Thesis (PhD (Conservation Ecology and Entomology))--Stellenbosch University, 2008. / The Okavango Delta, Botswana, is a unique ecosystem and this is reflected in its extraordinary biodiversity. The Nile crocodile (Crocodylus niloticus Laurenti) is the apex predator, and performs a number of vital functions in this system, making it a keystone species. The panhandle crocodile population has declined significantly over the last 80 years and is now threatened as a result of past over-exploitation and present human disturbance. In order to effectively conserve this species and in turn the health of this important region it is imperative to gain an understanding of their ecology and population dynamics. The population status of the Nile crocodile in the panhandle region of the Okavango Delta, Botswana, was assessed using a combination of capture-mark-recapture surveys, spotlight surveys and aerial surveys. The capture-mark-recapture experiment was conducted continuously from 2002 - 2006. A total of 1717 individuals, ranging in size from 136 mm – 2780 mm SVL, were captured, of which 224 animals were recaptured. Using a Bayesian technique, the total annual population in the panhandle region of the Okavango Delta was estimated to be 2 570 ± 151.06 individuals, with an adult population of 649.2 individuals with the number of breeding females estimated to be 364 individuals. It was concluded that this population cannot sustain the further harvest of breeding animals prior to the increase and stabilization of the population. Spotlight counts revealed a decline in the encounter-rate of crocodiles on the Okavango River with time, although more long-term data needs to be collected to confirm this trend. During the low-water season (September - February), 22.34 % of all crocodiles were observed, while during the flood-season only 13.34 % were observed, yielding correction factors for spotlight surveys of 4.46 (low-water) and 7.49 (high-water) for all animals in the panhandle. Two aerial surveys, conducted at the low-water and high-water peaks yielded total estimates of 588 (77.7 % of adults) during the low-water period and 350 (56.7 % of adults) during the high-water period. Correction factors of 1.28 (low-water) and 1.77 (high-water) were calculated for aerial surveys. An annual average of 50 nests was located in the panhandle, representing a 50 - 60 % decrease over the last 20 years. In regions with high human disturbance, breeding females situated their nests in hidden locations, away from accessible channels. Hatchlings exhibited elongation of the jaw in order to capture smaller prey items and morphometric shifts in jaw shape coincided with a dietary change at 400 mm SVL. The jaw became broader and deeper as animals matured, presumably in preparation for larger mammalian prey. The average growth rate of recaptured yearlings was 0.198 ± 0.116 mm.d-1 SVL and was closely correlated to the amount of time an individual spent in above-average water temperatures. Body condition (RCF) was significantly and positively correlated with a rise in water-level and negatively correlated to time spent in above-average water and air temperatures. Average RCF values were intersected when animals had spent 50 % of their time in above-average temperatures and water level. Generally crocodiles in the panhandle showed no significant sex-related differences in their sizes or the distances they travelled. The majority of recaptures (62.5 %) moved less than 500 m from the initial capture site. Adults in the panhandle occupied definite ranges, within which were preferred core areas where the majority of their time was spent. The panhandle crocodile population has declined significantly over the last 80 years, and is now threatened as a result of past over-exploitation and present human disturbance. The management of this population, including both its conservation and sustainable commercial utilisation, requires an adaptive strategy based on accurate monitoring procedures.

Thesis (MSc(Conservation Ecology and Entomology)--University of Stellenbosch, 2006. / The stomachs of 286 crocodiles (17 cm to 166 cm snout to vent length) were lavaged over a two year period. Crocodylus niloticus has a similar ontogenetic shift in diet to that of other crocodilians. Yearlings predated primarily on aquatic insecta and arachnida, as crocodile size increased (juveniles) the diet became more diverse including crustacea, amphibia and fish. The largest size class (sub-adults) consumed primarily fish. Yearlings fed consistently throughout the year, however a higher proportion of empty stomachs occurred within the juvenile and sub-adult size classes during the winter months. A captive experiment with wild caught crocodiles (0.7 kg - 20 kg) indicated a decrease in satiation rate (maximum mass of food eaten as a percentage of crocodile body mass) from 11.3 % to 6.5 % with an increase in crocodile size. Wild yearlings consumed 0.48 % of their body mass in prey per day, and juveniles, 0.23 %. The wild food conversion rates of natural prey were low, 32 % and 40 % for yearlings and juveniles respectively. This was possibly due to a high percentage of indigestible (chitinous) invertebrate mass in the diet. The percentage of stomach stone mass to crocodile body mass increased with crocodile size. Seven species of nematodes were found within the stomachs, four of which represent new geographic records.

Le Delta de l'Okavango est un système endorhéique formant un cône alluvial dans la dépression du Kalahari. L'écosystème y est rythmé par une crue annuelle en provenance de l'Angola et entravée par l'escarpement des failles normales du graben de l'Okavango. Ce régime annuel est très variable, car la répartition de la crue annuelle diffère tous les ans. À plus grande échelle temporelle, du millénaire au Ma, l'endorhéisme du Delta peut varier entre des marais (actuel), un mega-lac ou perdre son caractère endorhéique (rivière). Les processus pouvant contrôler ces variations de régime sont 1) l'hydrologie du système, 2) l'activité des failles du graben, 3) l'apport sédimentaire et 4) l'écosystème. Cette étude apporte des contraintes quantifiées éclairant deux de ces processus à travers l'observation de la déformation de la surface terrestre par l'enregistrement de stations GPS permanentes. Le signal observé est annuellement impacté par la charge hydraulique résultant de la saison des pluies, et inter-annuellement par les variations de cette charge, ainsi que l'activité tectonique du graben. Les satellites GRACE fournissent un enregistrement quasi-continu de la variation du stock d'eau continentale, rendant possible la modélisation de la déformation élastique de la surface terrestre sous l'effet de cette charge. Le signal de déformation ainsi modélisé est comparable au signal saisonnier enregistré, validant les données satellitaires GRACE et révélant un important aquifère dans le bassin de l'Okavango. GRACE fournit ainsi un nouveau proxy permettant de suivre l'évolution de l'aquifère, et de valider le modèle hydrologique calibré précédemment pour le bassin. Les variations inter-annuelles d'eau souterraine dans le bassin validées par GRACE confirment le rôle de tampon des variations climatiques joué par l'aquifère sur la modulation des variations climatiques. La phase des variations du stock d'eau met de plus en évidence un effet de seuil dans la recharge de ce stock, en fonction de l'intensité des premières pluies. La faible résolution spatiale des variations du stock d'eau continental fournie par GRACE peut finalement être mieux contrainte par un examen plus détaillé des signaux GPS dans le bassin. Les vitesses de déformation horizontales de part et d'autre du graben révèlent une déformation tectonique faible sur l'ensemble du graben, de 1 mm/an exclusivement décrochante dextre. Une si faible déformation exclut une influence significative de l'activité tectonique et sismique du graben sur la variabilité de la distribution de la crue. Cette déformation observée remet en question le modèle géodynamique admis dans la région, soit une phase précoce de rifting liée à la propagation d'une branche SW du Rift Est-Africain. Un recensement des nombreuses études géophysiques réalisées dans la région dans la dernière décennie et un réexamen de la sismicité de l'Afrique australe n'amène aucune preuve significative de rifting dans le graben de l'Okavango. Un autre modèle géodynamique pour l'Afrique australe est proposé, basé sur l'accommodation lointaine de la déformation associée à l'ouverture à taux différentiels du Rift Est-Africain et le déplacement du craton du Kalahari par rapport au reste de la plaque nubienne. / The Okavango Delta is an endorheic system forming an alluvial fan in the Kalahari depression. The local ecosystem is paced by the annual flood coming from the Angolan highlands, blocked downstream by the normal faults scarps of the Okavango graben. This annual regime is highly variable, with the spatial distribution of the flood differing every year. At the geological time scale, from millenary to mega-annual, the Delta's endorheism can also vary between wetlands (current regime), mega-lake or loose its endorhism to turn into a river. Processes driving these regime variations are 1) the hydrological system, 2) the faulting of the graben, 3) the sedimentary input and 4) the ecosystem. This study brings quantitative constraints regarding two of these processes through the deformation of the Earth's ground surface monitored by permanent GPS stations. The observed signal is impacted seasonally by the hydrological loading resulting from the rainy season, and inter-annually by the variations of the terrestrial water storage, as well as the tectonic activity of the graben. GRACE satellites provide a quasi-continuous record of the variations of the Earth's continental water storage, allowing the modelling of the elastic deformation imposed on the Earth's crust by loading. The resulting modelled deformation signal is well correlated to the observed seasonal signal, hence validating the GRACE data products and revealing a large aquifer in the Okavango basin. GRACE thus provides a new proxy to monitor the evolution of water storage, and to validate more robustly the hydrological model calibrated for the basin. The variations in terrestrial water storage (TWS) in the basin validated by GRACE confirm the buffer effect of the aquifer in the modulation of climatic variations. The phase of the TWS variations moreover highlights a threshold in the recharge of TWS during the rainy season, depending on the intensity of the first rains. Finally, the poor spatial resolution of TWS variations provided by GRACE can be further constrained by a detail examination of the GPS signals in the basin. Horizontal displacements rates across the graben reveal a low tectonic deformation rate, of 1 mm/yr, exclusively along a dextral strike-slip component. Such a low deformation rate excludes a significant influence of the graben faulting on the variability of the flood distribution. The observed deformation calls into question the current geodynamic model for the area, i.e. incipient rifitng due to the propagation of a southwestern branch of the East African Rift System. A review of the numerous geophysical studies in the study area over the last decade, as well as a re-examination of the seismic data in southern Africa does not provide significant evidence for rifting in the Okavango graben. An alternative geodynamic model is proposed, based on the far-field accommodation of the deformation resulting from the differential extension rates in the Rift Valley, and the displacement of the Kalahari craton relative to the Nubian plate.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters in Science Centre for African Ecology, School of Plant Animal and Environmental Sciences, 2011. / Habitat selection occurs across a range of different spatial scales and is influenced by a variety of factors, ultimately determining how animals distribute themselves across the landscape. Studying the decisions that an individual animal makes across different levels of selection, from its choice in dietary item to predator avoidance strategies, is a fundamental link in understanding the response of groups of animals and ultimately entire populations that may provide insight into population performance. The study formed part of a broader study focused on the decline of rare antelope species. Specifically this study was aimed at establishing the home range and resource use of sable antelope in a region where they were initially expected to be thriving. The levels of selection covered in this study are: the location of home ranges of an individual or social group within the landscape; the use of various habitat components within the home range; and the procurement of food items within those habitats. At the highest level, the broad objectives were (1) to determine where sable occupied home ranges within the landscape, indicating the suitability of various landscape units to sustain sable populations and (2) to determine the relative use of habitat types within those home ranges that may enable sable to avoid predation and acquire resources required to survive and reproduce. At the lowest level of selection the characteristics of forage selection and how the grass quality in the different vegetation types during different seasons affects the success of sable herds was explored. The broad objectives were (1) to determine the effect of seasonal flooding and rainfall on grass greenness in the floodplains and upland vegetation types and the consequent use of those vegetation types by sable antelope and (2) to determine how exploitation of resources in the floodplains and in the uplands contributed to the nutritional status of sable. I additionally quantified the time spent browsing and determined the composition of the browse component of the diet of sable. Adult female sable from each of three adjacent sable herds were fitted with GPS collars providing hourly GPS co-ordinates. Adaptive LoCoH was used to determine home range location and annual, seasonal and core home range extents. A vegetation map was created and the number of GPS locations within each vegetation type was counted to determine their relative use in relation to availability within the home ranges. GPS collars were used to locate herds daily so that foraging observations of browsing and characteristics of the grasses grazed could be attained. Acceptability and dietary contributions of grass species and browse were determined for each sable herd during different seasons. The availability of grass species on the floodplain grasslands and in the upland grasslands and woodlands was estimated. Water and the availability of key resources posed a constraint on where sable home ranges were established. Sable simply did not occupy the region in the north of the study area further than 7 km from permanent water and floodplain grasslands. Herds generally avoided open savanna, mopane woodlands and Kalahari apple-leaf woodlands characterised by sparse grass cover, particularly during the dry season. Home ranges were relatively small compared 4 to the range estimates from herds in Kruger National Park. There was no obvious seasonal difference in home range extent nor were there large areas of overlap between home ranges of adjacent herds. Observations during the study indicated that competitor species, including zebra and wildebeest, concentrated on the floodplain grasslands. Throughout the year H. dissoluta was the most strongly favoured grass species and contributed most to the diet of the sable herds in both the wet and dry season. During the dry season sable herds expanded their diet to include Aristida stipitata and Aristida meridionalis which are generally considered poor forage value species for cattle but that retained some greenness. Additionally, the contribution of browse, especially the leaves of Croton megalobotrys, Philenoptera nelsii and Combretum mossambicense and the flowers of Kigelia africana, constituted an important bridging resource during the extended dry season. Crude faecal protein levels remained above the suggested maintenance levels throughout the annual cycle. Crude faecal protein levels were elevated prior to calving when sable spent more time foraging on the floodplain grasslands where high value forage species such as Paspalum scrobiculatum, Panicum repens and Urochloa mossambicense and sedges were eaten. Indications are that the constraint posed by the distribution of water within the landscape, rather than resource limitations within occupied home ranges, are the primary limitation to population performance in the Kwedi concession. / Wilderness Safari, the Wilderness Wildlife Trust, the Conservation Foundation, Classic Africa and National Research Foundation (NRF) of South Africa.

<p>This thesis presents models applicable for aquatic ecosystems. Geographical Information Systems (GIS) form an important part of the thesis. The dynamic mass balance models focus on nutrient fluxes, biotic/abiotic interactions and operate on different temporal and spatial scales (site, local, regional and international). The relevance and role of scale in mass balance modelling is a focal point of the thesis.</p><p>A mesocosm experiment was used to construct a model to estimate the nutrient load of phosphorus and nitrogen from net cage fish farming (i.e., the site scale). The model was used to estimate what feeding conditions that are required for a sustainable aquaculture scenario, i.e., a zero nutrient load situation (a linkage between the site scale and the regional scale). </p><p>A dynamic model was constructed for suspended particulate matter (SPM) and sedimentation in coastal areas (i.e., the local scale) with different morphometric characteristics and distances to the Sea. The results demonstrate that the conditions in the Sea (the regional and international scale) are of fundamental importance, also for the most enclosed coastal areas.</p><p>A foodweb model for lakes was transformed and recalibrated for Baltic Sea conditions (i.e., the international scale). The model also includes a mass balance model for phosphorus and accounts for key environmental factors that regulate the presuppositions for production and biomasses of key functional groups of organisms. The potential use of the new model for setting fish quotas of cod was examined.</p><p>For the intermittent (i.e., regional) scale, topographically complex areas can be difficult to define and model. Therefore, an attempt was made to construct a waterscape subbasin identification program (WASUBI). The method was tested for the Finnish Archipelago Sea and the Okavango Delta in Botswana. A comparison to results from a semi-random delineation method showed that more enclosed basins was created with the WASUBI method.</p>

This thesis presents models applicable for aquatic ecosystems. Geographical Information Systems (GIS) form an important part of the thesis. The dynamic mass balance models focus on nutrient fluxes, biotic/abiotic interactions and operate on different temporal and spatial scales (site, local, regional and international). The relevance and role of scale in mass balance modelling is a focal point of the thesis. A mesocosm experiment was used to construct a model to estimate the nutrient load of phosphorus and nitrogen from net cage fish farming (i.e., the site scale). The model was used to estimate what feeding conditions that are required for a sustainable aquaculture scenario, i.e., a zero nutrient load situation (a linkage between the site scale and the regional scale). A dynamic model was constructed for suspended particulate matter (SPM) and sedimentation in coastal areas (i.e., the local scale) with different morphometric characteristics and distances to the Sea. The results demonstrate that the conditions in the Sea (the regional and international scale) are of fundamental importance, also for the most enclosed coastal areas. A foodweb model for lakes was transformed and recalibrated for Baltic Sea conditions (i.e., the international scale). The model also includes a mass balance model for phosphorus and accounts for key environmental factors that regulate the presuppositions for production and biomasses of key functional groups of organisms. The potential use of the new model for setting fish quotas of cod was examined. For the intermittent (i.e., regional) scale, topographically complex areas can be difficult to define and model. Therefore, an attempt was made to construct a waterscape subbasin identification program (WASUBI). The method was tested for the Finnish Archipelago Sea and the Okavango Delta in Botswana. A comparison to results from a semi-random delineation method showed that more enclosed basins was created with the WASUBI method.

No description available.

Intraguild competition and predation have been recognised as important ecological factors influencing the population dynamics of carnivores. The effects of these interactions are often asymmetrical due to a size-related dominancy hierarchy. However, it has been suggested that competitively subordinate carnivores can minimise the costs of predation and competition through spatial and temporal avoidance. Here I investigate the ecological and behavioural mechanisms by which cheetahs (Acinonyx jubatus) coexist with competitively stronger lions (Panthera leo) and spotted hyaenas (Crocuta crocuta). Fieldwork was carried out in the Okavango Delta, northern Botswana, between October 2008 and August 2011. A total of 20 Global Positioning System (GPS) radio-collars were fitted on all known cheetahs (n=6), lion prides (n=5) and spotted hyaena clans (n=6) in the study area (approx. 3 000 km²). Pre-programmed radio-collars recorded locations and activity continuously for each individual and these data were complemented with direct behavioural observations. Cheetah data were analysed with respect to the temporal and spatial likelihood of encountering lions and spotted hyaenas. Results suggest that the response to the risks posed by other predators is species-specific, habitat-specific and dependent on the immediacy of the risk. Resource partitioning was not the main mechanism for coexistence as cheetahs overlapped extensively with lions and spotted hyaenas in time, space and habitat use. Instead, cheetahs adjusted their spatial distribution in response to immediate risks or adapted their habitat use depending on their vulnerability (e.g. behaviours such as feeding or with differing levels of moonlight at night). In general, cheetah temporal and spatial distribution is a hierarchal process, firstly driven by resource acquisition and thereafter fine-tuned by predator avoidance. In addition, habitat heterogeneity seemed to be key in facilitating coexistence. Understanding the behavioural mechanisms that interacting apex predators adopt to regulate these negative interactions could be crucial to carnivore conservation, especially as human-related habitat loss is forcing species into ever smaller areas.