Browsing by Author "White, Joseph Douglas Mandla"

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    Analysis of long-term changes in populations of the Clanwiliam Cedar (Widdringtonia cedarbergensis) using repeat photography
    (2013) White, Joseph Douglas Mandla; Hoffman, Timm; Jack , Sam; February, Edmund C
    Repeat photography photosets were used to analyse 20th century changes in populations of Widdringtonia cedarbergensis from four sites in the Cederberg Mountains, South Africa. Systematic evaluation of 115 photosets was combined with field observation and environmental data to determine the age class and degree of foliage cover, as well as fire frequency, rockiness of habitat, altitude, aspect of slope, annual mean temperature and annual precipitation for each tree viewed in photosets. Mortality and recruitment events were documented in each of the photosets. A total of 1315 trees were recorded in historical photographs (1931- 1982), with 968 of these shown to have died in the repeat photographs (2007-2013), indicating 74% mortality. With only 45 (3.4%) recruits being recorded, the total current living population of W. cedarbergensis in the repeat photographs was 392. There was no significant difference found in mortality or recruitment across the sampled sites. From the historical to repeat photosets there was an 8% increase in the proportion of mature adults and a 13% increase in the proportion of W. cedarbergensis individuals with sparse foliage cover. A generalized linear model was used to determine the effects of environmental factors on W. cedarbergensis mortality in natural populations.
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    Seed predation and potential dispersal of Ceratocaryum argenteum (Restionaceae) nuts by the striped mouse (Rhabdomys pumilio) / Joseph Douglas Mandla White
    (2013) White, Joseph Douglas Mandla; Bronner, Gary N; Midgley Jeremy J
    This study aimed to better understand the role of rodents as seed predators and dispersers in the fynbos biome at De Hoop Nature Reserve, South Africa in May and June, 2013. Based on previous studies I hypothesised that the large, nut-like seeds of Ceratocaryum argenteum (Restionaceae) are scatter-hoarded by rodents and that rodent seed choices and seed fates are affected by seed size and hull thickness. Field trials using three seed types showed that smaller seeds with a high reward and low processing cost were consumed significantly(p<0.01) more than large, thick hulled seeds. Application of wire tags to facilitate discovery of relocated seeds had no significant influence on seed choice (p>0.05), but further research should be conducted to determine if spooling of C. argenteum seeds influences rodent seed choice. Smaller seeds with a high-reward and low processing cost showed a significantly greater percentage of usage (p<0.01) where seed stations were encountered and exploited. Rhabdomys pumilio was confirmed as being the most common murid at the study site, however, it seems unlikely that it scatter-hoards C. argenteum seeds, as no consumption or burial of seeds was observed. However, R. pumilio did show an interest in C. argenteum seeds and attempted to consume some seeds or carried seeds over distances not significantly different (p>0.05) from the observed distances between nearest neighbour C.argenteum plants before discarding them on the soil. Additionally, the maximum distance R. pumilio moved C. argenteum seeds was commensurate with the maximum distance between nearest neighbour C. argenteum stands, so the end fate of the seeds remains unknown. Further research should account for seasonal variability in scatter-hoarding behaviour.