Browsing by Author "Haiden, Sarah"

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    Topography as a determinant of range extent and overlap : a species level phylogenetic reconstruction and geographical range analysis of Syncarpha (Asteraceae)
    (2011) Haiden, Sarah; Verboom, George Anthony; Bergh, Nicola G
    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.
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    Why do wasp induced galls of Acacia longifolia photosynthesise?
    (2011) Haiden, Sarah; Cramer, Michael D; Hoffmann, John H
    While many stem and bud galls contain chlorophyll, and have the potential to photosynthesise, these insect-induced growths are generally thought to act as strong carbon sinks, manipulating the normal phloem transport of the host plant in order to serve the demands of the galling herbivore. This study investigated the photosynthetic capacity of bud galls induced by the wasp, Trichilogaster acaciae longifoliae (Pteromalidae) in the invasive Acacia longifolia. The role of this photosynthetic activity was examined in terms of its ability to subsidise carbon budgets, as well as to provide O₂ to the larvae and consume CO₂ in the dense gall tissue, thereby maintaining O₂ and CO₂ concentrations within the range of larval tolerance. Galls were found to contain an overall chlorophyll concentration that was less than half that of subtending phyllodes and a maximum stomata! conductance only 16% that of phyllodes. Gas exchange measurements indicated that while photosynthesis never fully compensated for the respiratory costs of the galls, light-induced carboxylation within galls contributed substantially to the maintenance and growth of galls, especially in the early stages of their development. Very low levels of O₂ were found within the larval chamber and internal tissues of galls, and these levels responded only marginally, if at all, to light, suggesting that the photosynthetic activity of galls does not play a critical role in providing 0 2 to the larvae. The percentage mortality and metabolic response of larvae in reaction to various atmospheres of reduced O₂ and elevated CO₂ indicated that larvae were tolerant of hypoxia and capable of rapidly reducing their respiratory rates to cope with hypercarbia, at least over the short term. Sustained metabolic arrest may, however, have toxic consequences for insects, causing cell damage or even death. The photosynthetic activity of galls substantially reduced internal CO₂ concentrations, thus preventing CO₂ from accumulating within galls over prolonged periods. Hence, the capacity of galls to photosynthesise has significant implications for the survival of the developing larvae by reducing the risk of hypercarbic_toxicity and supplying additional carbohydrates to the gall and its inhabitants, thereby creating a favourable microhabitat in which to live.