Browsing by Author "Distiller, Gregory"
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- ItemOpen AccessEvaluating occupancy and the range dynamics of invasive bird species in South Africa(2022) Swingler, James; Distiller, Gregory; Clark, AllanThere is great interest in the distribution of invasive species that threaten indigenous wildlife. All effective conservation management decisions need to be based on sound inference and predictions so that these species can be controlled and the risk posed to the local ecosystem minimized. Thus, there is significant benefit in the study of invasive species as a means of aiding those charged with protecting indigenous wildlife. The occupancy and population range dynamics of the Myna and Mallard species are individually investigated in the South African region by fitting static and dynamic occupancy models to a set of citizen science data for a 10-year study period between 2010-2019. The occupancy and detectability of the respective species is analysed using static occupancy models for the 2010 study season. The covariates included in the best fitting static models are used to estimate the initial occupancy and detection parameters for the dynamic models which now include estimates for colonization and local extinction. A sensitivity analysis pertaining to the dynamic models is implemented by altering the data structures in terms of the number of analysed sites and length of the detection histories. The results find the Myna's proximity to urban environments to play a significant role on its occupancy in 2010, and yearly changes in climatic and anthropogenic factors influence its 10-year range dynamics. The models fitted to the Mallard are inconclusive possibly due to the violation of the closure assumption potentially caused by migratory behaviour. The results are limited by the presence of a potentially migratory species when using a poorly designed study and highlights the difficulties of conducting an occupancy analysis on a highly mobile avian species as opposed to their sedentary counterpart. The workings of this dissertation support previous claims that an increase in the quantity of sites, and thus the degree of overlapping sites over the different seasons, will improve the precision of the model estimates. However, caution must be exercised when increasing the length of the seasonal detection histories and should generally be set to no more than 10 repeated visits to a site.
- ItemOpen AccessMultiscale patterns of mammal diversity and occurrence in the Karoo(2022) Woodgate, Zoë Anne; O'riain, Mannus; Distiller, GregoryThe transformation of natural habitat for urban, industrial, and agricultural activities is the leading driver of terrestrial biodiversity loss. Yet how such land–use changes will impact the global drylands is poorly understood, despite the vulnerability of these once isolated regions. One such region is South Africa's Karoo, which is characterised by low human density, high levels of endemism and extensive pastoralism. Whilst centuries of small–livestock farming have severely impacted the Karoo's biophysical environment it remains relatively intact from an ecological perspective. However, novel threats have emerged, including a proposal for the extraction of natural gas by hydraulic fracturing extensive shale reservoirs. A major impediment to understanding how this, and other land–use changes, may impact indigenous wildlife is the lack of updated, multiscale foundational biodiversity data. To address this knowledge gap, my thesis evaluates the distribution and ecology of the mammalian (>0.5kg) community across the Karoo, with a focus on developing potential methods that can effectively record both common and rare species. My primary aim was to understand how biophysical drivers and intraguild interactions have shaped the distribution of mammals at different spatial scales, relative to existing and proposed land–uses. I had three main objectives: 1) to provide a foundational understanding of the mammalian community present throughout a 171 811km2 area under consideration for shale gas extraction, 2) to compare species occurrence and diversity across different land–use types (i.e., farmland, protected area [PA] and private protected areas [PPA]), and 3) to understand the drivers of the Karoo's rarest mammal species, namely the critically endangered riverine rabbit (Bunolagus monticularis), presence. To accomplish these objectives, I deployed three camera trap surveys at different geographic extents, with differing array designs. At the broadest geographical extent, I utilised an array consisting of 25 sites (125 camera traps total) that were selected using the Latin hypercube method. At the intermediate extent I compared three different land–uses, each sampled using a standardised 2km2 grid of sites (451 camera traps total). Lastly, at the finest scale (223.24km2 ), I deployed a stratified random design (30 clusters of 5 camera traps deployed within 400m) with the explicit goal of detecting riverine rabbits. I found that at the broadest scale wildlife diversity and occupancy was largely influenced by landscape–level abiotic processes, namely longitude and precipitation. None of the wide–ranging large predator (e.g., leopard or brown hyena) or endangered species (i.e., riverine rabbit) known to occur in the region were detected, suggesting the survey design was too coarse, and that protected areas were underrepresented. At the intermediate extent I used a hierarchical multi–species single–season occupancy model that incorporates species–specific responses to management treatments to show that land–use has a significant impact on species richness and occupancy, both of which were highest in the PPA and lowest in commercial small livestock farms. It was only at the finest sampling scale, with numerous camera traps arranged in discrete clusters at independent sites, that I detected all of the rare species present in the region, including the targeted riverine rabbit. Using a multispecies occupancy model that accounts for ≥2 interacting species, I showed that riverine rabbit occurrence was conditional on the absence of close competitors– namely scrub and cape hares– and smooth terrain. These results will better align ongoing conservation efforts for the species, which currently focus exclusively on environmental restoration of assumed preferred habitat. Overall, my findings reveal a diverse community of mostly medium sized generalist species that have persisted throughout the Karoo despite hundreds of years of sustained human impact. Larger predators and herbivores were almost entirely restricted to protected areas, and the PPA provided the only refuge for the critically endangered riverine rabbit. Whilst the predicted impacts of climate change and extractive mining on the mammals of the Karoo remain poorly understood, this thesis provides an important baseline of the extant mammal communities across the three dominant land–uses. Long term monitoring of select sites would provide a better understanding of how local and global anthropogenic impacts may affect the future of mammals in the drylands of South Africa.
- ItemOpen AccessTails through time: leopard population dynamics in the Little Karoo(2024) Steyn, Lawrence; Williams, Kathryn; Distiller, Gregory; Hofmeyr, Sally; Mann, Gareth; Wilkinson, AnitaLarge carnivores play a vital role in structuring our ecosystems, yet they face mounting threats such as habitat loss, prey reduction and persecution. These threats reduce their global distribution and impacts their population numbers. Protected areas can offer refuge for large carnivores, however leopards (Panthera pardus), can persist outside of these areas and often occupy mixed-use landscapes. Our understanding of how leopards persist over time in mixed-use landscapes is limited, especially in the semi-arid regions of southern Africa. This study, to the best of my knowledge, is the only multi-session maximum likelihood spatial capture-recapture (SCR) analysis to have been conducted in a semi-arid environment outside of a protected area in Southern Africa. The study aimed to estimate leopard population changes over time and to investigate the possible drivers affecting density, using three surveys (2012, 2017, 2022), in the mixed-use landscape of the Little Karoo in the Western Cape, South Africa. In 2012, a total of 141 paired camera stations were used for a total of 13,050 trap days resulting in 29 unique leopard captures. In 2017, a total of 40 paired camera stations were used for a total of 2,128 trap days resulting in 18 unique leopard captures and in 2022 a total of 64 paired camera stations were used for a total of 8,997 trap days resulting in 37 unique leopard captures. The best performing density model indicated an increasing population trend over the study period which included a trend term on density (D~year) and an interaction term (individual session*sex) on λ0 (capture rate) and σ (spatial decay). Density estimates (Standard Error) for leopard populations for the three surveys 2012, 2017, and 2022, were 0.52 (± 0.11), 0.70 (± 0.08), and 0.95 (± 0.08) leopards per 100 km2, respectively. Terrain ruggedness, elevation, vegetation type and distance from major rivers were all important drivers in leopard density in the Little Karoo. Indicating that high lying areas provide suitable refuge for leopards and are key areas for movement corridor planning. These density estimates are similar to previous single maximum likelihood SCR density estimate studies in the Little Karoo and the Western Cape province. Results from this study indicate the leopards have persisted in the Little Karoo over the study period and suggest that the population may be increasing. Further research on what is driving this population shift is needed, but the results serve as an encouraging sign for leopard conservation in the Little Karoo
- ItemOpen AccessTails through time: leopard population dynamics in the Little Karoo(2024) Steyn, Lawrence; Williams, Kathryn; Distiller, Gregory; Hofmeyr, Sally; Mann, Gareth; Wilkinson, AnitaLarge carnivores play a vital role in structuring our ecosystems, yet they face mounting threats such as habitat loss, prey reduction and persecution. These threats reduce their global distribution and impacts their population numbers. Protected areas can offer refuge for large carnivores, however leopards (Panthera pardus), can persist outside of these areas and often occupy mixed-use landscapes. Our understanding of how leopards persist over time in mixed-use landscapes is limited, especially in the semi-arid regions of southern Africa. This study, to the best of my knowledge, is the only multi-session maximum likelihood spatial capture-recapture (SCR) analysis to have been conducted in a semi-arid environment outside of a protected area in Southern Africa. The study aimed to estimate leopard population changes over time and to investigate the possible drivers affecting density, using three surveys (2012, 2017, 2022), in the mixed-use landscape of the Little Karoo in the Western Cape, South Africa. In 2012, a total of 141 paired camera stations were used for a total of 13,050 trap days resulting in 29 unique leopard captures. In 2017, a total of 40 paired camera stations were used for a total of 2,128 trap days resulting in 18 unique leopard captures and in 2022 a total of 64 paired camera stations were used for a total of 8,997 trap days resulting in 37 unique leopard captures. The best performing density model indicated an increasing population trend over the study period which included a trend term on density (D~year) and an interaction term (individual session*sex) on λ0 (capture rate) and σ (spatial decay). Density estimates (Standard Error) for leopard populations for the three surveys 2012, 2017, and 2022, were 0.52 (± 0.11), 0.70 (± 0.08), and 0.95 (± 0.08) leopards per 100 km2, respectively. Terrain ruggedness, elevation, vegetation type and distance from major rivers were all important drivers in leopard density in the Little Karoo. Indicating that high lying areas provide suitable refuge for leopards and are key areas for movement corridor planning. These density estimates are similar to previous single maximum likelihood SCR density estimate studies in the Little Karoo and the Western Cape province. Results from this study indicate the leopards have persisted in the Little Karoo over the study period and suggest that the population may be increasing. Further research on what is driving this population shift is needed, but the results serve as an encouraging sign for leopard conservation in the Little Karoo
- ItemOpen AccessThe application of spatial capture-recapture models to investigate leopard ecology and conservation in South Africa(2021) Rogan, Matthew S; O'riain, Mannus; Distiller, GregoryPopulation monitoring is essential to wildlife conservation and management. Rare and elusive species are difficult to observe, and hence monitor, in wild populations. Leopards (Panthera pardus) are an iconic and threatened species whose conservation is hampered by a lack of robust population data, in part due to their sparse populations and cryptic nature. I used cameratrap surveys from 27 protected areas in northeastern South Africa to make inferences about the status and conservation needs of leopards. I first evaluated the relationship between leopard density and proportion of area used within protected areas to determine if the latter could serve as a more efficient yet robust proxy for the former. I found that the relationship was too imprecise to be informative, that many populations of varying density used all the space available, and that the scale of individual movement strongly influenced the proportion of area used regardless of density. I then fit multisession spatial capture-recapture models to time series data from seven of these leopard populations to assess their threat level based on the estimated probability of population declines. I found some evidence of decline in six of the seven populations and found that the population at one site has a 75% chance of declining by 80% over three leopard generations. Lastly, I investigated the relative influence of bottom-up ecological factors and top-down anthropogenic factors as possible determinants of leopard density to identify what conditions are most suitable for conserving leopard populations. I found that while habitat and management characteristics of protected areas matter, human impacts around and within protected areas are the primary drivers of variation in leopard density. Based on these analyses, I conclude that South African protected areas are not conferring sufficient protection to leopard populations and that more must be done to mitigate human impacts inside protected areas. I also show that the leopard monitoring program would benefit from longer surveys with more sampling locations to increase the statistical power for detecting changes. This thesis demonstrates the capacity for large-scale monitoring programs to greatly expand our understanding of the conservation needs of a cryptic species.