Browsing by Author "Musker, Seth"

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    Dispersal, gene flow, niche divergence and local adaptation in the hyper-diverse ruschioid Aizoaceae: testing alternative modes of speciation in the Knersvlakte quartz field flora of the Succulent Karoo, South Africa
    (2017) Musker, Seth; Verboom, George Anthony; Ellis, Allan G
    What are the roles of alternative modes of speciation in the generation of biological diversity? This question is fundamental to the debate surrounding the origins of extraordinarily diverse regions and clades. Diversification in the Cape flora of South Africa has been intensively studied owing to its remarkable diversity, for which significant roles for both ecological and non-ecological speciation have been invoked as drivers. However, much of this research has focused on the mesic fynbos vegetation, with far less attention paid to its neighboring biodiversity hotspot, the arid Succulent Karoo (SK), which has hosted the spectacular radiation of the ecologically dominant ruschioid Aizoaceae, a succulent group which exhibits extreme morphological diversity and convergence. This thesis focused on ruschioid Aizoaceae in the Knersvlakte, a small region of the SK which holds a diverse and endemic-rich flora specially adapted to the ecologically unusual quartz fields - whose patchy distribution in the landscape suggests that diversification may have been facilitated by divergence of populations isolated on these 'environmental islands' in a similar fashion to serpentine systems such as those in California - and continues from previous work on the Knersvlakte-endemic ruschioid genus Argyroderma which supported an adaptive radiation in allopatry hypothesis. A population genomic approach was used to investigate the scale of seed dispersal (which is thought to be very limited due to the group's highly specialized ballistic dispersal mechanism) as well as correlates of population divergence in two ruschioid quartz-field specialists with very different growth forms (the shrubby Ruschia burtoniae versus the dwarf Conophytum calculus) and which co-occurred at four sites distributed throughout the Knersvlakte. This, in combination with ecological and experimental transplant data to test the adaptive underpinning of edaphically-driven community structure, ecological isolation and niche divergence in these and other quartz field species, made it possible to tease apart the roles of local adaptation and limited dispersal in driving gene flow and speciation in the system. Quartz fields were found to be a highly insular habitat with strong internal edaphic community structure, suggesting that they represent an environmental island system. In addition, intrinsic dispersal ability was very poor in the specialist shrub, which showed complete genetic isolation between the four populations separated by just 17-42 km. This species showed strong local adaptation between the populations as well as some evidence that this inhibited gene flow, though it is more likely that dispersal limitation allowed for fundamental niche divergence. In contrast, the dwarf showed surprisingly good dispersal ability and consequent weak genetic structure, which accounted for the lack of edaphic local adaptation between the populations. The study showed that, contrary to expectation, not all ruschioid Aizoaceae are poor dispersers, and also suggested that the likelihood of ecological speciation in response to edaphic heterogeneity is contingent on dispersal ability. Strongly limited dispersal may thus have contributed to the group's diversification either through non-adaptive radiation or edaphically driven adaptive radiation, but other factors are more likely to have driven diversification in sections of ruschioid Aizoaceae that possess mechanisms of long-distance dispersal. Future work might focus on inferring the scale of dispersal (e.g. based on morphological traits) and whether it predicts diversification rates. Finally, the lack of morphological variation in R. burtoniae belies its strong ecological and genetic divergence; in light of this, systematists are encouraged to investigate cryptic speciation in the group.
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    Polyploid speciation in the Greater Cape Floristic Region : species limits within Ehrhara calycina
    (2013) Musker, Seth; Verboom, Tony
    Ehrharta calycina J. E. Sm. (Poaceae) is a widespread and often abundant species whose core distribution lies within the Greater Cape Floristic Region (GCFR). Importantly, it shows a high degree of morphological and ploidy level variation. Based on observations of distinct forms co-occurring at multiple sites, the hypothesis that E. calycina comprises more than one species was tested. Morphological analysis of herbarium specimens using a multivariate approach found strong evidence for the presence of two distinct phenetic clusters among specimens from sites of sympatry, and these were termed ‘robust’ and ‘slender’ forms. This translated to a broader sampling of specimens from across the GCFR (n = 516), of which over 91% could be attributed to one or other form with >90% confidence using linear discriminant analysis. Importantly, chromosome counts supported the hypothesis that these phenetic clusters represent distinct cytotypes, with slender forms being diploid (n = 12) and robust forms tetraploid (n = 24), thus providing the primary reproductive isolating mechanism between the forms. Furthermore, differences in climatic niche suggest that these forms represent ecologically independent entities. Finally, evidence that the forms differ in palatability has considerable practical implications given that E. calycina is regarded as having high pasture value. Together, our results provide multiple lines of evidence for the existence of at least two morphologically distinct, reproductively isolated species within E. calycina, and suggest that the role of polyploidy in generating floral diversity in the GCFR may be more important than previously thought.
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    Quantifying Shoebill (Balaeniceps rex) habitat suitability in the Bangweulu Wetlands, Zambia
    (2013) Musker, Seth; Mullers, Ralf; Amar, Arjun
    A quantitative analysis of suitable habitat for the Shoebill (Balaeniceps rex), a large waterbird confined to African swamps, was conducted by using a combination of aerial photographs and a previously determined habitat suitability model from the Bangweulu Wetlands, Zambia. The Shoebill is considered Vulnerable on the IUCN red list, but there are no pre-existing quantitative data on the composition of suitable habitat for this species. Both univariate and multivariate analyses revealed that non-wetland habitats as well as non-vegetated wetland habitats have low suitability compared to vegetated wetland habitats. Notably, reeds correlated significantly and positively with suitability (rs = 0.338, p < 0.001). We found little support for the hypothesis that floating vegetation is highly suitable for Shoebills, but the analysis for this habitat may have suffered as a result of confounding factors. The relationship between flooded grassland and suitability was highly variable and was not significant (rs = 0.009, p = 0.807). In contrast, dry grassland correlated positively with suitability (rs =0.289, p < 0.001), but its suitability scores were generally much lower than for flooded grassland. Quantitative data on Shoebill habitat suitability will be useful for future Shoebill population surveys, and will improve our ability to make informed decisions regarding its conservation.