Browsing by Subject "prediction"
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- ItemOpen AccessCollapse of an iconic conifer: long-term changes in the demography of Widdringtonia cedarbergensis using repeat photography(2016) White, J D M; Jack, S L; Hoffman, M T; Puttick, J; Bonora, D; Visser, V; February, E CBACKGROUND: Conifer populations appear disproportionately threatened by global change. Most examples are, however, drawn from the northern hemisphere and long-term rates of population decline are not well documented as historical data are often lacking. We use a large and long-term (1931-2013) repeat photography dataset together with environmental data and fire records to account for the decline of the critically endangered Widdringtonia cedarbergensis. Eighty-seven historical and repeat photo-pairs were analysed to establish 20th century changes in W. cedarbergensis demography. A generalized linear mixed-effects model was fitted to determine the relative importance of environmental factors and fire-return interval on mortality for the species. RESULTS: From an initial total of 1313 live trees in historical photographs, 74% had died and only 44 (3.4%) had recruited in the repeat photographs, leaving 387 live individuals. Juveniles (mature adults) had decreased (increased) from 27% (73%) to 8% (92%) over the intervening period. Our model demonstrates that mortality is related to greater fire frequency, higher temperatures, lower elevations, less rocky habitats and aspect (i.e. east-facing slopes had the least mortality). CONCLUSIONS: Our results show that W. cedarbergensis populations have declined significantly over the recorded period, with a pronounced decline in the last 30 years. Individuals that established in open habitats at lower, hotter elevations and experienced a greater fire frequency appear to be more vulnerable to mortality than individuals growing within protected, rocky environments at higher, cooler locations with less frequent fires. Climate models predict increasing temperatures for our study area (and likely increases in wildfires). If these predictions are realised, further declines in the species can be expected. Urgent management interventions, including seedling out-planting in fire-protected high elevation sites, reducing fire frequency in higher elevation populations, and assisted migration, should be considered.
- ItemOpen AccessHIV viral load as an independent risk factor for tuberculosis in South Africa: collaborative analysis of cohort studies(2017) Fenner, Lukas; Atkinson, Andrew; Boulle, Andrew; Fox, Matthew P; Prozesky, Hans; ZYrcher, Kathrin; Ballif, Marie; Furrer, Hansjakob; Zwahlen, Marcel; Davies, Mary-Ann; Egger, MatthiasIntroduction: Chronic immune activation due to ongoing HIV replication may lead to impaired immune responses against opportunistic infections such as tuberculosis (TB). We studied the role of HIV replication as a risk factor for incident TB after starting antiretroviral therapy (ART).
- ItemOpen AccessPatterns of stress and strain rate in southern Africa(2006) Bird, Peter; Ben-Avraham, Zvi; Schubert, Gerald; Andreoli, Marco; Viola, GiulioThe southward propagation of the East Africa rift presents an opportunity to study plate boundary formation. We tabulate orientation data which confirm the province of NW-SE directed most compressive horizontal principal stress (Wegener stress anomaly) earlier tentatively attributed to ridge push. We also collect information on stress regime, described by the associated Andersonian fault type(s). We use thin shell finite element models with realistic rheology to test three causes of stress: (1) lateral variations in density moment, (2) resistance of unbroken lithosphere to relative plate rotation, and (3) stress concentration ahead of a crack tip. Models with stress due primarily to variations in density moment are unsuccessful in their predictions (59-73% incorrect regimes; 32-40° azimuth errors). Models in which Africa-Somalia spreading is regulated at realistic rates by remote boundary conditions are more accurate (18-41% incorrect regimes; 25-35° azimuth errors). Treating the East Africa rift as a frictionless crack degrades the fit in either case. Apparently, the Wegener stress anomaly is caused primarily by resistance to the relative rotation between the Somalia and Africa plates. The East Africa rift north of 21°S may be weakened by strain but has residual friction ≥0.1. Greater strength of oceanic lithosphere is likely to cause stress increases, reorientations, and regime changes offshore. The predicted strain rate map has high rates along the rift, curving at 12°S into a western arc through Angola-Namibia-South Africa. Seismic hazard in Namibia may be greater than the instrumental catalog suggests. However, a number of unfit data indicate that these models represent only a first step.
- ItemRestrictedThe relationship between bullying behaviours and high school dropout in Cape Town, South Africa(2008) Townsend, Loraine; Flisher, Alan J; Chikobvu, Perpetual; Lombard, Carl; King, GaryBullying is generally defined as largely unprovoked, negative physical or psychological actions perpetrated repeatedly over time between bully/ies and victims. Bullying can lead to fear of school, absenteeism, and stunted academic progress, which in turn are precursors to dropping out of school. This paper's aim is to report rates of bullying behaviour, and to investigate whether bullying behaviour predicts high school dropout in Cape Town, South Africa. Stratified, proportional sampling yielded 39 from a total of 214 schools, from which 40 learners were randomly selected from the combined class list of two, randomly chosen, Grade 8 classes in each of the 39 participating schools. Thus 1 470 learners (from a total of 181 018) completed a self-report questionnaire in 1997, and were followed-up in 2001. This report focuses on those learners who had dropped out of school between 1997 and 2001 (n = 776; 55.2%). Univariate and multiple logistic regression models were used to investigate the relationship between bullying behaviours and dropout, controlling for factors known to be strongly related to high school dropout, namely age, socioeconomic status, race, or ethnicity, being raised by a single parent, repeating a grade, and substance use. Odds ratios and 95 per cent confidence intervals were calculated, taking the clustering of schools into account. In 1997, 52% of the boys and 37% of the girls had been involved in bullying behaviours. Of the three bullying categories (bully, victim, and bully-victim), girls but not boys in the 'bully-victim' category were significantly more likely to drop out of school (OR 1.82; CI 1.09-3.04, and when controlling for confounders OR 2.60; CI 1.32-5.10). The pervasiveness of both high school dropout and bullying behaviour points to an urgent need for future research, and intervention in these areas.