Browsing by Author "Rebelo, Anthony G"

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    Changes in abundance and distribution of Protea caffra in the central and northern Drakensberg as a consequence of climate and land use change
    (2014) Poultney, Daniel Mika-Nsimbi; Hoffman, Timm; February, Edmund C; Rebelo, Anthony G; Puttick, James; Jack,Sam
    The study investigated changes in populations of Protea caffra at five sites in the central and northern regions of the uKhlahlamba Drakensberg Park, a UNESCO world heritage site. Based on an archaeological study in 1994 that used charcoal remains to compare the size of Protea species populations in the Drakensberg to a much earlier era (between 1260 to 1880 BP), it was hypothesized that there would be a further decline in the number of individuals. A total of 76 historical photographs (dated from 1950-1995) were re-photographed in 2014 and the number of individual P. caffra plants in the two sets of photographs counted to assess the extent and rate of population change. The landform units were demarcated on each photograph according to their catenal position, aspect, elevation and the extent of rocky cover. Geological parameters for each landform unit were obtained from GoogleEarth, climatic data from the South African Atlas of Climatology and Agrohydrology and fire data from MODIS. In 76% of the landform units there was an increase in the number of individuals over time, in 17% a decrease and in 7% there was no change over time. Several hypotheses were generated from a conceptual model based on ecological insight into the study area to explain change in the abundance and distribution of P. caffra. Using an Akaike test, the input models were ranked according to how representative the explanatory variables were of the observed change. The model with aspect and elevation as explanatory variables was ranked the best predictor of change. A greater proportion of increase in the size of the population was found on lower slopes. With increasing atmospheric CO2, P. caffra on the warmer lower slopes are likely to have increased photosynthetic rates and increased productivity. A greater proportion of landform units showing a decrease in P. caffra was shown at higher elevations. The reason could be higher fire intensity at higher elevations. The variation in change influenced by fire suggests it is an important regulator of P. caffra numbers. The fact that there is no relationship between fire frequency and landform units showing an increase in P. caffra implies that the majority increase in the species is more likely to be driven by climatic changes, i.e. increases in atmospheric CO2 rather than a change in the fire regime. Increasing atmospheric CO2 levels lead to enhanced growth rates, postburn recovery and resprouting of tree species in grasslands and savannas, which could account for the majority increase in P. caffra populations in the grassland of the Drakensberg. This has important conservation management implications for the reserve, in that the current fire regimes do not appear to be having a negative impact on the abundance in P. caffra. Further, fire will continue to be an important management tool in maintaining the vegetation structure and grasstree codominance in the reserve.
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    The influence of rainfall seasonality and climate change on the demography of Aloe Dichotoma, a long-lived succulent tree from semi-arid southern Africa
    (2014) Gallaher,Kirsten Sarah Leilani; Hoffman, Timm; Rebelo, Anthony G; Jack,Sam
    In the arid and semi-arid western parts of southern Africa, Aloe dichotoma Masson, a long-lived succulent tree species, is thought to be responding to anthropogenic climate change. However, differences in response across its distribution are likely to be related to rainfall seasonality. This study investigated change in ten Aloe dichotoma populations within winter and summer rainfall zones in South Africa and southern Namibia. Using repeat photography over a timespan of approximately 30 years, demographic patterns, population dynamics (including mortality, recruitment and overall population change) and growth were assessed and modelled with climatic variables. Long-term patterns of recruitment and longevity were also investigated by using individual plant growth data to reconstruct recruitment histories for each population. Finally, the influences of climatic conditions on recruitment were evaluated by superimposing historical rainfall and temperature data. Differences in response between rainfall zones are clearly evident throughout. Generalised linear models revealed lower mortality, higher recruitment and positive population change at winter rainfall sites, while summer rainfall sites showed negative population change. Growth data revealed more rapid growth in height of juveniles than adults, and slower growth in height in the winter rainfall zone, most likely related to differences in tree architecture. It is evident that biotic and anthropogenic factors such as herbivory, nurse plants and theft are likely to moderate observable patterns as opposed to driving them. Recruitment modelling suggested that A. dichotoma attains a maximum age of 300 to 350 years, and revealed recent recruitment peaks in the winter rainfall zone and peaks around the turn of the 19th century in the summer rainfall zone. Changes in temperature and rainfall are likely to be the main drivers. Rising temperatures within the last century may have driven increased recruitment and low mortality in the cold-limited winter rainfall zone, while decreasing rainfall within the summer rainfall zone, combined with increasing temperatures, may explain this region's low recruitment and high mortality. The advancement of knowledge of broad spatial and temporal patterns in A. dichotoma and the likely causes, coupled with fine-scale future studies, will enable more detailed prediction of the species' response to future global change.
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    The preservation of the Cape flora : status, causes of rarity, ideals and priorities
    (1992) Rebelo, Anthony G; Siegfried, W R
    The Cape Floristic Region (CFR), covering 90 000 km2, comprises one of the world's six floral kingdoms. With 8600 species, of which 68 per cent are endemic, it ranks amongst the richest of temperate and tropical floras. Although 19 per cent of the CFR occurs in nature reserves, by far the majority of the preserved area comprises Mountain Fynbos. Only 0.5 and 3 per cent of the original extent of Renoster Shrub/and Lowland Fynbos is preserved, respectively. In this study Fynbos vegetation is identified as the richest habitat for Red Data Book (RDB) plant, freshwater fish, amphibian, butterfly, and reptile species in southern Africa. The greater Cape Town metropolitan area is identified as containing by far the highest richness of RDB plant, butterfly, reptile and amphibian species in the CFR Thus, this area ranks globally as one of the most urgent conservation priorities. The study also illustrates that previously used methods for evaluating priority conservation areas have under-rated species-poor areas containing a high proportion of RDB species. By collecting for species richness, a far more realistic picture of threatened areas can be obtained from RDB taxa. This study predicts, using a priori hypotheses based on ecological traits, and finds, that seed dispersal and regeneration strategies are most strongly correlated with rarity, most specifically with distributional area. Using distributional data for the Proteaceae, this study estimates that 95 per cent of all vascular plant species in Fynbos can be preserved in 16 per cent of the area. It also identifies the sites that require preservation if the maximum protection of floral diversity is to be realized. Two null models for evaluating the efficiency of a spatial configuration of reserves are proposed. Utilizing an iterating selection procedure, this study explores various algorithms, based on species richness and rarity, to construct ideal reserve configurations. This study provides the first empirical confirmation that the ideal approach to designing a reserve configuration is to identify areas of high endemism and richness in distinctive vegetation types within particular biogeographical regions. Thus, this study pioneers the use of RDB data to identify priority conservation regions, provides one of the first assessments of the causes of rarity in plants and establishes useful null models and algorithms for the identification and testing of ideal reserve locations in the design of integrated reserve networks. Not only does this study contribute towards theoretical reserve selection procedures, but it provides one of the most advanced frameworks for the preservation of a top conservation priority in the world, the CFR.