Dissecting biodiversity in a global hotspot: Uneven dynamics of immigration and diversification within the Cape Floristic Region of South Africa

Abstract

Aim:Fragmented distributions should show immigration and diversification dynamics consistent with the predictions of island biogeography theory. We test whether this applies to the fragmented Cape fynbos vegetation. Location: Southern Africa, Cape Floristic Region (CFR).Taxon: Angiosperms, Restionaceae (restios). Methods:We used a large occurrence dataset and environmental layers to characterize an existing regionalization and the intervals between the regions ecologically and spatially. We extended the available phylogeny for restios and inferred their historical biogeography using models implemented in BioGeoBEARS. We then measured the relative contribution of immigration and in situ speciation to the species richness of each region within the CFR. We used standard statistical methods to test the predictions of the island biogeography theory. Results:The area and environmental heterogeneity of the seven regions of the CFR are positively correlated with in situ speciation rate. Furthermore, more isolated areas, and areas colonized more recently, have proportionally higher immigration rates, and more central and older areas proportionally higher in situ speciation rates. Main conclusions: The variation in immigration and diversification dynamics among the regions within the CFR is extensive and consistent with the archipelago model of island biography theory. This dynamic may contribute significantly to the diversity of the Cape flora. Such a model could be generally useful for understanding the generation and maintenance of diversity in biodiversity hotspots, and may even scale up to explain continental biodiversity. K E Y W O R D S Cape flora, dispersal, hotspot, immigration-diversification dynamics, island biogeography theory, metapopulation, Restionaceae, semi-permeable barriers Editor: Rosemary Gillespie | 1937 WÜEST ET al. | 1947 WÜEST ET al. B I OS K E TCHRafael O Wüest explores the generation, preservation and future fate of the diverse facets of biodiversity. He assesses community structure and assembly, models diversity along environmental gradients, and analyses how evolution and biogeography contribute to shape biodiversity patterns across scales.

Disentangling the drivers of local species richness using probabilistic species pools

“…Cowling et al (2017) referred to this east-west diversity gradient as 'Levyns' Law', and discussed the possible reasons for this richness gradient. This pattern has also been repeatedly shown for the restios, first by Oliver et al (1983), subsequently by Linder and Mann (1998) The processes leading to these regional pool differences (immigration, speciation and extinction) are discussed in Wüest et al (2019). Although our probabilistic species pools contain information on the composition of the restio flora, we focus only on the species richness in this study.…”

“…Geological data were obtained from the South African Council for Geoscience (downloaded in January 2018 from http://www.geoscience.org.za), and describe the geology of South Africa at a resolution of 1:1,000,000 using 496 classes. We simplified these to six classes – calcrete, granite, recent sand, silcrete, sandstones, shale and others (including water‐bodies and classes outside the CFR; details of conversion see (Wüest et al, ) – which we assume to reflect important soil characteristics for plants.…”

“…The processes leading to these regional pool differences (immigration, speciation and extinction) are discussed in Wüest et al (2019). Although our probabilistic species pools contain information on the composition of the restio flora, we focus only on the species richness in this study.…”

“…The implications are that, given the current climate, ranges in the centre of the Cape Floristic Region are environmentally limited, whereas in the northern and eastern extremes they are dispersal limited. However, it seems more likely that during the glacials climate change at the extremes may have had a relatively greater impact on the restios, than the centre (summarized in Wüest et al, 2019). The temporal context is that the Last Glacial Maximum was less than 10,000 generations ago.…”

“…We do not explore however what forces regional richness (e.g. the drivers of speciation, extinction and dispersal -see Wüest, Boucher, Bouchenak-Khelladi, Boucher, Bouchenak-Khelladi, Karger, and Linder (2019). Specifically, we first disentangle the relative contributions of the environment and dispersal in shaping the spatial variation in local restio richness.…”