Browsing by Author "Distiller, Greg"

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    Camera traps as sensor networks for space-time exploration of terrestrial mammal communities
    (2018) Bussière, Elsa Marion Sylvie; Underhill, Leslie G; Distiller, Greg
    Most of the conservation issues which ecologists are called on to help resolve are essentially about ecological communities. Camera trapping technology has led to a surge in the collection of large ecological datasets, which provides an unmissable opportunity to attain deeper knowledge of animal community assembly and structure. Using extensive camera trap data, this thesis examines whether camera traps can be used as sensor networks for a space-time exploration of the terrestrial mammal community that occurs in the Little Karoo of South Africa. In Chapter 1, the species-habitat relationship along a ruggedness gradient was studied. Using resource selection functions and multivariate statistics, this chapter showed that the strength of affinities, which mammals developed with specific terrain roughness, varied among species. It also enabled the recognition of subtle and continuous nuances in the spectrum of habitat preferences, providing a novel tool to explore the forces driving species coexistence in local animal communities. The theme of Chapter 2 was to consider patterns of seasonal occurrence within species circadian rhythms. Using kernel density functions with descriptive and multivariate statistics, this chapter showed that most mammal species responded to the ecological variability brought about by seasonality by adjusting their diel activity rhythms between winter and summer, resulting in a reduction of time exposure to a physiologically stressful environment caused by high temperatures in summer. It also highlighted that while some shifts only result from photoperiodism alignment, most are driven by other factors too. Chapter 3 examined temporal-partitioning as a mechanism driving sympatry. Using kernel density functions and mutivariate statistical analyses, this chapter enabled subtle nuances in the spectrum of diel activity rhythms to be visualised, highlighting the variety of temporal niche breadths and of activity onset/offset timings, which allowed diel activity rhythms to diversify and the mammal community to partition the temporal resources. Finally, in Chapter 4, topics dealing with leopard habitat preferences and leopard population density were explored. Using spatially explicit capture-recapture models, this chapter showed that leopard density remained low but varied with topographic relief; it increased with ruggedness of the terrain up to an optimum, and followed a reversed trend as the terrain roughness kept increasing. The population was composed of two groups of individuals with significantly different home range sizes, potentially explained by gender duality in movement. The chapter provided leopard density estimates ranging from 0.49 to 0.82 individual per 100 km2 . Local communities, such as that of the mammal species of the Little Karoo, are neither closed nor isolated. Therefore, it would be insightful if future studies were to embrace the metacommunity concept and explain these patterns of species distribution, abundance and interaction at multiple scales of spatio-temporal organisation.
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    A continuous-time formulation for spatial capture-recapture models
    (2016) Distiller, Greg; Borchers, David; Erni, Birgit
    Spatial capture-recapture (SCR) models are relatively new but have become the standard approach used to estimate animal density from capture-recapture data. It has in the past been impractical to obtain sufficient data for analysis on species that are very difficult to capture such as elusive carnivores that occur at low density and range very widely. Advances in technology have led to alternative ways to virtually "capture" individuals without having to physically hold them. Some examples of these new non-invasive sampling methods include scat or hair collection for genetic analysis, acoustic detection and camera trapping. In traditional capture-recapture (CR) and SCR studies populations are sampled at discrete points in time leading to clear and well defined occasions whereas the new detector types mentioned above sample populations continuously in time. Re- searchers with data collected continuously currently need to define an appropriate occasion and aggregate their data accordingly thereby imposing an artificial construct on their data for analytical convenience. This research develops a continuous-time (CT) framework for SCR models by treating detections as a temporal non homogeneous Poisson process (NHPP) and replacing the usual SCR detection function with a continuous detection hazard func- tion. The general CT likelihood is first developed for data from passive (also called "proximity") detectors like camera traps that do not physically hold individuals. The likelihood is then modified to produce a likelihood for single-catch traps (traps that are taken out of action by capturing an animal) that has proven difficult to develop with a discrete-occasion approach. The lack of a suitable single-catch trap likelihood has led to researchers using a discrete-time (DT) multi-catch trap estimator to analyse single-catch trap data. Previous work has found the DT multi-catch estimator to be robust despite the fact that it is known to be based on the wrong model for single-catch traps (it assumes that the traps continue operating after catching an individual). Simulation studies in this work confirm that the multi-catch estimator is robust for estimating density when density is constant or does not vary much in space. However, there are scenarios with non-constant density surfaces when the multi-catch estimator is not able to correctly identify regions of high density. Furthermore, the multi-catch estimator is known to be negatively biased for the intercept parameter of SCR detection functions and there may be interest in the detection function in its own right. On the other hand the CT single-catch estimator is unbiased or nearly so for all parameters of interest including those in the detection function and those in the model for density. When one assumes that the detection hazard is constant through time there is no impact of ignoring capture times and using only the detection frequencies. This is of course a special case and in reality detection hazards will tend to vary in time. However when one assumes that the effects of time and distance in the time-varying hazard are independent, then similarly there is no information in the capture times about density and detection function parameters. The work here uses a detection hazard that assumes independence between time and distance. Different forms for the detection hazard are explored with the most exible choice being that of a cyclic regression spline. Extensive simulation studies suggest as expected that a DT proximity estimator is unbiased for the estimation of density even when the detection hazard varies though time. However there are indirect benefits of incorporating capture times because doing so will lead to a better fitting detection component of the model, and this can prevent unexplained variation being erroneously attributed to the wrong covariate. The analysis of two real datasets supports this assertion because the models with the best fitting detection hazard have different effects to the other models. In addition, modelling the detection process in continuous-time leads to a more parsimonious approach compared to using DT models when the detection hazard varies in time. The underlying process is occurring in continuous-time and so using CT models allows inferences to be drawn about the underlying process, for example the time- varying detection hazard can be viewed as a proxy for animal activity. The CT formulation is able to model the underlying detection hazard accurately and provides a formal modelling framework to explore different hypotheses about activity patterns. There is scope to integrate the CT models developed here with models for space usage and landscape connectivity to explore these processes on a finer temporal scale. SCR models are experiencing a rapid growth in both application and method development. The data generating process occurs in CT and hence a CT modelling approach is a natural fit and opens up several opportunities that are not possible with a DT formulation. The work here makes a contribution by developing and exploring the utility of such a CT SCR formulation.
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    Enumerating and estimating maternal and neonatal deaths in the Western Cape Province, South Africa
    (2022) Damian, Damian Jeremia; Boulle, Andrew; Distiller, Greg
    Measuring and monitoring progress towards global development goals requires valid and reliable estimates of maternal and child mortality. This thesis has aimed at enumerating and estimating maternal and neonatal deaths from 2010 to 2013 in the Western Cape Province; and determining factors associated with these outcomes during the same period. This thesis comprises nine chapters, of which six present the research findings. The first results chapter has presented the findings from a systematic review, determining trends of maternal and neonatal mortality from 1990 to 2015 in South Africa. The review found that estimates of maternal and neonatal mortality are widely divergent across data sources and estimation methods, with conflicting trends over the analysis period. The second results chapter compared the performance of the existing decision-rule based linkage approach (provincial linkage) which uses fuzzy linkage to an independent fully probabilistic record linkage (PRL) implementation for identifying mortality records across the Western Cape Provincial Health Data. The PRL was shown to be a feasible method for future implementation, while the existing linkage performed similarly to the independent linkage exercise, providing reassurance on the adequacy of the linked datasets on which the subsequent chapters were based. The third and fourth results chapters involved the applications of three-source capture-recapture methods, to estimate maternal and neonatal mortality under-reporting in the Western Cape province. Based on these models, maternal and neonatal mortality under-reporting were estimated at 45.6% and 17.7% over the full 4-year period respectively. The last two results chapters focused on determining factors associated with maternal and neonatal mortality in this setting and exploring whether the estimates of association were altered through using an expanded number of outcome events based on database linkage across multiple data sources. Most findings were consistent with known associations, as well as estimates from single-source analyses in the same setting. The thesis concludes that estimates of maternal and neonatal mortality are widely divergent in South Africa, and single-source reporting likely under-estimates the event rates. The application of capture recapture methods is a viable approach in South Africa to resolve the problems of under ascertainment in estimation of these outcomes.
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    Nonlinear mixed effects modeling of gametocyte carriage in patients with uncomplicated malaria
    (BioMed Central Ltd, 2010) Distiller, Greg; Little, Francesca; Barnes, Karen
    BACKGROUND:Gametocytes are the sexual form of the malaria parasite and the main agents of transmission. While there are several factors that influence host infectivity, the density of gametocytes appears to be the best single measure that is related to the human host's infectivity to mosquitoes. Despite the obviously important role that gametocytes play in the transmission of malaria and spread of anti-malarial resistance, it is common to estimate gametocyte carriage indirectly based on asexual parasite measurements. The objective of this research was to directly model observed gametocyte densities over time, during the primary infection. METHODS: Of 447 patients enrolled in sulphadoxine-pyrimethamine therapeutic efficacy studies in South Africa and Mozambique, a subset of 103 patients who had no gametocytes pre-treatment and who had at least three non-zero gametocyte densities over the 42-day follow up period were included in this analysis. RESULTS: A variety of different functions were examined. A modified version of the critical exponential function was selected for the final model given its robustness across different datasets and its flexibility in assuming a variety of different shapes. Age, site, initial asexual parasite density (logged to the base 10), and an empirical patient category were the co-variates that were found to improve the model. CONCLUSIONS: A population nonlinear modeling approach seems promising and produced a flexible function whose estimates were stable across various different datasets. Surprisingly, dihydrofolate reductase and dihydropteroate synthetase mutation prevalence did not enter the model. This is probably related to a lack of power (quintuple mutations n = 12), and informative censoring; treatment failures were withdrawn from the study and given rescue treatment, usually prior to completion of follow up.
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    South African quantity surveyors: issues of gender and race in the workplace
    (2008) Bowen, Paul; Cattell, Keith; Distiller, Greg
    A web-based questionnaire survey of the opinions of SA quantity surveyors was undertaken to establish gender- and race-based differences in job satisfaction. Issues explored included demographic factors, issues of gender and race in the workplace, and gender and racial harassment and discrimination at work. 'Significant' differences on the basis of gender exist on a number of issues. Women, more than men, have strong positive feelings regarding their levels of job satisfaction, feel that their career expectations have been fulfilled, would choose the same career again, and would unequivocally recommend the career to others. Females see QS practices as male-dominated, see themselves as being blocked from advancement to managerial ranks, participating less in decision-making, and remunerated at a lower level than equivalent colleagues. Issues important to women include : gender representivity in the profession, flexible working hours and maternity leave above the statutory minimum. Although both gender groups report racial harassment and discrimination at work, women experience significantly more sexual and gender harassment and religious and gender discrimination than do males. 'Significant' differences on the basis of race are evident concerning : feelings of job satisfaction and views on maternity / paternity leave above statutory minima. 'Highly significant' differences on the basis of race arise over issues of : being subjected to greater supervision because of race, not being allowed to contribute meaningfully to the decision-making process, viewing PDI status as a valid basis for promotion, seeing race representivity in the profession as important in combating discrimination at work, having personally experienced racial harassment and discrimination at work, and seeing respect for individual diversity in the workplace as important - with 'Whites' viewing these issues less 'empathically' than their 'Non-white' counterparts. The results provide valuable indicators for how the quantity surveying firms can create a more conducive work environment for professional staff, particularly females.
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    The scientific and sociocultural value of citizen science
    (2025) Daniel, Karis Amrita; Underhill, Leslie; Loos, Jacqueline; Cameron, Alison; Distiller, Greg; Winter, Kevin
    At its best, conservation science serves as an interface between multiple disciplines, bringing together innovation and insight from the humanities, arts, business and economics, health, natural and social sciences to address the most pressing challenges facing the planet and its biodiversity. Conservation is also unique in its ability to transcend the academic. Subdisciplines such as biological citizen science involve non-academic individuals in data collection and as co-creators in research design and implementation. There is wide theoretical appeal to these initiatives, as they may simultaneously generate enough data for fine-scale biological monitoring and, through community engagement, promote the democratic generation and dissemination of knowledge. However, despite their potential within both the scientific and sociocultural realms, the regional scale impacts of citizen science initiatives in continental Africa remain poorly understood. Studying local citizen science initiatives affords opportunities to gain an understanding of their impact; within the African context, citizen science participants contribute to African Bird Atlas Projects in fifteen countries by compiling comprehensive avian species checklists at a fine geographic scale. Two projects are of particular interest for analysis: The Second Southern African Bird Atlas Project (SABAP2), and the Nigerian Bird Atlas Project (NiBAP). SABAP2 occupies a unique position as a long-term African citizen science project with ample data for statistical analyses. NiBAP employs an innovative grassroots approach to atlasing and hosts an enthusiastic and growing community of citizen science participants. In addition to impressive datasets, 'atlasers' in South Africa and Nigeria are a wealth of information regarding the personal- and community-level values surrounding participation and motivation. Furthermore, variations in project structure and cultural contexts between SABAP2 and NiBAP allow for some comparison of the values of participants between subprojects and contribute towards an understanding of the broader values of African Bird Atlas Project participants. In light of this potential, this thesis aims to examine the scientific and sociocultural contributions of SABAP2 and NiBAP to the work of conservation—namely achieving fine-scale species monitoring and advancing the democratic generation and sharing of scientific knowledge—within continental Africa. I examine the quality of data collected by a community of SABAP2 atlasers in Hessequa, South Africa, and assess their value in monitoring species population trends. I ask 1. Does systematic atlasing improve the temporal quality of atlas data for monitoring? and 2. Can these data detect trends in species populations and inform local response? Then, I explore values to nature expressed by atlasers and non-citizen science participants in both Hessequa, South Africa, and Jos, Nigeria. I ask the following two questions: 1. Does the type and frequency of values (instrumental, intrinsic, and relational) differ between participants and non-participants, and between cultural contexts?, and 2. what relational values (if any) are linked to participant motivation? Results demonstrate the ability of SABAP2 data to support biodiversity monitoring at a local scale, and show the potential for democratising research by including participants in data collection, analysis and application. The study of values and motivations emphasises the particular importance of relational values in connection with participant motivations and human-nature relationships broadly. Overall, the findings of this research demonstrate the ability of African Bird Atlas Projects and similar initiatives to meet the challenges of contemporary conservation, and lay a foundation for future research into the practical application of co-created monitoring schemes and incorporating values into the design of both citizen science projects and conservation interventions