Topographyas a Determinant of Range Extent and Overlap:A Species
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BOLUS LI BRARY C31 0000 1405 111111111111., 111 HONOURS THESIS SARAH HAIDEN SUPERVISORS: DR G.A. VERBOOM AND DR N.G. BERGH DEPARTMENT OF BOTANY: UNIVERSITY OF CAPE TOWN TITLE: Topography as a determinant of range extent and overlap: A species-level phylogenetic reconstruction and geographical range analysis of Syncarpha (Asteraceae) University of Cape Town ~ !\ 1 The copyright of this thesis vests in the author. No quotation from it or information derived from it is to be published without full acknowledgement of the source. The thesis is to be used for private study or non- commercial research purposes only. Published by the University of Cape Town (UCT) in terms of the non-exclusive license granted to UCT by the author. University of Cape Town Topography as a determinant of range extent and overlap: A species-level phylogenetic reconstruction and geographical range analysis of Syncarpha (Asteraceae) Abstract Understanding what determines species' geographic range extents has several implications for questions in ecology, evolution and conservation biology. The Cape Floristic Region of South Africa is noted for its remarkably high geographic species turnover, often attributed to the exceptional environmental heterogeneity of the region. The complex and highly dissected topography of the CFR provides a model environment in which to investigate the relationship between altitude and species range extent, as well as explore the role of topography in speciation and current range overlap. I examined these questions in the context of Syncarpha, a genus within the Asteraceae tribe Gnaphalieae (paper daisies). A Bayesian analysis of combined plastid and nuclear genes provided the robust, dated phylogenetic hypothesis required to assess the monophyly of the genus, as well as reconstruct the signal of geographic speciation within the lineage. The phylogeny recovered Syncarpha as polyphyletic, comprising two clades with good support, placing the small CFR-endemic genus Edmondia as sister to the larger Syncarpha clade. Using realised range extent estimates and modelled potential distributions of Syncarpha and Edmondia species, this study confirms the importance of topography as a factor constraining species' distributions, and thereby enhancing the scope for their allopatric isolation. The relationship between altitude and realised range extent was found to be unimodal, with ranges being restricted at both high- and low-altitudes, and more extensive at intermediate altitudes. Range filling (the ratio between realised and potential range extent) was also lower in high- and low-altitude taxa compared to mid-altitude taxa. Dispersal limitation, owing to the insular nature of montane habitats, seems the most likely mechanism to restrict the ranges of high altitude taxa, whereas edaphic factors are more likely responsible for the restricted ranges of low altitude taxa. Furthermore, age-range correlations confirm the role of altitude in maintaining a stronger signal of allopatry among recently diverged clades, where montane clades present lower levels of range overlap than those at intermediate altitudes. Thus, the role of topography in limiting dispersal, and hence constraining species distributions, has consequences for understanding the historical diversification of a lineage, as well as implications for management practices in light of climate change-induced range shifts. 2 J Introduction As a basic unit of biogeography, the geographic range of a species has implications for many questions in ecology, evolution and conservation biology. However, our understanding of the ecological and evolutionary determinants of the extent, occupancy, overlap and limits of geographic range, remains incomplete and rather system-specific (Brown et al. 1996; Angbert 2009). Range size has been hypothesised to correlate with various properties of species and features of their environment (Gaston 2003). Numerous studies have demonstrated how variation in population density, dispersal mode and latitude are associated with variation in geographical range extent (Brown et al. 1996), though few have examined the effect of topography in this regard. Patterns of range size and overlap between sister species can also be used to examine predictions about the predominant geographic mode of speciation (allopatry or sympatry), providing greater insight into the evolutionary history of a particular clade (Barraclough and Vogler 1999). There exists plenty of evidence for significant interplay between resource availability and the physiological tolerances of species, and perhaps the most durable explanation for what limits geographic range is simply that those tolerances and capacities are constrained by certain physiological and phylogenetic bounds, and that species are unable to persist in areas where environmental pressures exceed these (Gaston 2003). A species' distribution can be regarded , therefore, as a spatial expression of its niche - i.e. the complete set of environmental conditions or resources that allow for the species' long-term survival (Angbert 2009, Phillips et al. 2006). As such, the geographic range of a species represents a mapping of its fitness as a function of the biotic and abiotic environment, onto a heterogeneous landscape, where range edges occur at points along environmental gradients at which colonization no longer exceeds extinction (Angbert 2009). Complexity, however, is added to the problem of relating species' distributions to the niche when limitations to dispersal are considered . The realised niche (the area that is actually occupied) very seldom matches the fundamental niche of a species (Phillips et al. 2006). It has been argued that widespread, abundant taxa often possess well-developed dispersal abilities, while narrowly distributed, rare taxa have poor dispersal abilities, resulting in substantial variation in the extent to which plant species fill their potential range (Gaston 1998; Schurr et al. 2007). Dispersal is dependent on a number of factors. For example, models of seed dispersal by wind typically include parameters describing wind speed and turbulence, seed size and weight (related to terminal falling velocity), release height and the presence of dispersal aids, like wings or hairs, as well as the mean basal radius and density of obstacles impeding seed movement along the ground during secondary dispersal (Schurr et al. 2007). These parameters have been found to vary significantly between related species (Schurr et al. 2007) and with altitude (Pluess et al. 2005; Tackenberg and Stocklin 2008). 3 While the ability to disperse over long distances is an important determinant of range extent, the ability to persist in the new post-dispersal environment is equally important. It is, therefore, essential to understand the role of niche conservatism (i.e. the retention of niche related ecological traits over time) when considering range dynamics (Wiens et al. 2010). If, for example, the climatic tolerance of a species is not wide enough to encompass a new set of conditions or to allow for acclimatization to them, species with strong niche conservatism will go extinct, whereas species that are more evolutionarily labile have the potential to adapt and undergo range expansion (Wiens et al. 2010). Both dispersal limitation (gene flow) and niche conservatism can strongly influence population genetic structure and promote speciation (Kozak and Wiens 2006). While allopatric speciation is widely viewed to be the most common geographic mode of speciation (Barraclough and Vogler 1999; Coyne and Orr 2004), its ecological basis is poorly understood and remains largely unstudied (Kozak and Wiens 2006). Kozak and Wiens (2007) have, however, investigated how climatic barriers to dispersal in montane regions may drive geographical isolation, divergence and speciation. They considered two models, referred to as the refuge and gradient models of speciation. The first describes how high altitude habitats separated by valleys (or lowland habitats divided by mountains) might serve as refugia, where populations become geographically isolated as they track suitable habitats to higher or lower elevations in response to climate change; the second model describes how new species may evolve as populations adapt to different climates in distinct altitudinal bands. These models predict opposing roles for natural selection in speciation (Kozak and Wiens 2007). The inability to adapt to new environmental conditions through niche conservatism plays the key role in geographic isolation during refuge speciation; contrastingly, with gradient speciation, population isolation is driven by niche divergence and adaptation to new environmental conditions in distinct elevational zones (Moritz et al. 2000; Kozak and Wiens 2007). In terms of range dynamics, the refuge/niche conservatism model predicts that sister species should occupy similar altitudinal bands and possess similar niche requirements (Wiens 2004 ), whereas the gradient model predicts that sister species will occupy divergent niche-spaces and have non-overlapping geographical ranges (Moritz et al. 2000). The species-rich and topographically-complex Cape Floristic Region (CFR) of South Africa provides a model environment in which to study the relationship between topography and horizontal range extent, and investigate the role of geography in speciation using range-overlap analyses and species-level phylogenies (Linder 2003; Barraclough and Vogler 1999). The CFR possesses exceptional plant species