Doctoral Degrees (Ecology)

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Ecology and evolution of the specialized hemipepsis-wasp (Hymenoptera : Pompilidae) pollination guild in South Africa.
(2011) Shuttleworth, Adam.; Johnson, Steven Dene.; Brothers, Denis John.
Pollinators are believed to have played a key role in the radiation of flowering plants. The Grant-Stebbins model of pollinator-mediated speciation, in which evolutionary shifts between pollinators result in phenotypic diversification and enforce reproductive isolation, is one of the most compelling hypotheses for the rapid diversification of angiosperms. A key principle in this model is that plant pollination systems tend towards specialization, resulting in convergent suites of floral traits (syndromes) associated with particular types of pollinators. However, the expectation of pollination system specialization is not always supported by ecological data and has also been questioned on theoretical grounds. In this thesis, I examine pollination by Hemipepsis spider-hunting wasps (Hymenoptera, Pompilidae, Pepsinae) and use this system to address questions about levels and proximal mechanisms of floral specialization, floral shifts and convergent evolution of floral traits. Specialized pollination by Hemipepsis wasps is a newly described pollination system within the angiosperms. I document pollination by these wasps for the first time in 15 South African grassland plant species, including two species of Eucomis (Hyacinthaceae) and 13 asclepiads (Apocynaceae: Asclepiadoideae). In one of the asclepiads, Xysmalobium undulatum, I describe a bimodal pollination system involving both Hemipepsis wasps and a cetoniine beetle. I also describe an unusual and potentially antagonistic pollination mechanism whereby wasps are systematically dismembered during the insertion of pollinia in the two asclepiads Pachycarpus asperifolius and P. appendiculatus. I have used these and previous case studies to establish the existence of a new pollination guild, consisting of at least 21 plant species (across 10 genera and three families), that are reliant on four functionally similar species of Hemipepsis wasp for pollination. Plants in the guild are distributed throughout the moist grasslands of eastern South Africa and flower from September through until early May, peaking in December/January. The Hemipepsis-wasp pollination guild is characterized by high levels of functional specialization (17 of the 21 known guild members are pollinated exclusively by Hemipepsis wasps), despite the absence of morphological adaptations to prevent non-pollinating insects from accessing nectar. I used field and laboratory based experiments to explore the function of floral traits in enforcing specialization. These showed that Hemipepsis wasps primarily use scent, rather than visual cues, to locate flowers, but I was unable to firmly identify specific compounds responsible for the attraction of these wasps (compounds that elicited antennal responses in preliminary GC-EAD experiments did not attract wasps in bioassays). The chemical composition of the floral scents of guild members was examined for 71 individuals representing 14 species in addition to previous studies, and found to comprise complex blends of volatiles (usually containing between 30 and 50 compounds), typically dominated by aliphatics and monoterpenes with small amounts of aromatics. I also showed that the floral colours of guild members are similar to background vegetation, suggesting that floral colours are adapted for crypsis to avoid detection by non-pollinating insects. Palatability choice experiments with honeybees showed that non-pollinating insects find the nectars of at least three of the asclepiad guild members distasteful. Plants in this guild thus appear to achieve specialization through biochemical filters (scent as an attractant and differentially palatable nectar) and cryptic coloration. Pollinator-mediated convergence in floral traits is the fundamental basis for pollination syndromes, but has seldom been rigorously analyzed. Flowers in the Hemipepsis-wasp pollination guild share several qualitative traits, including dull greenish- or brownish-white colour, often with purple blotches, exposed sucrose dominant nectar with a relatively high sugar concentration (typically over 50% sugar by weight) and a sweet/spicy fragrance to the human nose. To test for convergent evolution in guild members, I compared scent, nectar and colour traits of guild members to those of congeners with different pollinators. Although traits often differed between guild members and their congeners, I found little evidence for overall convergence in floral scent profiles and nectar properties, but floral colours in the guild were significantly closer to the colour of background vegetation than those of congeners. At this stage, the lack of knowledge about specific floral volatiles that influence Hemipepsis-wasp behaviour and secondary nectar constituents that limit non-pollinator visits makes it difficult to identify the extent of biochemical convergent evolution within the guild. The directions and functional traits involved in evolutionary transitions between pollination by Hemipepsis wasps and other vectors are currently difficult to ascertain as there is limited phylogenetic data for the plant families concerned. In the genus Eucomis, fly and Hemipepsis-wasp pollinated species are very similar in floral morphology and colour, but differ strongly in floral scent. Using manipulative field experiments in conjunction with detailed analyses of colour, scent and morphology, I was able to show that a shift between wasp and fly pollination could be induced simply by manipulating oligosulphides in the scent emission from inflorescences. When considered in combination with other experiments highlighting the importance of scent as a pollinator attractant for all guild members, this suggests that scent properties may have played a key role in the evolutionary transitions between pollination by Hemipepsis wasps and other vectors. This research has established that pollination by Hemipepsis spider-hunting wasps is more geographically and phylogenetically widespread than was previously known, and has confirmed that these wasps are important and consistent pollinators in southern African grassland ecosystems. I have shown that a distinct guild of plants is specialized for pollination by these wasps. The high levels of specialization within this guild highlight the effectiveness of biochemical filters and cryptic coloration in limiting the spectrum of flower visitors. The major challenge ahead will be to identify the floral volatiles that attract Hemipepsis wasps and the non-sugar constituents that make the nectars of some guild members differentially palatable. These would both contribute greatly to our understanding of floral specialization and the mechanisms involved in the radiation of the angiosperms.
The ecology and parasitology of small mammals from selected sites in Swaziland.
(2007) Mahlaba, Themb'alilahlwa A. M.; Perrin, Michael Richard.
The study was initially a long term study of the ecology of small mammals at Mlawula Nature Reserve in the eastern part of Swaziland. Due to the drought and dwindling numbers of rodents in the reserve the study was redirected to determining the factors resulting in the low numbers. The impacts of food and cover and grazing by the larger ungulates were studied. In addition, the age class distribution and gastro-intestinal parasites of small mammals were studied. A study of the small mammals in the Siphiso Valley of Mlawula Nature Reserve was conducted over four years from August 2000. The population density, biomass and composition of the small mammal community in the area were studied. The community comprised of Mus minutoides A. Smith, 1834, Mastomys natalensis (A. Smith, 1834), Lemniscomys rosalia (Thomas, 1904), Crocidura hirta Peters, 1952, Steatomys pratensis Peters, 1846 and Graphiurus murinus (Desmarest, 1822). Mus minutoides was the dominant species with pregnant females caught from November to May. Species richness varied significantly with the time of the year. The biomass, density and numbers of small mammals were low and by the end of the second year of the study, small mammal density was close to zero. Mastomys natalensis from a Middleveld study site, Luyengo, Swaziland were used to study the age structure of the population by means of eye lenses. The eye lens to age (in days) curve determined by Leirs (1994) was applied. A large percentage of M. natalensis in winter (June) were 2 months old while in spring/summer (October to March) the population consisted mainly of 3 month old specimens. A very low number of specimens were older than 4 months. This suggests a high mortality/removal rate of the young especially in the winter months. The impact of grazing pressure and rainfall on small mammal densities were investigated. High grazing pressure by ungulates rendered the habitat unsuitable for small mammals as it removed cover and encouraged colonization by alien invasive plant species. This effect was exacerbated by diminishing and unpredictable rains, such that mild grazing pressure negatively impacted on small mammal communities and on individual species. When the small mammals disappeared from the study site, M. natalensis was reintroduced to determine the factors that led to the disappearance. Supplementary food resulted in the longest persistence of the reintroduced mice while the impact of additional cover was small. Predation was likely responsible for the rapid decline of the reintroduced mice. Small mammals were examined for ectoparasites and gut parasites as these were thought to negatively impact on their physiology and reproduction. Ectoparasites collected included the ticks Ixodes sp. and Boophilus sp., the mite Allodermanyssus sp. and another species of mite. The gastrointestinal tracts contained the helminths Syphacia sp., Heligmonina sp., Trichuris sp., Protospirura sp., two unidentified nematode species and different cestode species. A new species of heligmosomoid nematode is described and named.
The comparative ecology of Natal's smaller estuaries.
(1983) Begg, George William.; Hanks, John.; Bowmaker, A. P.
Fifty one of Natal's 73 estuaries have been almost completely overlooked in terms of any scientific study, despite which a marine nursery function has commonly been attributed to each of them. In the knowledge that many of these systems were normally closed and others were in a seriously degraded state, this study was undertaken with the aims of examining their present day community structure in order to provide a basis upon which their future condition can be monitored and to provide a classification of these coastal resources. The study area incorporated 62 different systems extending over 240 km of the Natal coastline south of the Tugela River. During the three year study period (Sep 1979 - Nov 1982) 82 515 specimens, comprising an assemblage of 125 different species, were caught by means of a small beam trawl. These comprised 86 species of fish, 21 species of crabs and 18 species of prawns. The data obtained were correlated to abiotic variables such as mouth condition, salinity, temperature, dissolved oxygen, water transparency, depth, nature of the substratum and peripheral vegetation. Based on the ability of biota to synthesize environmental variables into one common response, multi-variate analysis is used to demonstrate the similarity in community structure between open and closed systems (for example) or between fresh and saline systems, and thereby resolve an age-old argument about estuary classification. The data also suggest that in their present day condition only six of the systems studied make a significant contribution to the recruitment of estuarine-dependent marine stocks (sensu stricto) principally because of their open mouth condition. Closed systems, deliberately classified as lagoons, have a different resource value, being utilized primarily by resident species that can complete their life cycle within the system. An appreciation of this salient difference helps to reinforce the critical need for an effective management strategy to be implemented to prevent Natal's dwindling estuarine resources from deteriorating any further. The term 'estuarine-dependence' is critically examined in this context to show that a species more dependent on estuaries than any other, is man. It is argued that man's continued abuse of these resources is shortsighted, and that the most serious threat of all is sedimentation, accelerated in this instance beyond the geological norm by catchment mismanagement. The practical application and value of classification to planning and management is demonstrated and a methodology proposed, based on community responses, for the monitoring of the future environmental condition of each estuary and lagoon in Natal.
Key factors driving the foraging ecology of Oribi : fear, cattle and the quality and quantity of food.
(2015) Stears, Keenan.; Shrader, Adrian Morgan.
Body size and digestive physiology are two factors that influence herbivores foraging behaviour, habitat use and potential to compete with other animals. The aim of this study was to determine how seasonal changes in grass quality, interactions with cattle, and perceived predation risk influenced oribi (Ourebia ourebi) foraging and landscape use. Oribi are one of the smallest pure grazing ruminants. As a result, they should select high quality vegetation. In line with this, I found that throughout the study oribi fed selectively over multiple spatial scales ranging from plant parts to habitats. By focussing on green grass within these different scales, oribi were able to maintain their crude protein intake needed for maintenance. Throughout South Africa, oribi frequently interact with cattle. Due to their differences in body size and nutritional requirements, there should be sufficient resource partitioning to avoid competition. However, I found that the nature of the interspecific interactions (i.e. competition or facilitation) between these species depended on season and cattle stocking rates. During the wet season, cattle facilitated oribi by providing high quality regrowth. However, at high stocking rates, cattle indirectly competed with oribi during the dry season via the impacts of their wet season grazing. Specifically, intense wet season grazing by cattle reduced the availability of high quality grass for oribi in the dry season. This was not the case at low and intermediate stocking rates. Differences in food availability and predation risk across habitats can influence how herbivores utilise landscapes. I found that predation risk greatly affected oribi foraging behaviour, with oribi preferring to feed in safer rather than riskier habitats. However, when food availability increased in these risky habitats, oribi increased their risk-taking behaviour at both small- and large-scales within these risky areas. Ultimately, this suggests that oribi trade-off between predation risk and food availability. Finally, to link my results to the management and conservation of this vulnerable antelope, I applied the knowledge I gained from the above research to a case study.
Spatial and reproductive ecology and population status of the Nile crocodile (Crocodylus niloticus) in the Lake St Lucia estuarine system, South Africa.
(2014) Combrink, Alexander Stanislou.; Downs, Colleen Thelma.
The Lake St Lucia estuarine system, Africa’s largest and oldest protected estuary, also contains the largest Nile Crocodile (Crocodylus niloticus) population in a single waterbody in South Africa. We investigated the species’ spatial and reproductive ecology as well as population status in order to make management recommendations. During the first decade of the 21st century, the St Lucia estuarine lake experienced a prolonged drought, streams ceased flowing and in 2006 more than 90% of the total water area evaporated. We conducted 10 aerial surveys from 2009 - 2013 and recorded the majority of crocodiles in the Narrows, a ~27 km low salinity channel south of the lake. Above average rainfall at the end of 2010 resulted in the refilling of the lake, and most crocodiles moved north to the lake. We estimated the sub-adult and adult population at 1005 ± 137 individuals. We investigated detailed movements and activity for 18 Nile Crocodiles using GPS-satellite transmitters. The overall activity level was 41.0 %, and it differed significantly throughout the day. There was a significant seasonal effect on activity, peaking during autumn (52.0 %), while crocodiles were most inactive in winter (30.5 %). Crocodile size and mobility were positively correlated with mean daily movement (1244 m). Adults moved more at night, but sub-adults were significantly more mobile during the day. There was a considerable seasonal variation in mobility, with the longest movements during autumn and the shortest in winter. About 60 % of total daily movements were < 1 km per day, but for sub-adults this calculation was 96 %. We recorded complex and varied home range patterns for 14 Nile Crocodiles, resulting from differences in size, sex, reproductive status and habitat. The median home range and core-use area of adults were significantly greater than sub-adults. Three size- related patterns of home range behaviour emerged for adult males; transient, (< 3.0 m TL), topographically confined (3.5 - 4.0 m TL) and “territorial” (> 4.0 m TL). Adult males revealed an inverse correlation between home range size and crocodile size, while the home range sizes of adult females were generally more homogeneous. All nesting females displayed an explosive increase in mobility and space-use subsequent to the nesting period, and all adults, except one female in the central lake, moved during winter in the drought period to large crocodile congregations south of the lake. Sub-adults occupied significantly smaller home ranges than adults, which were habitat specific with strict spatial partitioning. They remained in shallow vegetated areas adjacent to deep water, avoiding open deep water altogether. Nile Crocodile nests have been monitored since 1982, with mean nest abundance = 76.19 ± 6.42, range: 29 - 141. The macro-level heterogeneity of nesting habitats reflects the spatio-temporal diversity of the Lake St Lucia system, and is possibly unique within a single Nile Crocodile population. Changes in nest abundance and distribution were seemingly related to increased human disturbance and habitat transformation in the northern and southern parts of the lake. Hydrological variability, especially during droughts, combined with the state of the estuary mouth (i.e. open or closed), affected prey abundance/availability contributing to large variation (6.9 % - 56.4 %) in nest effort from 1982 - 2013. All nests were located close to freshwater streams or seepage areas. We confirmed the re-use of the identical nest-site by a female, while other females oviposited in nest-sites occupied by different females during previous years. Despite variable nest effort, the St Lucia nesting population remains the largest recorded nesting population in South Africa, and least vulnerable to flooding. The mean home range of nesting Nile Crocodile females (0.85 ha) was significantly smaller than non-nesting females (108.41 ha) during the nesting season. Activity levels and mean daily movements on the nest were 8.1 ± 2.5 % and 213 ± 64 m, respectively, and increased to 47.9 ± 11.7 % and 2176 ± 708 m during the post-nesting period. Overall levels of nest fidelity were 82.8 ± 11.7 %, which increased to 87.3 ± 7.8 % at night. The highest nest fidelity recorded during incubation was 99.7 % over a 96 day period. We investigated nest predation, hatchling liberation and nest-guarding activities of nesting Nile Crocodile females using remote camera traps. We captured 4305 photographs of 19 nest-guarding females over four years. Seven nests (36.8 %) were raided by the egg predators Water Monitors (Varanus niloticus) and Marsh Mongooses (Atilax paludinosus), on average 12.1 days ± 6.2 subsequent to trap camera employment. All females settled back on the nest following the first predation event and on average, females returned to their nests three times ± 0.8 between nest raids before finally abandoning the nest. Nest raids continued on average 5.9 days ± 1.6 while on average 18.8 ± 4.0 raids per nest were recorded. Five females were captured by trap cameras liberating hatchlings. During the day females were almost never photographed on the nest, but during the late afternoon or early evening females moved onto the nest and continued to stay there during the night. Females always defended their nests aggressively against non-human intruders. We investigated homing behaviour and specific movements using a GPS-satellite transmitter by translocating an adult female (2.7 m), with a known home range, ~50 km north (straight line distance) to the False Bay area of Lake St Lucia. Following release, the individual moved a total distance of 178.3 km over 136 days (mean daily movement = 1311 ± 207 m), compared with 60.4 km (mean daily movement = 444 ± 32 m) for the identical time period the previous year. Homing movement was not continuous, but characterised by periods of extensive and directed mobility followed by prolonged periods of inactivity associated with freshwater or low salinity habitats. The translocated crocodile displayed remarkable navigational abilities, even though this required negotiating complex habitat challenges including extensive areas of the lake that were either hypersaline or completely dry, resulting in frequent and extensive overland movements. On 14 September 2012, the individual returned to the same freshwater pool where it was captured 136 days previous. This is the first study to confirm homing behaviour for Nile Crocodiles, and supports growing evidence that crocodilians and other ectothermic taxa possess complex navigational abilities. Our study revealed numerous novel insights into the ecology and behaviour of Nile Crocodiles and some of the findings may be applicable to other crocodilian taxa. We hope the results will guide the management and conservation of this threatened species.
The ecology and conservation biology of Lilian's lovebird Agapornis lilianae in Malawi.
(2014) Mzumara, Tiwonge I.; Downs, Colleen Thelma.
Despite their popularity, parrots are the world’s most threatened birds. Lovebirds (Agapornis) are very popular pet and aviary birds and as ecologically specialised species in the wild, they are also among the most threatened group of birds. Lilian’s lovebird Agapornis lilianae is a mopane Colophospermum mopane woodland specialist. This study represents the first detailed investigation of the species ecology in the wild. The current distribution of Lilian’s lovebird in Malawi was explored. Furthermore, the extent of the largest resident population in Liwonde National Park (LNP) was investigated. Five new atlas records are reported; three within 40-56 km of the LNP population, and two were over 150 km south and north of LNP respectively. One of them in Kasungu National Park is about 66 km from the Lilian’s lovebird population in Luangwa Valley, Zambia. Lilian’s lovebirds occurred throughout LNP with the highest abundance in the central section. Seasonal movements to areas outside the park were recorded. A variety of vegetation types were used by the lovebirds. The strongest vegetation associations were with seasonally wet grasslands and not mopane woodlands. The abundance and density of the Lilian’s lovebird in LNP was investigated. The highest density estimates of 17 ± 4.8 lovebirds km-2 were recorded in LNP’s mopane woodland. However, number of observations per transect differed significantly. Waterhole counts had the lowest estimates (10 ± 3.5 lovebirds). Flyway counts had the intermediate estimate (14 ± 3.0 lovebirds). The total population of Lilian’s lovebirds in LNP is therefore estimated to be about 4000 individuals. The use of line transect counts at the end of the rainy season is recommended for continued monitoring of Lilian’s lovebirds abundance in LNP. Lilian’s lovebird is a secondary cavity user adapted to mopane woodlands. We investigated its roost characteristics and roosting behaviour. We quantified tree and roost site variables for roost and non-roost trees. Roosting behaviour was observed during the morning and late afternoon. Lilian’s lovebirds’ roosts were located in large tall mopane trees with a mean diameter at breast height (dbh) of 57.4 ± 1.64 m, a mean height of 16.5 ± 0.42 m, and with a mean cavity height of 10.0 ± 0.05 m. All roosts were located in mopane trees within mopane woodland with 10 – 50 % tree cover. Non-roost areas had significantly smaller trees (mean dbh = 39.4 ± 1.72 m) and were located significantly closer together. Human disturbance was low in both areas, however, evidence of elephant Loxodonta africana browsing was high with large areas of stunted mopane woodland recorded in non-roost areas. We recommend that the current LNP vegetation map be updated to highlight areas of stunted mopane woodland unsuitable for Lilian’s lovebird roosts. The impact of elephant browsing on large mopane trees should be assessed to understand its impact on the availability of suitable cavities for lovebirds and other tree cavity-reliant vertebrate species. Investigations into the diet and foraging behaviour of the Lilian’s lovebird revealed they fed on 30 different plant species. These occurred in six habitat types, two of which were outside LNP (agriculture fields and dambos). In the wet season majority of Lilian’s lovebirds (23 %) foraged in dambo areas, whilst in the dry season (August – November) the lovebirds mainly foraged in grasslands with tree cover (18 %). In mopane woodland feeding flock sizes differed significantly between the wet (mean = 20 ± 1.0 lovebirds) and dry season (mean = 34 ± 2.3 lovebirds). Grass seeds were their main food source from December to June. Lilian’s lovebirds diet was more diverse from July to November and included leaves, leaf buds, fruits, fruit seeds and herbs. Grass seeds fed on during the wet season had a high protein and energy content. The Lilian’s lovebirds foraging habitat is protected within LNP, however, early burning in areas outside the park needs to be monitored. The breeding biology of the Lilian’s lovebird was investigated. Data were collected through a combination of direct observations and infrared camera traps during three breeding seasons. Results show large similarities with the black-cheeked lovebird A. nigrigens in Zambia. The breeding season was from February to May. Lilian’s lovebirds nested mainly in south-east oriented deep cavities (≥ 1 m) located in large mopane trees (mean dbh = 57.6 ± 2.35 cm). Nests were located in loose clusters in the areas they roosted (mean distance to nearest nest = 24.2 m). Nest fidelity was observed. Clutch size ranged from 3 – 6 eggs, (mean 5.0 ± 0.22). We recorded 49 % hatching success and 69 % fledging success. Results suggest a low breeding success mainly due to the loss of eggs to predation. The use of poison to kill wildlife is a threat to biodiversity. In LNP illegal hunters poison naturally occurring waterholes to catch mammals and birds for food. Lilian’s lovebirds are among the victims at these poisoned waterholes. Lilian’s lovebird population in LNP represents about 20 % of the global population. The drinking habits of the Lilian’s lovebird, the availability of natural waterholes and the occurrence of poisoning incidents in LNP were investigated. Results showed Lilian’s lovebirds congregate at waterholes in the dry season with flock sizes ranging from 1 to 100 individuals. Significantly larger flock sizes were seen in the dry season compared with the wet season. The number of poisoning incidents/year ranged from 1 to 8. The dry season had the highest numbers of poisoning incidents. Lilian’s lovebirds were killed at approximately four poisoning incidents each year between 2000 and 2012. The number of lovebirds found dead at a poisoned pool ranged from 5 to 50 individuals. A list of other species affected by the poisoning is provided. There is need for increased efforts in preventing this lethal activity in the park. Avian diseases are considered to be one of the key threats to bird conservation. Psittacine beak and feather disease (PBFD) is the most significant infectious disease in psittacines. It is caused by the beak and feather disease virus (BFDV) and currently has no cure. PBFD threatens the survival of wild populations of endangered parrots in Africa. The occurrence of BFDV was investigated in wild populations of Lilian’s lovebird. In addition, evidence of blood parasites presence was also investigated to determine their general health. All samples (n = 48) tested negative for BFDV. Blood parasites were observed in 13 of the 48 samples (27 %). Investigation of virus occurrence in other known populations of the species is recommended to assess the conservation risk faced. Lilian’s lovebirds (n = 55) were mist-netted and ringed in LNP. Measurements showed that females were significantly larger than males. About 50 % of the birds ringed in October were half way through their primary moult indicating that moulting starts in earlier months possibly just after the breeding season in April. This study highlights three of the key threats (waterhole poisoning, habitat loss and predation) to the conservation of Lilian’s lovebirds in LNP and provides proposed actions to address these threats.
Flexibility in metabolic rate in a small Afrotropical bird Zosterops virens.
(2014) Thompson, Lindy Jane.; Downs, Colleen Thelma.; Brown, Mark.
The scientific literature contains hundreds of studies on avian basal metabolic rate (BMR), many of which assumed that BMR was fixed for each species. Yet those from the last few decades have shown avian BMR to be a flexible trait, changing temporarily and reversibly in response to numerous environmental variables. Given that birds from lower latitudes are relatively understudied compared with temperate and Holarctic species, and that seasonal trends in BMR of southern hemisphere birds are not well understood, we looked at seasonal variation in BMR of a small Afrotropical bird, the Cape white-eye (Zosterops virens), over two years, and found that small birds may reverse the direction and amplitude of seasonal change between years. We also looked at circannual rhythm in avian resting metabolism (RMR), and found that peaks and troughs in resting metabolic rate (RMR) may not necessarily correspond with peaks and troughs in ambient temperature, suggesting that some of the confusion regarding the direction and magnitude of seasonal change in avian BMR may be caused by timing of seasonal measurements. Since we were using captive birds for my work, and since captivity may have an effect on avian BMR, we compared the BMR of freshly wild-caught birds with that of long-term captives housed in outdoor aviaries. The captive birds had higher BMR, giving weight to the argument that some physiological data of captive birds should not be used as representative of wild conspecifics, however the direction of seasonal change was similar in freshly wild-caught and long-term captive birds. Along the same vein, acclimation to laboratory conditions, experimental procedure, and different thermal environments, may also affect avian BMR, and thus before we started the final experiment, Cape white-eyes were acclimated to two different thermal regimes, with no change in RMR over an eight-week period, although there was an increase in body mass over the first three weeks, presumably due to the captive diet being of higher quality than a wild one. These results suggested that in some instances, small birds that are freshly wild-caught may not need to be acclimated in terms of their metabolism, before respirometry trials begin. Finally, given that anthropogenic climate change is anticipated to eclipse all other threats to biodiversity, and since many current predictive models pay no heed to metabolic flexibility of birds, we investigated the effect of a 4°C increase in housing temperature on resting metabolism of the Cape white-eye. This temperature increase is equivalent to that predicted for the range of this species by 2080, and therefore gives an indication of the effect of a sustained increase in mean surface air temperature. The results showed only a marginal difference in various metabolic parameters, suggesting that these birds may cope with the mean temperature increase predicted for their range in the coming decades. Together, these results highlight the importance of considering phenotypic flexibility when studying avian resting or basal metabolic rate. This has special implications for seasonal studies that implicitly assume that summer and winter measurements provide snapshots of the maximum and minimum RMR of which birds are capable, and for comparative studies, which may incorporate metabolic data from both wild and captive populations, or from study birds that were acclimated for different periods.
Morphometrics, ecotoxicology and stable isotope ecology of Nile crocodiles (Crocodylus niloticus) in KwaZulu-Natal, South Africa.
(2015) Warner, Jonathan Kirk.; Downs, Colleen Thelma.; Myburgh, Jan.
Abstract available in PDF.
Urban ecology of the crowned eagle stephanoaetus coronatus in KwaZulu-Natal, South Africa.
(2015) McPherson, Shane Cameron.; Downs, Colleen Thelma.; Brown, Mark.
Urban environments comprise a complex and dynamic landscape, and urban sprawl is irreversibly transforming large areas of land globally. Increasingly, the need for incorporating ecosystem services into urban landscapes provides opportunities for green-space to benefit biodiversity and indigenous wildlife. Enhancing urban green-space maximises indigenous biodiversity and provides conservation value, and can also benefit people by enriching their experience and awareness of nature. Large charismatic species can stimulate awe and interest as emblematic representatives of the wilderness. As the global population becomes ever more urban, this enriches the human experience. The crowned eagle (Stephanoaetus coronatus) is a large predatory raptor and a threatened species that is increasingly known to inhabit the Durban Metropolitan Open Space System (D’MOSS), within eThekwini municipality, KwaZulu-Natal (KZN), South Africa. This research investigated the ecology of the crowned eagle in the urban environment and suggests opportunities for enhancing the urban landscape for conservation benefits. Globally, dramatic land use change typical of urbanisation negatively affects biodiversity, especially for top predators. The D’MOSS design faces the challenge of conserving biodiversity in a regional hotspot in the face of rapid urban growth in one of Africa’s major commercial hubs. Understanding habitat use of keystone and apex species provides urban planners with an opportunity to integrate biodiversity in a growing city. Consequently, we investigated habitat use and nest site selection of crowned eagles on various spatial scales within this urban mosaic. Unexpectedly the inter-nest distances were small in this human-dominated landscape. However, breeding sites were not evenly distributed through the landscape and were closely associated with natural forest, while nest trees were most frequently in patches of exotic large riverine Sydney blue gum (Eucalyptus saligna, Smith 1797) within the D’MOSS planning zones. Crowned eagles showed a strong tendency to avoid informal settlement areas, however they were tolerant of proximity to established formal settlements and occupied dwellings. Further, home range and habitat selection were investigated with GPS telemetry, albeit with a limited sample size (n =5) due to the limitations of abundance and dispersion of this apex predator. The 350 km2 urban core study area comprised a matrix of mainly formal settlements (44%), and DMOSS green space areas (29%). The study area was occupied by up to 22 active breeding pairs of crowned eagles. We documented a mean (n = 4) annual home range of 13 km2 (hull100%) containing 6.3 km2 of territory per pair (LKDE HLSCV 95%). These relatively small home ranges for a large eagle included shared territorial boundaries. Rapid replacement of vacancies at breeding sites suggests a saturated population. Habitat selection within the home range, thresholds of critical habitat, exotic trees, and correlation with DMOSS show the importance of pockets of indigenous forest in this urban mosaic landscape. These forests are fragmented and fragmentation increases the available edge habitats and landscape heterogeneity, potentially enhancing resource availability for crowned eagles in a highly modified landscape. The presence of remnant patches of mature Eucalyptus was more preferred than monotypic timber plantation stands. Consequently, continued protection of the D’MOSS system, and a considered approach to management of Eucalyptus are required for the persistence of the crowned eagle in this landscape The study of diet is pivotal in understanding a species, particularly for quantifying a predatory raptors’ economic niche and potential for human-wildlife conflict. In close association with urban development, the local population of crowned eagles has the potential to be a concern to the safety of domestic stock and pets. Time-lapse cameras were positioned at urban nest sites (n = 11) to identify the prey composition during breeding, particularly in regards to taxa with human associations. This was the first use of this technique for this species. The numerical proportion of avian prey, particularly hadeda ibis (Bostrychia hagedash) pulli, was several times greater than any previous diet description. The methodology used and the abundance of hadeda ibis in these urban environments are potential contributing factors. Rock hyrax (Procavia capensis) was the primary prey and where hyrax were unavailable, the diet composition was broader and included more vervet monkeys (Chlorocebus pygerythrus). Domestic stock comprised only 6% of the identifiable prey. Contrary to popular belief, no dogs (Canis familiaris) and few cats (Felis catus) were delivered to the nest by breeding eagles in this study. In situ pet attacks are most frequently attributed to juvenile and immature crowned eagles in winter and spring. Attacks on pets by crowned eagles, especially on small dog breeds, although relatively rare have a substantial influence on human-wildlife conflict and public perceptions. Pet attacks are generally attributed to juvenile and immature crowned eagles during periods of limited resources, particularly winter and during dispersal in the juveniles’ first spring. Negative social perceptions have resulted in persecution (n = 5), one of the main causes of recorded injury and mortality to crowned eagles in the region. Gunshot persecution, electrocution and collisions with anthropogenic structures have the greatest impact on juvenile and immature survival in the region. We provide management recommendations regarding various categories of crowned eagle human-wildlife interactions. Collaboration of wildlife authorities with NGO’s and public stakeholder input creates an environment for successful crowned eagle conservation and management of human-wildlife conflicts. Public awareness is an important aspect to the sustainability of the urban crowned eagle population. This study demonstrates that urban mosaic landscapes can provide conservation benefits for the crowned eagle. The land planning strategies enacted in Durban can guide urban expansion in tropical forest biomes to enhance indigenous biodiversity in urban mosaic landscapes in Africa, and globally.
The ecology of large-spotted genets within an urban landscape.
(2015) Widdows, Craig David.; Downs, Colleen Thelma.
Urbanization is one of the most damaging and rapidly expanding forms of anthropogenic landscape modification and is having profound consequences on biodiversity worldwide. The global increase in urbanization has resulted in exclusion of many carnivore species from human-altered landscapes due to a variety of anthropogenic impacts. However, despite the negative impacts of urbanization on carnivores, certain species such as large-spotted genets (Genetta tigrina) exhibit an ability to persist within urban areas. Despite their extensive distribution range, large-spotted genets are poorly studied in comparison to other African carnivores, with a handful of studies conducted on genetics, activity patterns and diet. Furthermore, no studies have focused on their ecology in an urban environment. There have been increasing reports of large-spotted genets within urban areas throughout KwaZulu-Natal, South Africa. The mosaic of patches of native vegetation within this urban landscape provides habitats for a variety of wildlife species. The main aim of the study was to investigate the ecology of large-spotted genets within an urban landscape and to determine what factors facilitate their ability to persist in an urban environment. Residential interviews were conducted to ascertain information pertaining to behavioural observations, land use as well as wildlife conflict and public perceptions of genets. Chi-square (x2) goodness-of-fit tests were used to determine significant differences in the frequency of responses. Domestic pet food was the main food item genets were observed eating. The most cited cause of genet fatalities were attacks by domestic animals and collisions with vehicles. Genets were reported using roof spaces for resting during daylight hours and as locales for breeding. The majority of respondents expressed positive attitudes towards genets in urban areas; negative views stemmed from concerns of disease transfer and impacts on wild bird populations. We investigated the relationship between occurrence of large-spotted genets with various environmental variables believed to influence their site occupancy and detection in an urban environment. Presence/absence data was collected from 28 camera trap stations between June 2012 and October 2013 in Kloof, KwaZulu-Natal, South Africa. Average estimated occupancy of urban genets was 0.62 ± 0.14 with a detection probability 0.19 ± 0.03. Model selection indicated that bush cover and placement of camera traps on wide paths negatively influenced large-spotted genet occupancy. Both winter and fringe habitats influenced the site occupancy of large-spotted genets positively. Furthermore, bush cover was negatively associated with detection probability of urban genets. The dietary composition and the influence of predictable feeding stations on urban large-spotted genets were investigated through scat analysis. Scats were collected on a monthly basis over a year from eleven midden sites. Invertebrates were the dominant prey items consumed with small mammals also forming an important component of the diet. Significant seasonal variation was recorded with the presence of birds and pollution in the scats with highest recorded frequencies during spring and winter respectively. During winter, when invertebrate abundance declined, urban genets increased their intake of anthropogenic refuse resulting in plastic, elastic bands and various other waste products present in the scats. We investigated the effect of anthropogenic structures on the roost temperatures of large-spotted genets. Roost temperatures were recorded using i-Button® temperature loggers at known genet roosts in anthropogenic structures as well as in natural roost sites (tree hollows and rocky overhangs). Over the seasons temperatures varied significantly between months and among different roosts. However, anthropogenic roost temperatures were significantly higher than ambient temperatures throughout the study period. Furthermore, anthropogenic roosts had higher temperatures (with lower variability) than natural roost sites. This study indicated the importance of anthropogenic structures as daytime roosts and for breeding for large-spotted genets within an urban matrix. Given their adaptability and apparent success within the urban environment, we investigated the movement ecology of large-spotted genets within the urban landscape. Large-spotted genets were captured and fitted with global positioning system mobile transmitters. Although seven individuals were trapped, detailed data were only obtained for two individuals. Minimum convex polygons (MCPs), 95% and 50% kernel density estimates (KDEs) were calculated for a male and female large-spotted genet. The 95% kernel density home range sizes showed relatively small home range sized for both individuals, with an area of 48.2 ha (female) and 17.5 ha (male). Habitat selection within home ranges indicated that the male large-spotted genet preferred urban residential habitat while the female genet avoided nearby croplands. Both individuals used most of the habitats within their study area. Variable habitat used by both individuals in this study confirmed the species’ ability to adapt to the urban mosaic of habitats. This thesis showed the importance of a variety of factors on the distribution of large-spotted genets in landscapes where natural habitats are threatened by changing land use and increasing human populations. Furthermore, this thesis illustrated the ability of genets to live in an urban mosaic landscape by using a variety of anthropogenic resources.
Ecosystem carbon change of shrub-encroached grasslands across a precipitation gradient in South Africa.
(2015) Mureva, Admore.; Ward, David Mercer.
Abstract available in PDF file.
Assessing anthropogenic impacts on the persistence of forest mammals within the Indian Ocean coastal belt of southern KwaZulu-Natal province.
(2016) Ehlers Smith, Yvette Cathrine.; Downs, Colleen Thelma.
Abstract available in PDF file.
Aspects of the ecology of trumpeter hornbill (bycanistes bucinator) across urban-forest mosaics in KwaZulu-Natal, South Africa.
(2016) Chibesa, Moses.; Downs, Colleen Thelma.
Abstract available in PDF file.
The influences of plant secondary metabolites on the foraging behaviour and carrying capacities of the african elephant, loxodonta africana.
(2017) Schmitt, Melissa Holbrook.; Shrader, Adrian Morgan.; Ward, David Mercer.
Herbivore diet selection is influenced by plant quality and availability. For browsing and mixed-feeding herbivores, such as elephants, which consume trees and shrubs, plant quality is influenced by the concentrations of crude protein, fibre, and plant secondary metabolites (PSMs). To date, the driving factors behind elephant diet selection have not been well understood. The broad aim of this study was to better understand how pre-ingestive cues such as volatile organic compounds (VOCs) and postingestive cues such as the effects of plant secondary metabolites (PSMs) influenced the foraging behaviour of African elephants. I also examined the salivary tannin-binding proteins of elephants and how these influenced carrying capacities. I found that elephants were able to differentiate between plant odours in the form of VOCs across multiple spatial scales. This could help elephants to reduce their search time while foraging across a landscape. I also found that these pre-ingestive cues were a better indicator of diet preference than the tested postingestive cues such as digestibility, crude protein, tannins (a type of total polyphenol) and the ratio of palatable: unpalatable indices. The VOCs in preferred plant species had significantly lower concentrations and diversity of PSMs than avoided species, particularly with respect to monoterpenes, a known anti-herbivory VOC. In contrast, avoided plant species were more digestible and had lower levels of polyphenols and tannins. Ultimately, I found that terpenes play a stronger role in elephant diet choice than phenolics (such as tannins) and other postingestive feedback measures. This is likely because terpenes are a pre-ingestive cue that also incur postingestive costs, which the elephants can use prior to making foraging decisions, unlike other postingestive feedback measures. While it seems counterintuitive that preferred plants have high concentrations of tannins, which reduces the amount of crude protein available, I found that elephants have salivary tannin-binding proteins, which neutralize the influences of tannins by an average of 75%. This neutralization influences the amount of available crude protein, which would, in turn, influence the carrying-capacity for elephants. I constructed an optimal-foraging model for carrying capacity that incorporated the negative effects of tannins and the neutralization of these chemicals by the tannin-binding affinity of elephant saliva. I also included diet breadth and dietary contribution of browsed species, browse quality, and the available standing crop of browsed species. As a result, my model produced more plausible estimates of elephant carrying capacity than conventional standing-crop models.
The habitat, nesting and foraging requirements of southern ground-hornbills in the Kruger National Park.
(2016) Combrink, Leigh.; Downs, Colleen Thelma.
Southern ground-hornbills Bucorvus leadbeateri are large, terrestrial, carnivorous birds that inhabit the savanna and bushveld habitats of much of Africa, south of the equator. They were once prevalent in north-eastern South Africa, but as a result of extensive habitat loss and persecution, their population has suffered a significant decline. They are currently listed nationally as Endangered and globally as Vulnerable. In an effort to curtail this decline in South Africa, a National Species Recovery Plan was developed, with reintroductions of the birds into suitable habitat outside of protected areas listed as a viable conservation intervention for the species. This plan also highlighted a number of knowledge gaps which need to be addressed and which are essential to the long-term conservation of the species. The exact habitat requirements (including specifics of nest cavities) and the foraging ecology of southern ground-hornbills were both listed as areas where data are lacking. Consequently the main aim of this study was to determine the habitat, nesting and foraging requirements of the southern ground-hornbills with the intention of developing management guidelines for areas planned as reintroduction sites for the species. Our study focused on the population of southern ground-hornbills located within the Kruger National Park. We found that the particular characteristics of the southern ground-hornbill nest (cavity dimensions, tree species, height of cavity etc.) did not affect nesting success of the birds. The proximity of roads was important, with more successful nests being situated closer to roads. Habitat structure and diversity of vegetation around the nest also influenced the success of the particular group, with nests with more open habitats and a wider variety of vegetation types being more successful. Nest cavity temperatures were significantly different to ambient for selected nests studied across the 2013-2014 and 2014-2015 breeding seasons. We also found that nest temperature did not affect their nesting success. Interestingly, the artificial nest within our study area showed extremes in temperature (significantly higher and lower than ambient maximum and minimum temperatures, respectively) despite this being one of the most successful nests studied. As southern ground-hornbills are carnivorous, they are known to take a variety of prey items and have been considered generalists. One of the important current questions in foraging ecology is whether generalist populations consist of individuals (or in our case, groups) that are all generalists, or if the generalist population comprises a number of dietary specialists. We tested this theory for southern ground-hornbills using stable isotope analyses of feather and bill samples. Our results show that they are obligate generalists at the group level, suggesting that they access and consume prey species in accordance with their availability in the landscape. At the individual level, based on the two bill samples obtained, there could be some form of specialization occurring. With our relatively small sample size we were unable to determine whether this was in fact specialization or whether these results were reflecting local environmental changes, affecting the isotopic signatures of the vegetation and thereby, prey species of southern ground-hornbills. We used satellite tracking technology to investigate home range sizes and habitat use of southern ground-hornbills within the Kruger National Park and surrounding conservation areas. We then used first-passage time analysis to determine whether certain movement behaviours were influenced by habitat type. We found that there were marked seasonal differences in home range size and that all groups showed a range restriction during the wetter months (coinciding with the breeding season), where activities are concentrated around the nest site. Grassland, open woodland and dense thicket habitats were found to be important habitats for foraging and grassland and open woodland areas were used in accordance with their availability within the groups’ respective territories year-round. The results from this study have been consolidated into recommendations for areas being considered as potential release sites for captive-reared southern ground-hornbills. This research investigated what the habitat, nesting and foraging requirements of southern ground-hornbills are with the aim of adding to the current data on the species as well as addressing these knowledge gaps as highlighted in the National Species Recovery Plan. Each aspect of this PhD study provided results that can be used in current and future conservation interventions, and in particular, reintroductions in areas outside of protected areas. These results are applicable to populations within South Africa, but can also be applied to the species across its range.
Varying degrees of fear : how do large herbivores adjust their anti-predator behaviour in response to different predators?
(2016) Makin, Douglas Ferguson.; Shrader, Adrian Morgan.; Chamaillé-Jammes, Simon.
Large predators are returning to areas where they have previously been absent through both predator reintroductions and natural range expansion. As a result, these re/introductions are changing the nature of predator-prey systems, with potential shifts from single to multi-predator environments. In response, prey species must make spatial (i.e. patch use) and behavioural (i.e. vigilance, feeding) adjustments to minimize risk from predators with varying hunting strategies, prey preferences and activity patterns. With this in mind, the broad aim of my study was to determine how perceived predation risk from different large predators affected the spatial and behavioural decisions made by individuals in a community of mammalian herbivore species. To achieve this, I first investigated, how kudu, sable and warthog responded to the introduction of wild dogs into an area with cheetah. In response to this shift from single to multiple predators, all three-herbivores decreased their feeding effort. However, sable (not hunted by cheetah) decreased their feeding intensity more than kudu (preyed upon by cheetah). In contrast, warthog (avoided by cheetah) showed a different feeding pattern, but then displayed a dramatic response to the wild dog introduction in that they disappeared from the study site. These patterns suggest that the magnitude for increasing anti-predator responses to predator introductions may be greater for prey (e.g. sable, warthog) living in initially low risk environments compared to prey species (e.g. kudu) that already have high perceived predation risk from the resident predator (e.g. cheetah). Despite decreasing their feeding effort, all three herbivore species preferred patches located in open grasslands compared to denser vegetation, before and after the wild dog introduction, possibly due to increased predator detection in open areas. In my second experiment, I explored how prey species employed a range of anti-predator behaviours (i.e. vigilance, grouping and temporal shifts in activity) of various combinations to minimize risk from predators with different hunting strategies (ambush vs cursorial) and prey preferences. Overall, herbivores had higher vigilance, moved in larger groups and avoided waterholes at night when exposed to predation risk from lions (ambush predator). In contrast, in response to cheetah and/or wild dogs (cursorial predators), only prey species within the accessible weight range of both these predators (warthog, red hartebeest) moved in larger groups compared to conspecifics in the lion section. They did not however, shift their waterhole use to night to avoid the largely diurnal cheetah and wild dogs. These results suggest that the potential threat of ambushing lions was greater than that of the cursorial cheetah and wild dogs. I then expanded on this experiment by investigating, how prey adjusted their anti-predator behaviour (i.e. vigilance) in response to the alarm calls of con/heterospecifics (i.e. zebra, wildebeest) or to non-hunting predator calls (i.e. lion roars). Overall, red hartebeest and wildebeest living with lions tended to show greater vigilance in response to the lion roars compared to the alarm calls. This suggests that these species perceived the direct cue of lions as a better indicator of risk than the alarm calls. I then compared these responses for prey species living with and without lions. I found that herbivores living with lions had higher vigilance than conspecifics living without lions. Despite a greater overall response in the lion section, herbivores in the lion-free section still significantly increased their vigilance in response to the lion roar. Yet, it was two of the lions’ preferred prey species (i.e. zebra, wildebeest) that showed the greatest response. This suggests that species under the greatest threat may maintain innate anti-predator responses to a dangerous but absent predator longer than less preferred prey. Ultimately, my findings indicate that simple cues from dangerous predators can have a greater effect on anti-predator behaviours of prey species than alarm calls. Overall, the results from my PhD highlight that prey respond to changing predation risk after a predator introduction by modifying their fine scale patch use and foraging behaviour. Moreover, prey responses to different predators are not uniform, but reflect differing degrees of danger posed by the predators, resulting in prey using varied combinations of anti-predator behaviours to minimize risk around risky areas. Lastly, non-hunting cues from dangerous predators can trigger strong anti-predator responses from prey living with and without the predator. Ultimately, the results of my three experiments highlight the behavioural plasticity of prey species to reduce predation risk across their fluctuating landscape of fear.
Olfactory communication of the white rhinoceros (Ceratotherium simum)
(2017) Marneweck, Courtney Jade.; Shrader, Adrian Morgan.; Jürgens, Andreas.
Many mammal species communicate olfactorily via specialised scent glandular secretions, urine and/or dung. Despite a large body of work on olfactory communication, the extent to which mammals communicate via dung odours, and what information is transmitted, is unknown. White rhinos (Ceratotherium simum) have poor eyesight but an acute sense of smell and therefore rely heavily on olfactory signals. Moreover, white rhinos of all ages and sex defecate communally in middens, thus it is possible that these middens act as olfactory information centres for male-male, female-male, male-female and female-female communication. To explore these possibilities, I first analysed the odours emitted from the dung of free-ranging white rhinos. In doing so, I identified distinct odour profiles that indicated an individual’s sex, age, male territorial status, and female oestrous state. Once I had identified the information transmitted, I then explored how long these signals lasted. In order for an olfactory signal to be effective it must persist in the environment for an extended period. To determine signal longevity I analysed the temporal changes of white rhino dung odours. I found that over a short period male dung odours had shorter longevity than female odours. Within males, territorial odours had shorter longevity than non-territorial, while non-oestrous female odours had a shorter longevity than oestrous odours. The high temperature and humidity of the wet season decreased the longevity of all adult dung odours. However, white rhinos did not adjust their visitation or defecation frequency during the wet season to counteract this decrease in longevity. Having identified the odours and how long they lasted, I then investigated the behaviour of white rhinos at middens to determine which individuals were primarily transmitting information and who were the intended targets. I found that middens were utilised predominately by adults. Moreover, the primary function of middens was for territorial males to transmit and obtain information (male-male and female-male communication), with secondary functions for non-territorial males to also assess female reproductive state, and females to assess the quality and number of potential mates (male-female communication). In addition to olfactory signals there was a spatial aspect to defecating in middens, where territorial males defecated in the centre of the midden and other individuals around the periphery. Further, territorial males regulated their dung output, with a higher defecation frequency and smaller dung volume than any other adult. Finally, I conducted an experiment to investigate the purpose of territorial male dung kicking. Using non-territorial adult male dung as a surrogate, I found that the dispersal of male white rhino dung caused olfactory signal amplification by increasing the emission of hydrocarbon acids. However, despite the benefits of odour amplification, dung dispersal also carried a cost of decreased odour longevity, ultimately decreasing signal longevity. Territorial males likely counteract this by defecating in middens during peak visitation times by other individuals. Ultimately, my results highlight the mechanism behind olfactory communication in white rhinos and the importance of middens in this communication system. Moreover, as many other mammal species defecate communally, olfactory communication via dung odours is likely a widespread phenomenon.
The conservation ecology of the African tigerfish Hydrocynus vittatus in the Kavango River, Namibia.
(2017) Jacobs, Francois Jakob.; Downs, Colleen Thelma.
Abstract available in PDF file.
Monitoring and assessment of macroinvertebrate communities in support of river health management in KwaZulu-Natal, South Africa.
(2017) Agboola, Olalekan A.; O’Brien, Gordon.; Downs, Colleen Thelma.
Conservation of freshwater systems is globally weak and generally declining, with rivers and wetlands being the most threatened ecosystems by anthropogenic impacts. Though they are highly import, freshwater ecosystems remain poorly understood and insufficient data often limit conservation efforts on many freshwater ecosystems. KwaZulu-Natal (KZN) Province is an important high water yield area of South Africa, but the sustainability of the rivers is being threatened. Macroinvertebrates are good indicators of water quality and ecosystem degradation, but their biodiversity and ecosystem conservation depend largely on the quality of the available data and the efficiency of the methods used in the data analysis. Each aspect of the research provides results that can be used in current and future conservation planning for rivers and aquatic macroinvertebrates. The reference condition approach is an effective bioassessment technique closely related to the biological/ecological integrity concept, which is based on the evaluation of the deviation of the ecological quality of a test site’s biological community from that of a near-pristine “reference” condition having very similar characteristics. Although the term reference condition, is used to describe near-natural or pristine condition, several practitioners believe that only a few pristine ecosystems still exist in the world. Hence, the reference condition (RC) defines the representative of a group of undisturbed or minimally disturbed sites by anthropogenic activities, while biological reference condition is the description of the biological elements that exist under no or very minor anthropogenic activities. This study applied the multivariate method of selecting and validating reference conditions, using ecoregions, river types and seasonal changes as grouping criteria for the reference sites. The ecoregions and river types were more adequate than the seasonal variations in the selection of reference conditions. Although there is currently no consensus about the most appropriate and informative index, biodiversity indices are essential for environmental monitoring and conservation management decisions. This study compared a series of macroinvertebrate data from the rivers of KZN according to nine diversity indices (total number of species/taxa, total number of individuals, Margalef’s, Pielou’s, Brillouin’s, Hill’s, Simpson’s, Fisher’s and Shannon’s indices), one similarity index (similarity percentage – SIMPER) and three biotic indices (SASS5, ASPT, and MIRAI). There were clear connections between water quality, and abundance of macroinvertebrates with the decrease in the diversity values of macroinvertebrates along pollution gradients. Fisher’s index, similarity percentage, SASS5, ASPT and MIRAI were suitable indices for comparing degraded and least degreaded sites in this study. However, small changes in community compositions were better revealed by the Fisher’s diversity index, similarity percentage and SASS5. The MIRAI was better than SASS5 as an ecological tool for the rivers of KZN, but it can further be improved by incorporating measures of diversity and taxa richness into the model. Also, this study examined the effectiveness of macroinvertebrate taxa composition metrics to assess the ecological health of the rivers in KZN. Nine taxa metrics were able to distinguish between reference and impaired sites, through correlation strength with environmental variables and their reliability. The nine metrics were total number of taxa, total number of Diptera taxa, total number of Plecoptera individuals, percentage of Ephemeroptera, Plecoptera and Trichoptera taxa, percentage of Odonata taxa, total number of Trichoptera individuals, total number of Gastropoda individuals, total number of Oligochaeta individuals and total number of Coleoptera individuals. This study showed increasing water quality deterioration along the longitudinal gradients of the rivers in KwaZulu-Natal, from the upper reaches towards the lower reaches of the rivers. We found that macroinvertebrate community composition metrics could detect nutrient pollution, organic pollution and physical habitat degradation in KZN rivers. Thus it is recommended that more studies and validation of macroinvertebrate community-based metrics in the assessment of rivers in KZN are conducted. Furthermore, they are relatively cheap and easy to use. Macroinvertebrate community-based indices could be an effective alternative assessment method in the case of the lowland rivers where the lack of quality data often have negative impacts on the use of the biotic indices (SASS5, ASPT and MIRAI). In addition, this study demonstrated how Bayesian networks can be used to conduct an environmental risk assessment of macroinvertebrate biodiversity and their associated river ecosystem to assess the overall effects of multiple anthropogenic stressors in rivers of KZN. Cause-effect exposure pathways were established between the sources of stressors, habitats and endpoints (macroinvertebrate biodiversity and river ecosystem wellbeing) using using a conceptual model. The resulting conceptual model was then used to construct the Bayesian network models for each study site (risk regions) to estimate the overall risk from water quality, flow and habitat stressors. The model outputs and sensitivity analysis showed ecosystem threat and river health (represented by MIRAI) as the top factors posing the highest risks to macroinvertebrate biodiversity and the river ecosystem wellbeing respectively. The Bayesian network model was used to estimate the risk across the sites in the current scenario and three other scenarios that could occur if there were inadequate management practices. The current scenario was developed from field data collected during this study, while the other three scenarios were simulated to predict potential risk to the selected endpoints. We further simulated the low and high risks to the endpoints in order to demonstrate that the Bayesian network can be an effective adaptive management tool for decision making. The results of this study demonstrated that Bayesian networks can be used to calculate risk for multiple stressors, and that they are a powerful tool for informing future management strategies for achieving best management practices and policy making in the rivers of KZN.
Understanding the effects of changing climate and land use on woody plant encroachment in South African grasslands and savannas.
(2017) Pillay, Tiffany Prileeni.; Ward, David Mercer.
The increase in dominance of woody species, observed in many grasslands and savannas worldwide, highlights the sensitivity of woody herbaceous biomass ratios to changed environmental conditions. While the major factors driving the tree–grass continuum have been identified as precipitation, nutrients, increased CO2 concentrations, fire and herbivory, the interaction of these driving forces determines the tree: grass ratio, and ultimately the occurrence/ absence of woody plant encroachment. Furthermore, with forecasted alterations to the global climate (such as an increase in the frequency and magnitude of climate extremes), and increased nutrient availability (either through extensive use of fertilizer or increased nitrogen deposition), more knowledge is required on the factors which promote woody plant performance. We conducted a series of greenhouse experiments to manipulate these factors, and a two-year field trial to disentangle the effects of varying climate and land use on woody plant encroachment. These studies are summarized below: (1) We investigated the effects of manipulated water availability and passive warming on the productivity of the commonly occurring grass species Eragrostis curvula, and the emergence and performance of seedlings of the woody encroacher, Acacia sieberiana. We simulated a typical savanna microhabitat by planting tree species within a grass matrix. Watering treatments altered to represent drought (minus 40% of the average), normal (same amount as the long-term average of the area), and excess (addition of 40% of the long term average) precipitation. Passive warming was achieved using open-top chambers. Tree seedling emergence was monitored in the first two weeks. Thereafter, weekly tree growth rate measurements were recorded. The final above-ground grass biomass, tree root: shoot ratio and specific leaf area were quantified. We found that water availability was the main limiting factor for grass productivity, with low grass biomass attained under the drought treatments. Warming had no significant effect on grass productivity, but had a large positive effect on tree growth and performance when coupled with excess or normal water treatments. Tree seedling root:shoot ratios were highest in the drought treatment, possibly as an adaptation to low water supply. Higher specific leaf area was recorded with warming, further elucidating the positive effect of increased temperature for tree growth. (2) Using a completely randomized experimental design in the greenhouse, we investigated the effects of fertilizer, fire and grazing on the survival, growth and biomass of Acacia sieberiana. We simulated a typical savanna microhabitat by planting a common savanna grass species, Eragrostis curvula, together with saplings of A. sieberiana. Treatments of fertilizer, simulated grazing and fire were applied. Weekly measurements of relative growth rate (RGRheight and RGRdiameter) were recorded. At the end of 24 weeks, survival, total above- and below-ground biomass, and key functional traits (specific leaf area and average thorn length) were recorded to assess differences in tree performance. We found that fertilization was beneficial to grasses only, increasing their total biomass. Consequently, we observed increased tree sapling mortality with high grass biomass. Fire increased the saplings’ investment in shoot growth and stem diameter. However, we found no evidence of differential post-fire allocation to roots or shoots when grass competition was absent. Grass biomass was also found to decrease the specific leaf area of tree saplings. (3) We conducted a two-year field fertilizer trial, using pairs of open grassland and encroached plots at four sites across a precipitation gradient, ranging from 300–1500 mm mean annual precipitation (MAP). We assessed the effects of encroachment, N, P and N+P addition on soil N stocks and soil P, litter % N and C:N ratio (index of decomposition), plant productivity and species richness. We found that soil N stocks were on average 15 times higher in the higher precipitation sites regardless of nutrient addition or encroachment. Larger variation in soil P was noted at the high precipitation sites, with some evidence of P-limitation in the encroached area with the highest precipitation. We found significant positive effects of encroachment on litter % N (quality), C:N ratios and forb cover across the precipitation gradient. Fertilization increased grass biomass and reduced species richness in the high precipitation sites only. The purportedly beneficial effects of encroachment on litter and plant species richness was more pronounced at the high-precipitation sites. Overall, increased nutrient availability was found to be of importance in high precipitation sites only, possibly due to water scarcity at the low-precipitation sites. (4) To elucidate the effects of fertilizer addition, simulated fire and grazing on the rate of nitrogen (N) fixation and soil respiration we conducted a greenhouse experiment using a woody encroaching species, Acacia sieberiana, and the commonly occurring grass species Eragrostis curvula. Treatments of fertilizer, simulated grazing and fire were applied. Thereafter the Acetylene Reduction Assay was used to determine the rate of biological Nitrogen fixation (BNF). We found a significant decrease in BNF with fertilizer addition, and increases in BNF after fire application. Soil respiration increased with fertilizer addition and decreased after fire application. Grazing had no independent effect on any of the response variables. However, decreased grass biomass resulted in increased BNF across all treatments. Furthermore we found that larger saplings achieved a higher rate of BNF, with a positive correlation between the rate of BNF and both the number and weight of root nodules. The implications of these studies are: (1) Future climate-change predictions of increased drought may constrain grass biomass, thereby promoting woody plant success. Predicted warming is likely to further enhance woody plant performance. (2) Increased nutrient availability, whether as fertilizer addition or increased nitrogen deposition, may promote the competitive ability of the grass component, thereby limiting woody plant invasion. This is based on the assumption that grass productivity is not limited by frequent fire or intensive grazing. (3) Encroachment of leguminous woody plants is purportedly beneficial in higher precipitation areas, due to increased soil nitrogen, higher grass foliar quality under trees, greater forb cover and increased decomposition. Hence, encroachment control should be prioritized in low precipitation sites, particularly in rangelands where woody plants are undesirable. (4) Low intensity or infrequent fire is ineffective at controlling woody plant proliferation, particularly if the invader is able to resprout and is capable of biological nitrogen fixation.

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