OpenUCT :: Browsing by Author "Bond, William J"

Browsing by Author "Bond, William J"

Now showing 1 - 20 of 80

Open Access
The adaptive geometrey {i.e. geometry} of savanna trees : a comparative study of the architecture and life history of Acacia karroo Hayne. in savanna, forest, and arid karoo shrubland environments
(1998) Archibald, Sally; Bond, William J
Various morphological and life history attributes of Acacia karroo Hayne. were investigated in five populations from three environments in South Africa: forest (Cape Vidal, Natal), arid shrubland (Prince Albert, karoo) and savanna (Hluhluwe-Umfolozi Game reserve and ltala Game reserve, Natal). I aimed to elucidate on the distinctive characteristics required for trees to survive in savannas. The combination of frequent fires and intense herbivory in savanna environments creates a disturbance regime not encountered by trees in other situations and it was expected that A.karroo would display specific adaptations of growth form, life history, and reproductive ecology - adaptations which would reflect the strong selective pressure imposed by fire and herbivory on juvenile savanna trees to grow above flame height and browse limit in order to recruit into the mature canopy. A. karroo in savannas was found to have an unbranched, vertical growth form, and is thus maximising height gain. Reproduction was delayed until the trees were above the reach of flames and herbivores. Savanna A.karroo trees had fewer, smaller spines than trees in the other two environments, even though spines are generally considered to be effective in defense against mammalian herbivores. It is therefore suggested that a combination of low 'apparency' to browsers and rapid growth rate is an effective herbivore avoidance strategy in conditions where the need to escape from the fire trap makes a trade-off of growth rate for defensive structures unacceptable. The architecture of mature savanna trees reflects their history of rapid growth in earlier stages and they have smaller canopies, and thus lower reproductive capabilities, than equivalently sized trees in other environments.
Open Access
Aspects of the biology of climbers in southern Africa
(1991) Balfour, David Andrew; Bond, William J
This study examines factors that may limit the abundance and distribution of climbers and the influence that climbers may have on the regeneration of forests. The abundance of climbing plants (climbers) was established over a soil nutrient gradient, under standardised light conditions. Abundance was found to be positively correlated with the potassium concentration and the soil S-value, but not with other nutrients. Thus, the association between climbers and soil nutrient levels is challenged. Instead the abundance of climbers was found to be positively correlated with tree canopy architecture and trellis availability. More climbers entered host trees with low canopies (< 15 m) than those with high canopies. Trees with low canopies had more climbers entering vertically into the canopy while trees with high canopies had more climbers entering the canopy horizontally. Evidence was found to suggest that climbers facilitate the entry of each other into tree canopies. Tree and climber saplings were grown under three nutrient and two light treatments to compare the relative growth rates of their terminal shoots under the different conditions. While climbers were found to have greater shoot extension rates than trees, they did not grow relatively faster than trees under any of the combination of treatments. Thus light levels and soil nutrient availability may not influence the competitive ability of climbers compared to trees. Three aspects of the carbon gain capacity (maximum photosynthetic rate, leaf nitrogen concentration and leaf water use efficiency) were compared between climbers and trees. While the climbers had a significantly higher leaf nitrogen concentration than trees, as well as higher leaf water use efficiency; the maximum carbon assimilation rates of climbers and trees did not differ. These differences between trees and climbers are slight and suggest that differences in growth rates between the two growth forms are largely due to differences in carbon allocation in trees (to support structures) and in climbers (shoot elongation), rather than carbon gain mechanisms. Patterns of canopy tree regeneration in gaps and sub-canopy plots were determined for lowland and highland forests to determine whether climbers influenced the regeneration of canopy trees. More canopy trees regenerate in the sub-canopy of highland forests, while in lowland forests treefall gaps are the major sites of canopy tree regeneration. These patterns of regeneration were found not to be related to the density of the ground layer, the extent of the lateral infill of the gap-forming trees or the abundance of woody climbers. Instead, a negative correlation was found between the percentage of regenerating canopy species and the density of the herbaceous and shrubby understory layer in both lowland and highland forests. Thus in both forests the understory layer has an important influence on the regeneration of canopy trees. The role of vertebrate herbivores may be important in providing the necessary release from suppressive effect of the dense understory layer and to enable the regeneration of canopy tree species in the lowland forests.
Open Access
Burning for birds : the response of bird communities to recent fire history in the Serengeti ecosystem
(2014) Kinsey, Elliot; Beale, Colin; Bond, William J
Fire is a ubiquitous ecosystem process with important implications for savanna ecology and conservation. In this study we aimed to explore the short-term and long-term effects of recent fire history on bird communities in the greater Serengeti savanna ecosystem in Tanzania. The study investigated the influence of immediate fire event history ( < one year), cumulative fire regime history (over 14 years), and fire seasonality on bird assemblages. We predicted that the occurrence of recent fire events and cumulative fire regime history would influence bird communities, particularly through its impact on vegetation structure. We mapped fire activity from remote sensing data (2000-2013) and extracted mean rainfall and nitrogen estimates from interpolated maps. We conducted field sampling of bird community composition and vegetation structure at 207 sample sites distributed across the western Serengeti ecosystem. We tested the structuring effect and regression trends of recent fire event occurrence, cumulative fires regimes, rainfall, and nitrogen on bird community structure and vegetation, and tested the response of birds to vegetation. Despite a high degree of variation in bird communities and vegetation structure, our results suggested that fire regimes do indeed impact bird communities with important conservation implications. We detected the strongest changes in bird community structure, diversity, and density in response to the immediate impact of a fire event and post-fire regeneration processes, and weaker effects to cumulative fire regime history. Short fire season burning (Feb-April) appears to have significant influence on birds and vegetation structure, and these preliminary findings warrant further investigation. While our findings do not have alarming conservation implications, the effects of seasonality and frequency of fires are not negligible. We recommend against extensive burning practices over short periods to allow continuity in the landscape. We recommend greater forethought among conservation managers, bearing in mind both short-term and longterm habitat changes caused by burning and the consequent bird assemblages impacted. We advise focused research on species and functional groups of conservation interest.
Open Access
The burning questions about Hluhluwe : causes and consequences of a severe wildfire
(2009) Browne, Catherine; Bond, William J; Midgley, Jeremy J
The biophysical drivers of fire; ignition, fire weather conditions, fuel biomass, and flammability, differ in varying ecosystems. The rates of occurrence of these factors influence fire regimes. This study investigated the causes and consequences of a severe wildfire that swept through the Hluhluwe Game Reserve, KwaZulu-Natal South Africa in September 2008. This fire was an extreme event that seems only to have been possible due to the combination of circumstances that occurred in the days prior to and during the 14th/15th September 2008. The event was considered extreme because it burnt from savanna into thicket and forest patches, which is atypical of fires. The fire caused large structural change in tree demography, however, not much change in densities. The results of this study indicate that coupled weather conditions conducive to fire; namely low relative humidity, high temperatures and high wind speeds, occurred at the time of the fire. The synergistic influences of fire weather conditions and the state of available fuel caused this severe fire. The fire continued to bum for 48 hours before weather conditions relaxed and became less dangerous. These data suggest extreme fires such as the September 2008 event may be exploited by managers to reclaim former grasslands and savannas that have suffered the effects of bush encroachment and/or create open areas allowing grasslands to develop. However, severe fires run the risk of leading to the loss of forests and the biodiversity that they support. This study has identified the conditions likely to promote such severe fires.
Open Access
A comparison of the seed dispersal service offered by chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla)
(2005) Bunney, Katherine; Bond, William J
The handful of studies that have investigated chimpanzee and gorilla seed-dispersal identify these primates as important dispersal agents. These studies do not, however, make any measure of the 'quality' of the dispersal service offered by chimpanzees and gorillas. Determining 'quality' requires a measure of the dispersal distance and the microsite to which the seeds are dispersed. In this study, I report the first estimate of seed dispersal curves for chimpanzees and gorillas. Seed dispersal curves were produced by combining ape movement data with gut passage curves from literature. The derived dispersal distances for chimpanzees and gorillas are similar c. 7.7 km; this is surprisingly large when compared with other seed dispersal agents. This is likely due to a combination of foraging behaviour and gut physiology. At a species level, chimpanzees (Pan troglodytes) were shown to direct dispersal of Uapaca palidosa to favourable microsites even though gorillas (Gorilla gorilla) were responsible for moving a greater number of seeds. This study presents a novel method for the rapid derivation of dispersal curves and highlights the importance of incorporating species. level as well as community level studies to assess the quality of seed dispersal agents. It is my hope that the methods presented here be applied elsewhere so that the role of extant megaherbivores as seed dispersal agents be incorporated into future models that investigate forest dynamics.
Open Access
The competitive dynamics of two Protea shrubs growing in a mixed fynbos stand
(1996) Anderson, Pippin; Bond, William J
Patterns of segregation among Protea species were investigated. Interspecific competition was studied as a cause of these patterns. Two different methods were used for measuring competition in a mixed stand of P. lepidocarpodendron and P. corona/a growing on the Cape Peninsula. These were the plot-based averaging method, which considers stand density, and the nearest neighbour approach, which considers competitive impacts as a function of the immediate neighbouring plant. Both methods demonstrated negligible interspecific, and strong intraspecific competitive effects on fecundity. However strong interspecific competition appears to be occurring at earlier stages of the life cycle and may account for habitat segregation of P. coronata and P. lepidocarpodendron. P. lepidocarpodendron juveniles appear to outgrow and suppress P. coronata plants. Survivors of this interaction grow to full maturity, giving an apparent lack of species interactions when measured in terms of fecundity. The same results were demonstrated in a mixed stand of P. lepidocarpodendron and H. suaveolens, where H. suaveolens suppresses P. lepidocarpodendron. The study indicates the importance of shrub architecture in reducing species interactions, and juvenile phase properties in determining competitive displacement. Soil preferences and variable fire responses between the species were studied in an attempt to explain the restricted distribution of the stronger competitor, P. lepidocarpodendron at the study site. No conclusions were reached, but the restriction of this species from certain sites appears to be a fire response.
Open Access
The conservation genetics of the Clanwilliam cedar (Widdringtonia cedarbergensis)
(1995) Thomas, Janet Clare; Bond, William J
Widdringtonia cedarbergensis is an endangered conifer species in the western Cape, South Africa. This species is under serious threat of extinction and is being actively managed by Cape Nature Conservation in a small section of the Cedarberg mountains in an attempt to boost population numbers with a seedling replanting scheme and preservation of adults from fires. This study set out to determine levels of genetic diversity and fitness within and among populations of the Clanwilliam cedar for the following reasons: (1) to assess the level of genetic diversity; (2) to screen the seed source for the replanting programme; (3) to locate vigorous seedling sources for replanting and (4) to determine the effect of population size on genetic diversity and fitness. Starch gel electrophoresis was employed to assess levels of genetic variation within and among seven populations of W. cedarbergensis. W. nodiflora and W. schwarzii, two congeneric species, were incorporated into this section of the thesis as benchmarks against which to compare levels of genetic variation in W. cedarbergensis. The three species are different with regard to biology and distribution and predictions as to their population genetic structures were set up accordingly. A major difference in their biology is the resprouting behaviour of W. nodiflora in response to fire. Fitness components in populations of W. cedarbergensis were divided into reproductive and "ecological" traits, and seedling growth traits. Reproductive and ecological traits were measured in the field and seedling growth traits were obtained from a seedling growth experiment carried out in the glasshouse. Overall, 15 potential fitness traits were measured. The same seven populations were tested for differences in these fitness traits. This enabled an assessment of the seed source used for replanting, as well as alternative seed sources. All fitness variables were tested for a significant relationship with genetic variation measured as heterozygosity. The populations were rated according to population size, density and isolation and correlated with heterozygosity to determine whether there was any relationship. Seventeen enzyme loci were resolved for each species. Estimates of genetic diversity showed that W. cedarbergensis and W. schwarzii had low allelic variation. This was attributed to the effect of bottlenecks. High levels of inbreeding and population substructuring were found in W. cedarbergensis which suggested that trees were selfing possibly due to limited pollen movement between trees as a result of tree isolation and fine-scale fragmentation incurred by fires. Although allelic diversity in W. nodiflora was extremely high, high levels of inbreeding were found within populations which was attributed to selfing among resprouted ramets of the same genet. Tests for differences in fitness traits between populations revealed no population as the most consistently fit for reproductive and ecological traits for the fittest population, although two populations, DG and CPS, were consistently found to have the most vigorous seedlings. The replanting seed source, MB, showed adequately vigorous seedlings. Four out of fifteen measures of fitness were found to co-vary with heterozygosity. These were embryo abortion fraction, germination rate, shoot biomass and total biomass. Seeds and seedlings were more vulnerable to the effects of inbreeding than traits related to fecundity. These four traits, in tum, co-varied with other traits. The relationship between germination rate and seedling:parent ratio, in particular, indicated that genetic phenomena are impacting the demography of populations. Reductions in fitness occurred in several traits below 30% heterozygosity and became critical below 25% heterozygosity. 30% heterozygosity occurred below a population size scale of 4 (4000 individuals) and 25% heterozygosity occurred below a population size scale of 1 (250 individuals). Population size, therefore, seemed important in maintaining the genetic diversity of populations. The effect of population density on fitness was not effectively examined in this thesis and deserves further attention. The results of this thesis had several implications for conservation and active management. My recommendations were the following: (1) two populations, DG and CPS, are the best seed sources for the replanting programme and should supplement the current and most accessible seed source (MB); (2) Replanting should be aimed at boosting seedling:parent ratios in small populations such as WB and KK, as well as closing gaps between trees and clumps of trees as far as possible to facilitate pollen movement and therefore outcrossing; (3) the bottleneck is at a critical stage where adult tree survival is of profound importance in ensuring seedlings are outbred and every effort should be made to reduce mortality due to fire.
Open Access
Decomposition of grasses in a South African savanna
(2007) Masubelele, Mmoto Leonard; Bond, William J; Stock, WD
Tropical and subtropical grasslands ecologists have ignored decomposition, because of the important role it plays in nutrient cycling, in terms of understanding how grass litter gets to the soil during the dry season. The major flaw is in the methods employed to investigate decomposition. In the prairies and steppes, litterbag methods employed investigate decomposition when litter is on the soil, even though the importance of carryover from standing litter is recognized. Standing litter results in high carryover which slows down decomposition thereby hindering productivity in the next growing season. I therefore decided to investigate the processes of photodegradation and microbial decomposition, as well as carryover and also how these processes differ between mesic and semi-arid grasslands. Since previous studies suggest that tallgrass prairies experience more carryover and slower decomposition than shortgrass prairies, I therefore hypothesized in this thesis that mesic grasslands will experience carryover and slower decomposition than semi-arid grasslands.
Open Access
The demographic impacts of browsing on woody plants in savannas : from individual branches to whole populations
(2010) Moncrieff, Glenn R; Bond, William J
Browsing ungulates can potentially have drastic impacts on vegetation patterns. This is particularly true in African savannas where many large browsers persist at high densities. Most of the theory and models outlining mechanisms of impact on plants and predicting responses are framed in terms of biomass impacts and responses. However, for trees in African savannas, fitness is more closely linked to height than above ground biomass. I evaluate the demographic impacts of browsing, making explicit contrasts with impacts on biomass. The results highlight under- explored intrinsic aspects of plants and browsers that determine the degree of browser impact on plant demography, aspects that have been under-explored due to an emphasis on biomass responses, and provide novel methods to measure and evaluate large-scale browser impacts, which have proved difficult before.
Open Access
Does tree size matter? : giraffe influence on African savanna ecosystem properties
(2010) Soutschka, Nadine; Bond, William J
Trees alter the environment by affecting microclimate, soil properties and adding structural complexity to an otherwise single stratum system. It is known that the herbaceous understory of Savanna ecosystems is most productive at intermediate tree density, where the shade and nutrients from the tree are more influential than the competition with the tree. Tree density varies in savannas due to disturbances, such as fire and herbivory. Yet, disturbances do not only alter density but can affect entire tree structure. It has been found that chronic ungulate browsing influences whole tree size, as well as canopy size and shape. Giraffe in south of Kruger National Park browse heavily on Acacia nigrescens found on basalt-derived soils. I studied the influence of trees above the giraffe browse trap (>6 m) as opposed to trees in the browse trap (< 6 m) on savanna ecosystem properties. I compared grass biomass, soil moisture, soil organic matter, soil carbon and soil nitrogen beneath tall trees vs. short trees, as well as in the open vs. underneath trees. Ecosystem properties were estimated and compared between a giraffe impacted to a giraffe absent area. Giraffe impacted area overall yielded lower grass biomass, soil moisture, soil nitrogen and soil carbon. The consequence is a trophic cascade of events from increased giraffe decreasing fires and in tum increasing tree density and hence carbon sequestration. As giraffes replace a single large tree by several small trees, shows that their population size is key determinant of ecosystem properties.
Open Access
The ecological effects of grazing by the White Rhino (Ceratotherium simum simum) at a landscape scale
(2005) Waldram, Matthew; Bond, William J; Stock, WD
In this thesis I generated hypotheses concerning the top down effect of grazing ungulates on grass communities and fire behavior from work done within grazing exclosures in Hluhluwe iMfolozi Park. White Rhino had a large influence in controlling grass biomass in Hluhluwe, a high rainfall mesic savanna. Other smaller species of grazers could not replicate the effect of White Rhino when their grazing was removed. In Umfolozi, a semi-arid savanna, other species of grazer could replace the effect of White Rhino grazing and exert a controlling influence on grass biomass. Hence the relative importance of different species of grazers changed along a rainfall gradient. When examined at a larger spatial scale I found that the removal of White Rhino led to a detectable change in grass biomass and in the grazing behavior of other species in the area of the removal. The effect that herbivores exerted on the grass layer also had consequences for the movement of fire through the landscape by reducing fuel loads. Burnt areas were larger and less patchy in areas from which White Rhino had been removed in comparison to control areas. This effect was larger in Hluhluwe but still significant in Umfolozi. I suggest that both fire and grazing are in competition for the same resource, grass, and that each results in conditions favorable to the recurrence of that event (fire or grazing). This allows the system to switch between mammal and fire dominated states. Rainfall shifts the balance of this competition and in mesic savannas White Rhino appear to be the only animal capable of competing successfully with fire. This work has application for the management of ecosystems that are influenced by top down control and for the maintenance of heterogeneity in mesic savannas.
Open Access
Ecological significance of variation in Themeda triandra Forsk : a case of intra-specific divergence in life history strategies?
(2006) Janse van Rensburg, Susan; Bond, William J
Rangelands are a important resource for commercial, subsistence and game enterprises. However, their variabiligy poses a challenge for their effective management. This thesis was motivated by the need to contribute to a functional classification system for rangelands to guide managers in a way that adequately, yet simply, addresses ecological variation in South African rangelands. The approach was to investigate life history characteristics of rangelands at four sites representing climate extremes. These differed in the amount, and predictability, of rainfall and the incidence of frost. The approach was novel in that a single species, Themeda triandra, was used in an attempt to test predictions on sets of key traits associated with the different rainfall regimes. The underlying hypothesis was that density-dependent processes would be most important in mesic grasslands, requiring frequent defoliation by fire or grazing to maintain a productive Themeda sward. Climate variabiligy would be the most important determinant of grass growth in semi-arid grasslands. Vegetive and reproductive traits of Themeda would be expected to have diverged in response to these different selection pressures in the different populations. Observations were made on reproductive and vegetative traits in field populations in the four study areas and on plants grown from clonal material and seeds in a glasshouse to determine key axes of variation. Population responses to varying light and moisture levels were compared in a glasshouse experiment simulating the effects of different moisture regimes and competitive environments of source populations. Fecundity, seedbank characteristics, seedling numbers and population size structure were compared in field and glasshouse experiments. Production characteristics were compared in a clipping experiment conducted in the field. Results showed that different populations of Themeda exhibit divergent life history characteristics associated with different rainfall and temperature regimes. These differences are evident in the morphology, allocation patterns, leaf traits, germination biology and phenology of populations. The ecological significance and implications of trait divergence were clearly evident in demographic characteristics of the populations, in experimental responses to watering and shading, and in divergent growth responses to clipping experiments. The results were consistent with classic life history theory. In low, unpredictable rainfall regions, populations have evolved r-selected strategies whereas populations from high and predictable rainfall are characterized by K-selected traits. However the incidence of frost is also a factor influencing growth form divergence.
Open Access
The effect of grassland shifts on the avifauna of a South African savanna
(2005) Krook, Kristine; Bond, William J; Hockey, Phil A R
Two distinct grassland types occur within Hluhluwe-iMfolozi Park (HiP); short stoloniferous grazing lawns and tall, tussock-like 'bunch' grasslands. Grazing lawns are maintained by grazing mammals, among which White Rhinoceros Ceratotherium simum is of major importance. By contrast, tall bunch grasslands are promoted by frequent burning. The extent of each grassland type within the park is highly dynamic and can be altered by changes in mammal numbers and/or fire regimes. Such changes may have cascading consequences on other components of the ecosystem if they show specialisation towards one or other grassland state. This study compared avifaunal assemblages of grazing lawns and bunch grasslands to assess how bird species of the park might change with shifts in the grassland mosaic. Distinct bird communities were associated with each grassland type, including several specialists, and bird distribution as linked to vegetation structure, rather than floristics.
Open Access
The effect of light-limitation on spinescent structural defence and its implications on resistance to herbivory in the shade
(2013) Adams, Ismat; Bond, William J; Dominique, Tristan-Charles
Plants can resist herbivore pressure through structural or chemical defence or both. The ultimate goal of defence against herbivory is to reduce the amount of damage to biomass, but more specifically to protect against damage to meristematic tissue. The defences employed depend on the type of herbivory experienced, which is contingent on the herbivore and its mouthparts. This investigation was concerned with structural defence presented by spines. This type of defence protects against mammalian herbivores such as browsers. Spiny species do not dominate in low-light deep forest environments. Therefore the aim of this study was to determine the constraints on spines as a defence strategy under shaded conditions by assessing the effect of reduced light on spine efficiency. Spine efficiency was defined as the amount of defence afforded the plant given the resources available. Spines require carbon to be built and need to be arranged properly in order to present an adequate defence. Thus two non-mutually exclusive hypotheses were proposed: Light limitation reduces the ability of spines to present an adequate defence against browsers due to the architectural strategy employed and/or its influence on carbon gain. The spinescent plant chosen for study was Carissa macrocarpa (Ecklon) A.DC. Light condition of plants was determined using hemispherical photography. Spine efficiency of sun and shade plants was determined using a bite test and was evaluated using architectural and physiological analyses. Architectural analysis involved identifying levels of organisation within the plant across ontogeny and indentifying sun and shade growth strategies. Physiological analysis involved determining carbon gain of sun and shade individuals using gas-exchange measurements, as well as the measurement of biomass allocation by harvesting and oven drying different plant parts. Results showed that biomass allocation patterns of C.macrocarpa did not change in sun and shade but total biomass increased from shade to sunlit conditions. Architectural analysis revealed that in the sun the plant adopted a stout dense structure with high spine efficiency, while in the shade it was more elongated with lower spine efficiency. Therefore C.macrocarpa adapts to the light environment by adopting either the shade or the sun architectural strategy. The way in which this works is that light affects carbon gain, which either increases or decreases biomass and in turn leads the plant to adopt the sun or shade architectural strategy. The architectural strategy affects spine efficiency such that plants in the sun have higher spine efficiency than plants in the shade. Thus, spinescent plants do not do well in light limited environments because they are architecturally constrained to elongate in such conditions. This constraint would put them at higher risk of browser damage than plants in light-sufficient conditions, ultimately decreasing their fitness. If the patterns observed in C. macrocarpa prove to be general, then it helps to explain why spiny plants are more commonly found in open, sunlit environments than in deep shade.
Open Access
The effects of different burning regimes on grassland phytodiversity
(2000) Uys, Roger Gregory; Bond, William J; Everson, Terry
The southern African grassland biome has a diverse flora including many rare and endangered species deserving a high conservation priority. With less than two percent currently conserved, proper management of this biome is crucial to maintaining its diversity. Fire, in particular, is important and although there has been extensive research into the grazeable component of the sward, little attention has been paid to the effects of burning on forbs or the overall phytodiversity. The aim of this study was examine the effects of different burning regimes on grassland phytodiversity. To do this, three long-term burning trials, widely located across the biome and consisting of a range of treatments randomly allocated to plots in replicated block designs, were sampled. Manipulated according to the frequency and season of burn, with fire protection controls, diversity was sampled at different scales across a range of treatments. In addition, a range of sward attributes was sampled in an attempt to explain diversity patterns.
Open Access
Effects of frequent burning on grass-grazer interactions in a mesic savanna
(2003) Archibald, Sally; Bond, William J
Fires are often used as a management tool in both game reserves and rangelands to manipulate food availability for grazing animals. After fires, large areas of the landscape are quickly covered with nutritious new regrowth, which grazers move into and utilise as a food resource. The effect of this change in animal grazing patterns on the grass communities is not yet well understood. Certain grass communities depend on heavy, continuous grazing for their persistence: they are out-competed by taller-growing species in the absence of grazing. Conversely, the taller-growing species die out under heavy grazing. Thus, in many savanna and grassland ecosystems, the grass community present in an area depends on how frequently and how intensively the area is grazed. Every year, fires in these systems are altering the distribution of grazing in space and time, by altering the proportions and distributions of short, palatable grass. In my MSc I present data describing how fire alters grazing patterns, and I show how this can result in the disappearance of intensively grazed patches in the landscape. I also use a model to illustrate how this effect might be mediated by rainfall and grazer density, and by different fire regimes. I investigate long-term consequences of this process on the distributions of alternative grassland states in Hluhluwe Umfolozi Park and show that lawn-grass-dominated areas are associated with a less-frequent fire regime. Thus, although large fires provide high-quality grazing in the short term, in the long term they could be limiting the amount of grazing in an area, because they prevent the initiation and spread of grazing-tolerant lawn-grasslands, which can support high grazer numbers and a high diversity of grazers.
Open Access
The effects of habitat change on bird diversity and community structure in a mesic Savanna landscape
(2000) Skowno, Andrew L; Bond, William J
Many conservation areas are effectively islands in a sea of agricultural or urban development in which natural disturbance regimes have been altered or limited. Managers of such areas often need to artificially maintain disturbance regimes in order to control ecosystem processes such as vegetation succession (Richards ef al. 1999). The relationship between disturbance, succession, vegetation change and habitat heterogeneity is crucial to managers because together they can influence biodiversity. For example, a break in the disturbance regime could promote woody plant encroachment. Though this might lead to an increase in habitat heterogeneity, the effects on biodiversity could be positive or negative. Habitat heterogeneity, in this context, can refer to vegetation variability or complexity at a range of spatial scales.
Open Access
The effects of population size on agathosma collina (rutaceae) and its conservation implications
(1997) Jeffery, Douglas John; Bond, William J
Agathosma collina is a relatively typical widespread, outcrossing shrub species, endemic to the coastal vegetation between the Agulhas Plain and Heidelberg in the Southern Cape. Although not a rare species, its endemism makes it a suitable candidate for a study of population size effects and the conservation implications for similar outbreeding species. Theory predicts that we should expect demographic problems to arise, for ecological or genetic reasons, when population size becomes too small. Initially, therefore, a study was carried out to assess if there was any discernible effect of population size on the population structure. As predicted by theory, the demographic status of small populations was worse than larger ones, measured in terms of seedling recruitment with small populations having relatively fewer seedlings than large. This effect was quite small and far from dramatic. Demographic aspects such as length of flowering, predation and seed set per capsule in different sized populations were studied. Inflorescence predation rates were found to increase to a peak during the peak flowering time of A. collina and medium sized populations showed the highest predation rate while lower predation percentages in large and small populations were probably a result of predator satiation (large populations) and crypsis (small populations). Seed set per capsule varied from mostly one seed per capsule in small and medium populations to two seeds per capsule in large populations in 1988. However, 1989 data showed an almost identical spread in the numbers of seeds per capsule in the different sized populations. None of the above results seemed sufficiently consistent over time or population size to be of importance regarding population structure differences between different sized populations. In order to obtain an indication of the amount of genetic exchange between populations, gene flow studies were carried out using pollen dispersal and seed dispersal. Although direct estimates of gene flow proved difficult to obtain, these studies indicated that gene flow was limited. Neighbourhood area resulting from estimates of pollen dispersal distances was only 2.12m² compared to ballistic seed dispersal neighbourhood area of 3.05m2, while that resulting from possible myrrnecochorous seed dispersal was 41.97m². Neighbourhood size proved to be heavily dependent on population density and was very specific to a population and care should be taken when extrapolating results to different populations or plant species. Cross pollination experiments were carried out to attempt to identify an optimal outbreeding distance. Although pollen from 7km away produced the highest capsule set of all the treatments this could have resulted from heterosis. The species is self-compatible but geitonogamous matings produced lower capsule set than outcrossed matings. Of the pollination distances which could be expected under natural circumstances the nearest neighbour crosses produced the most capsules. This corroborates the results of the gene flow studies which indicate that gene flow may be over very short distances. Inbreeding depression in small populations of A. collina seems likely since neighbourhood size is very small resulting in a low probability of pollen transfer from any great distance and since A. collina is self-compatible allowing geitonogamous matings in very small populations. Electrophoresis was then attempted to confirm the above results from a genetic aspect. Unfortunately this was not successful. This study provided some empirical support for theoretical predictions of the effects of population size on plants but suggests that the main problems may be genetic rather than ecological. The magnitude of the population size effects were small and then only in very small populations and are therefore probably only of concern under rare circumstances. Bibliography: pages 100-111.
Open Access
Effects of repeated fire on the savanna / forest boundary
(2012) Wills, Cameron; Bond, William J
Savanna and forest/thicket can exist as alternate stable states, among others, determined by fire ecology feedbacks. Bush encroachment has become an ever-increasing trend converting grassland and savanna biomes to forest/thicket. A severe firestorm occurred in the Hluhluwe-Umfolozi Game Reserve in north-eastern South Africa in September 2008. The fire penetrated closed thicket areas and opened up the landscape. The main aim of this study was to investigate whether repeated fire, following such an extreme fire event, could trigger a biome or regime shift; in this case from forest/thicket to savanna. Fire spread is determined by grassy fuel loads, primarily grass biomass in these ecosystems, and the fire weather at the time of the fire. Grass biomass was found to have significantly accumulated (p=0.0002) in the thicket areas in just three growing seasons since 2008, which allowed fires to burn the area again in 2012. In the 2012 fires, fire intensity, measured by char height on woody stems increased in relation to the increase in grass biomass up to a point after which increasing grass biomass had no effect. The 2012 fires were able to penetrate areas opened up by the 2003 firestorm despite high canopy cover created by tree resprouting. Tree mortality was cumulative with repeated burning (21% mortality post 2008 increasing to 47% mortality in 2012). This was linked to the vigour of post 2008 resprouting with much higher mortality of trees in 2012 that had few (<4) resprouting stems. Thus, this study indicates that, a fire regime of sufficient frequency and intensities could potentially cause a vegetation state shift from forest/thicket to savanna. Therefore, this may provide management options for wildlife parks and protected areas.
Open Access
Exploring the potential impacts of global change on the woody component of of South African savannas
(2014) Stevens, Nicola; Bond, William J; Archibald, Sally; Erasmus, B F N
Land cover change, elevated CO2 and associated climate change are driving changes in ecosystem structure and function across the world. These changes are best described and understood in northern hemisphere temperate systems. Studies, models and reports from these regions are therefore often used as the basis for understanding, predicting and managing change in other biomes, including African savannas. In savannas elevated CO2 has been implicated in driving an increase in woody cover, however attribution to global drivers is often confounded by land - use management which is also implicated as a driver of tree cover increases. Climate change is predicted to directly drive species range shifts in savannas, however several important gaps exist in the literature which prevent the development of a clear predictive framework to describe these changes. Whilst climate is often assumed to be the primary factor underlying savanna tree distribution this has not been demonstrated for savanna tree species. Additionally the determinants of individual savanna species distributions have seldom been investigated. The few existing studies are correlative species distribution models based on adult plant distribution and underlain by the assumption that climate sets plant distribution. Although a demographic approach is very important in understanding tree: grass coexistence in savannas, this approach is seldom used in understanding species distributions. The aim of this study was therefore to assess how global change might influence South African savannas. We place emphasis on the role of elevated CO2 and climate change. I first report on an analysis of historical changes in woody plant abundance at large spatial scales to assess the magnitude of tree cover change in South African savannas. I attempt to untangle the role of local drivers and global drivers in causing tree cover changes from 1940-2009. I then report on studies aimed at determining what limits the distribution range of two common savanna trees Acacia nigrescens and Colophospermum mopane, at three critical demographic stages. Here I used a series of field studies, transplant experiments, lab experiments and modelling to determine the critical drivers of the distribution of these plants so as to understand how savanna plants might respond to changing climates.