (e.g., all Alchemilla spp., Gehrke et al 2016; some Carex, Box 1, Gehrke 2011; Gizaw et al 2016b).
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