Browsing by Author "Duncan, John A"

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    Aloe Pillansii on Cornell's Kop : are population changes a result of intrinsic life history patterns or climate change?
    (2004) Duncan, John A; Hoffmann, Timm
    Aloe pillansii populations in the biodiversity hotspot of the Succulent Karoo in Southern Africa are thought to be under threat of extinction. This study investigated the population at the type locality; Cornell's Kop in the Richtersveld, South Africa. It has been suggested that theft, animal damage and more recently climate change have caused a decline in the population by over 50% in the last decade, however very little is known about this rare species. Repeat photography and surveys were used to analyse life history patterns and dynamics of the population and thus establish what the potential threats to this keystone species actually are. Repeat photography indicates that there have been high rates of adult mortality over the last fifty years (1.8% of the population dies annually), which results in an average predicted lifespan of 39 years for the remaining adult population on Cornell's Kop. However, a recent survey reported that over 40% of the population recorded were seedlings, which weren't found in a 1995 survey, which is indicative of a recent recruitment pulse on Cornell's Kop and that conditions on the hill are still habitable for A. pillansii. Growth analyses suggest that A. pillansii has an average annual growth rate of 20 mm.yr⁻¹, which in tum means that 8 m individuals may be up to 453 years old. This long-lived strategy would require A. pillansii to only recruit infrequently, during periods of high rainfall, in order to sustain a viable population, which is consistent with findings on other large desert succulents. Seedling ages were estimated from their heights and it was found that 50% of the seedlings appear to have germinated five to ten years ago; this is consistent with rainfall records from the area which indicate that rainfall was consistently above the annual average for this same period. The findings indicate that although the adult A. pillansii population is declining, the presence of 30 seedlings suggest that the population is entering a recruitment phase after just coming out of a lengthy senescent phase. Although A. pillansii 's extensive lifespan makes it a potentially useful indicator species of climate change, the evidence presented in this study does not suggest that climate change has affected the dynamics of this population.
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    Friend or foe? : the invasive potential and aquacultural application of the sporophytic Falkenbergia stage of Asparagopsis armata in South Africa
    (2004) Duncan, John A; Bolton, John J; Anderson, Robert J
    The presence of the sporophytic 'Falkenbergia rufolanosa' phase of the invasive algal species Asparagopsis armata was first recorded in South Africa 57 years ago. The introduction of this highly invasive alga, of Australian/New Zealand origins, to Europe in the 1920's has since led to a number of recorded invasions by the gametophytic Asparagopsis phase in both the Mediterranean and Atlantic oceans. Recently however, a number of commercial uses for both phases of A. armata have been identified, which have given rise to industrial interest in the species as a candidate for commercial cultivation. Previous studies on European strains of F. rufolanosa and A. armata have identified a number of life history traits, which not only increase A. armata 's invasive ability, but also make it a useful species for commercial tank cultivation. However, different strains are known to have different environmental parameters which regulate their survival, growth and reproduction capabilities. This study investigated the growth of the South African strain of F. rufolanosa in culture, in response to different environmental variables, as well as the conditions necessary for tetrasporogenesis to occur, in order to assess its potential impacts, both as an invasive threat and as a species for commercial cultivation in South Africa.