Browsing by Subject "Plant Ecology"
Now showing 1 - 20 of 37
Results Per Page
Sort Options
- ItemOpen AccessAcacia karoo and its effects on the succession of dune forests in the Eastern Shores, Kwazulu-Natal(1997) Maddams, C J; Midgley, Jeremy JAcacia karoo arises following disturbance in the Dune Forests of Northern KwaZulu-Natal. Patches of A. karoo which had arisen as a result of the abandonment of fields by subsistence farmers in a dune forest matrix were studied. Previous authors have considered A. karoo to be part of a successional mechanism which leads towards a climax forest canopy (Mentis and Ellery, 1994; von Maltitz et al, 1996). This succession is questioned as a number of the conclusions of these authors were not verified by my own work. The present paradigm holds that the A. karoo is even aged and will senesce after 25 years, while recruitment does not occur in the patches. I find no evidence of canopy senescence and microhabitats within which recruitment of A. karoo occurred were identified. Further the introduction of other species in the patches is facilitated not through the A. karoo canopy but was shown to be facilitated by the presence of multi-stemmed canopy-trees of other species which had not been removed by the farmers. However, this introduction of other species into the patches was still limited, with certain species not found in even the very oldest patches (37 years old). No discernible chronosequence could be identified when comparing patches of different age, which casts doubt on the belief that these patches will eventually revert to climax dune forest.
- ItemOpen AccessAn alternative hypothesis for explaining anomalies in the fine scale distribution patterns of Colophospermum mopane : Are shrub and tree forms genetically distinct ?(2004) Hempson, Gareth; Verboom, George Anthony; February, Edmund CAbiotic and top down control hypotheses do not adequately explain the fine scale distribution patterns of shrub and tree Colophospermum mopane (Caesalpinioideae). Genetic distinctiveness between growth forms is investigated as an alternative hypothesis. Tree and shrub C. mopane from the riparian and inland savanna zones were sampled at four sites in the northern Kruger National Park. Molecular DNA sequences were obtained for four plastid and one nuclear region, and the inter simple sequence repeat (ISSR) technique used to fingerprint individuals. Very low levels of sequence divergence were observed. The ISSR technique revealed no genetic structure between plants when grouped by growth form or by habitat in an analysis of molecular variance (AMOV A). Soil profile and xylem pressure potential data also did not explain the distribution of growth forms. A principle component analysis and a discriminant analysis of five leaf and branching characters identified a significant difference in the shape of shrub and tree C. mopane leaves. It is concluded that the C. mopane growth forms are not genetically distinct and that their fine scale distributions may be due to top down controls such as large mammals and fire.
- ItemOpen AccessAnalysis of cooking shelters in Paulshoek and the effects of harvesting on natural population of Polymita albiflora(2001) Evans, AdrianCooking shelters are found at all the 28 stocking posts in the Paulshoek village. They are round circular structures with walls made from plant materials to provide insulation. Food is prepared and cooked inside the shelters, providing protection from the elements mainly cold and wind. These shelters also serve as social meeting and entertainment venues. Due to the poverty of the area alternatives to constructing cooking shelters are too expensive. Thus the sustainable use of plants used to construct cooking shelters will prevent both species loss and economic expense. There are three types of cooking shelters: full canopy, half canopy and open shelters. 18 shelters including all three types were measured. The volume of plant material calculated and the species composition estimated. The full canopy shelters are generally the largest and well built. These shelters are used in winter and need to be sturdy to withstand the elements and last for a number of years [ 15 years]. Open shelters are less permanent and built more often. As they are not required to be as sturdy a higher proportion of less desirable but more available plant species were used. However in all types the major species used was Polymita albiflora. This species is desirable as a construction material as it packs tightly and the dense foliage supply insulation. Although most shelters were made up of more than one species, only Galenia africana and Euphorbia mauritanica are used consistently in small quantities. Both these species are common and widespread and the amounts being harvested are insufficient to cause any negative effects on the species population dynamics. However P. albiflora is restricted to quartzite slopes or shallow pockets in granite in Northern Namaqualand. Thus harvesting could be destructive to the natural populations. A cooking shelter was dismantled and a size class distribution of P. albiflora used for construction calculated. Transects were run in four separate P. albiflora populations and a size class distribution for all sites calculated. The majority of plants used to build cooking shelters are in the size class 2 0.005-0.015 m³. The size class distribution of natural populations of P. albiflora showed the smallest size class [ <0.005 m³] to have the highest frequency. There was a decrease in the frequency for the remaining size classes. Harvesting from those size classes could cause this reduction. However it is more likely that high mortality during the establishment phase causes this decrease. A regression between volume and number of capsules showed that the plants being removed are not the only reproductively active plants. Small plants [0.001m³] are capable of producing a few capsules each containing more than 100 seeds. Also the few large plants produce up to 300 capsules per plant. P. albiflora is also used as a poor quality fuelwood for baking. Baking occurs approximately once a week. However only populations within the immediate vicinity are threatened by overexploitation due to the efforts required for fetching these plants far outweighs their use as a fuelwoood. This study indicates that P. albiflora is being harvested well within the sustainable limit. P. albiflora grow in dense stand and removal of plants within these will most likely be replaced by small plants or seeds of this species.
- ItemOpen AccessApplications of air photo interpretation techniques in the Langebaan area, S. Africa(1977) Jarman, Margaret LouiseA fairly extensive introduction to the theory behind the use of remote sensing products for vegetation survey work is given. This includes the particular benefits of colour air photographs, the relationship between scale of survey and scale of mapping and remote sensing products, and more information about the 'semi-detailed' level of survey operation. The latter is the category of survey which the vegetation survey of the Langebaan area, South Africa, falls into. The importance of the timing of photography in the usefulness of air photographs in vegetation survey work is also emphasised. The particular example of the semi-detailed survey of the vegetation of the Langebaan Area using colour serial photographs of Job 195 was then used in describing the Phases of operation involved. Phase I is essentially the planning stage, Phase II the operational stage, divided into 5 separate sections of operation, and Phase III involves the final mapping and report compilation. Reference to the results is given, as is a short discussion on the merits of air photographic techniques.
- ItemOpen AccessA biogeographic analysis of the seaweed flora of the west coast of southern Africa, from Lüderitz to Cape Agulhas(1996) Anderson, Pippin; Bolton, John JA biogeographic analysis of the seaweed flora of the area from Lüderitz to Cape Agulhas was undertaken. Biogeographic patterns were reviewed across 15 geographic regions. A TWINSPAN analysis showed a clear division of the area into two separate species communities. These two communities are the Benguela province, and the western overlap region between the Benguela and Agulhas provinces. The western overlap region was found to be the most diverse. Diversity was shown to decrease with a decrease in latitude. Patterns in endemism among the brown seaweeds follow this trend. In contrast to this, red and green endemics increase with a decrease in latitude. Shore distribution patterns were reviewed, and demonstrated an increase in diversity with a progression down the shore. Among the red seaweeds, this increase, with greater depth was considerable. Species distribution patterns in both shore and shore pool zones were found to follow the same pattern, except for a drop in species in the subtidal fringe pools, which this study concluded was a meaningless concept. These results have been reviewed in relation to present conservation areas along this shore, and future recommendations were made for location of sites for the conservation of seaweed. These were the formation of reserves between Lüderitz and Port Nolloth, between Yzerfontein and Melkbosstrand, and from Scarborough to Cape Hangklip. In some of these areas existing reserves need to make policy adjustments while in others reserves need to be established for the conservation of both diversity and uniqueness of seaweed species.
- ItemOpen AccessBranch junction constriction and hydraulic limitation in two species in the Cape Proteaceae : a mechanism explaining the trade-off between longevity and degree of ramification in the Cape Proteaceae(2004) Slingsby, Jasper; February, Edmund CThe purpose of this study was to establish if hydraulic limitation and branch junction constriction explain the trade-off between longevity and increased ramification in species of the Cape Proteaceae. This was done by establishing if branch nodes are regions of greater resistance to xylem sap flow in the study species; attempting to quantify nodal resistance for a plant as a whole; and by determining if there is any evidence for hydraulic limitation or compensation in Leucadendron laureolum (Lam.) Fourc. and Leucospermum oleifolium (Berg.) R. Br. individuals. The extent of branch junction constriction was established by comparing the ratio of the xylem sap flow rates of adjacent branch segments of equal length. The ratio of a proximal intemodal to a distal intemodal segment was compared against the ratio of a proximal intemodal segment to a distal node. Branch nodes were found to create significant resistance to xylem sap flow rates in both species (p < .005 for L. laureolum, and p < .05 for L. oleifolium). Total nodal resistance in L. laureolum was modelled as equivalent extra branch length, and as loss of sap flow rate. Equivalent extra branch length increased exponentially with increased branching order, and increased much faster in more ramified plants. Loss of flow rate increased linearly with increased branching order and was unaffected by differences in ramification. This was because the nodes in more ramified plants had smaller basal sapwood areas, and the resistance imposed by nodes decreased with decreased basal sapwood area. As more ramified plants tend to branch earlier, and/or with greater temporal frequency, they will still accumulate greater nodal hydraulic resistance faster than less ramified plants, and thus may be limited to smaller size and younger maximum age. Branch specific conductivities (kₛ), leaf specific conductivities (LSC) and transpiration rates were investigated in higher (11 nodes) and lower (10 nodes) order branches of L. laureolum. All were found to be significantly lower in higher order branches (11 nodes), which can be related to greater nodal resistance to sap flow limiting the amount of water available to the area of developing xylem, causing the vessels to have narrower diameters. There was no significant difference in leaf to sapwood area ratios (Aₗ/As) between the two branching orders. Previous studies have suggested that changes in Aₗ/Aₛratios occur to compensate for hydraulic limitation. Reductions in Aₗ/Aₛin plants with greater hydraulic restriction allow LSC values, and thus transpiration rates, to stay as high as plants with less hydraulic restriction. As there was no such compensation, LSC values, and thus transpiration rates, were much lower in branches of higher order. Transpiration rate is a good indication of rates of photosynthesis. Lower transpiration rate in higher order branches is thus a good indication of hydraulic limitation on rates of photosynthesis with increased branching order. Hydraulic resistance in nodes thus imposes an important limitation on size and/or age of L. laureolum individuals, and explains the apparent trade-off between longevity and degree of ramification in the Cape Proteaceae.
- ItemOpen AccessCan nitrogen isotopes be used to detect the effects of burning on nitrogen cycling? : a study on Pinus resinosa in Minnesota, USA(2004) Ballantyne, Fiona; Craine, Joseph MPinus resinosa stands in Minnesota, USA were surveyed and fine and coarse roots from stands were analysed for nitrogen isotopes in an attempt to determine if fire effects nitrogen cycling. Fire increased the % nitrogen of coarse roots and changed the ratio of C: N. It may also decrease the depth of the A-horizon. It did not however affect the nitrogen isotope ratio of stands. Isotope values of coarse roots were affected by the depth of the A-horizon, which is consistent with other studies. A correlation between the ¹⁵N values of coarse and fine roots was found however fine roots were enriched proportionately to coarse roots. This suggests that they may be using a more enriched source of nitrogen such as ammonia although this is not conclusive. A PCA performed on site variables and isotope values determined the two major axes for sites were bum history and nitrogen cycle openness but that burning did not affect the second axis. A stepwise backwards regression showed the best predictors of a roots isotope value were, in decreasing order, the isotope value of coarse roots for fine roots and vice versa, the % nitrogen of roots and the age of stands. Coarse and fine roots showed opposite trends with % nitrogen. As the % nitrogen increased, fine roots were more ¹⁵N enriched, again pointing to different nitrogen sources for coarse and fine roots. Older stands do appear to have a more open nitrogen cycle, as roots were ¹⁵N enriched in these stands. The availability of different forms of nitrogen may influence the ¹⁵N signal of roots especially if fine roots are preferentially using ammonia. Therefore, it is not possible to say if isotope values are due to stand openness, or due to different resources use by roots.
- ItemOpen AccessComparative water relations of indigenous and invasive Australian Proteaceae in fynbos(1998) Bergh, Nicola G; Midgley, Jeremy JWater-use efficiency (WUE) as reflected in the leaf carbon stable isotope ratio was compared between co-occurring indigenous fynbos proteoids and invasive Australian hakeas H. sericea, H. gibbosa and H. suaveolens. At the driest site, H. suaveolens was slightly more WUE than several co-occurring proteoids; there was no significant difference between hakeas and proteas at the other sites. Transpiration rates of shoots and of whole trees were compared between Hakea sericea and Protea repens growing on Stellenboschberg northeast of Cape Town. Both measurements showed no real difference between the species and it is concluded that differences in water relations are not responsible for the highly competitive growth rates of hakeas in fynbos. It is hypothesised that hakeas may be able to vegetatively outcompete proteoids as a consequence of monopodial architecture and some ability to prevent shade-limitation of photosynthesis. A rough estimate of water loss due to transpiration and interception by H. sericea stands indicates that this species may have a significant effect on catchment water loss relative to open-canopy proteoid fynbos. This effect would be due not to transpiration rates of individual trees but to consistently high densities of mature hakea stands.
- ItemOpen AccessA comparison of the seed dispersal service offered by chimpanzees (Pan troglodytes) and gorillas (Gorilla gorilla)(2005) Bunney, Katherine; Bond, William JThe 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.
- ItemOpen AccessThe competitive dynamics of two Protea shrubs growing in a mixed fynbos stand(1996) Anderson, Pippin; Bond, William JPatterns 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.
- ItemOpen AccessThe demography of Acacia stands on the Hluhluwe-Umfolozi Game Reserve(2003) Mader, André Derek; Midgley, Jeremy JLittle is known about the demographics of the genus Acacia in Africa, despite its prominence and the economic and environmental importance of this group. The demographics and species composition of stands of four different Acacia species was investigated in the Hluhluwe-Umfolozi Game Reserve in northern Kwazulu-Natal in order to determine whether stands were self-replacing, or whether other Acacia species were invading them. Soil and stand density as well as the density and composition of the grass layer were investigated in order to determine whether any of these affected Acacia demography and species composition. In three out of four cases, the species whose large size class dominated the stand (known as the "stand species") was found to have a strongly bimodal size class distribution, skewed primarily towards the large size class and secondarily towards the small size class. Other Acacia species in the stands, with few or no large individuals present ("nonstand species") tended to have unimodal size class distributions, skewed primarily towards the small size class and secondarily towards the medium size class. Based on the proportion of small to large individuals, non-stand species are more likely to increase in overall numbers in future, suggesting that the species composition of the stands may be in a state of flux. Few relationships were found between stand density, grass density, grass composition and numbers of small and medium acacias. This could be as a result of differentiation between Acacia species, meaning that they cannot be analysed collectively. Furthermore, sample sizes of individual species may have been too small to analyse individually. Alternatively, it could mean that none of these factors have a significant effect on one another and that other explanations need to be found for the demographics of this genus.
- ItemOpen AccessDeterminants of pattern in fynbos vegetation: physical site factors, disturbance regime, species attributes and temporal change(1998) Privett, Sean David John; Cowling, Richard MThis study set out to explore the patterns and determinants of contemporary species distribution in a fynbos landscape based on information on 1) physical habitat characteristics, 2) past disturbance regime, 3) intrinsic properties of individual species and 4) temporal change in communities. The body of the thesis is divided into four parts covering each of these aspects individually. Each chapter has been written up as an individual paper and thus includes some repetition as well as cross-referencing. Each chapter includes a detailed rationale for the study in the introduction, as well as methods, results and a discussion of the findings. Chapter 2 describes the patterns of vegetation units in the Cape of Good Hope Nature Reserve and provides a quantitative assessment of the importance of physical site features as determinants of these units. These results are compared with the vegetation classifications of the area derived in earlier studies by Taylor (1984b) and Cowling et al. (1996a). Chapter 3 is the first study in fynbos to partition the variance in vegetation pattern into that explained by physical site factors and that explained by the recent disturbance regime. The study explores the role of 30 years of fire and alien plant infestations in influencing species distribution at the community and landscape scale. While a number of studies have explored spatial determinants of diversity and pattern in fynbos, temporal change within communities has been neglected. High levels of species turnover through time, as a result of colonisation and extinction, could be an important component of diversity at the landscape scale. Hence Chapters 4 and 5 both explore aspects of temporal dynamics in fynbos. The objectives of Chapter 4 are to determine the stability of populations at the landscape scale over a 30-year period and to establish the determinants of local extinction of species. Information on the stability of species over time and the attributes which enable species to persist or go extinct are important for understanding potential impacts of management practices as well as the importance of temporal dynamics in influencing spatial patterns. This is especially appropriate in the context of the results of Chapter 3, which suggest that a high proportion of the variance in species composition is unexplained despite the inclusion of physical factors, and past disturbance regime. The fifth chapter provides a descriptive account of the change in vegetation composition over a 30-year period. It is recommended that those readers not familiar with the fynbos system read it as it provides an overview of a variety of aspects of fynbos dynamics. Its objective is to show how the various components of the disturbance regime, as well as fluctuations in abundance of overstorey Proteaceae and their resultant competitive effects can influence community composition. The final chapter is a general discussion that summarises the major findings of the study.
- ItemOpen AccessThe ecology of rodent pollination in Liparia parva (Fabaceae)(2005) Letten, Andrew; Midgley, Jeremy JThe evolution of non-flying mammal pollination has given rise to a broad suite of adaptive plant traits including dull coloured and geoflorous flowers, copious sucrose rich nectar, nocturnal anthesis and nectar production and a musky odour. The Fynbos endemic, Liparia parva (Fabaceae) has been recognised to exhibit several of these traits. Based on this observation, field studies were carried out on the Cape Peninsula, South Africa, to investigate the prediction that L. parva is rodent pollinated. Several lines of evidence indicate that flowers of L. parva are visited by rodents. These include; the presence of Liparia pollen in the faeces of live trapped rodents; observations of captive rodents selectively foraging at flowers of L. parva; floral debris underneath L. parva plants; and video footage captured of a rodent visiting L. parva flowers in the field. However, a strong plant-pollinator relationship was apparent only in the Cape spiny mouse, Acomys subspinosis. Captive Acomys foraged non-destructively at flowers of L. parva, becoming visibly dusted with pollen. The exclusion of rodents from flowers resulted in a significant reduction in seedpod set, indicating rodents do contribute to pollination success in L. parva. Additional evidence that L. parva is adapted to pollination by rodents includes nocturnal floral anthesis and large amounts of total nectar in inflorescences. The findings of this study provide substantial evidence for rodent pollination in L. parva and thus represent the first report of rodent pollination in a legume.
- ItemOpen AccessThe effects of cloud moisture on Restions, Ericas and Proteas in the Cape Floristic region(2012) Gibson, Myfannwyn; West, AdamRecent studies on the interception and utilization of occult precipitation (fog, cloud-borne mist and dew) have revealed that the direct wetting of foliage provides a water subsidy to plants of various ecosystem types. In this study, we investigate the presence of foliar uptake, and the effects of misting on the plant water potential of species representing diverse functional types, namely ericoids, proteoids and restioids in Fynbos species occurring within the Cape Fold mist belt. In this study, foliar uptake after 180-min submergence in distilled water was demonstrated by five of the seven species investigated. These species included all the restioids and ericoids investigated in this study. By contrast, the proteoids L. conocarpodendron and L. laureolum were found to show no significant amount of foliar uptake or increased leaf water content (%). There was an increase in the average, normalized leaf water content in individuals subjected to misting treatments in both proteoids, L. laureolum and L. conocarpodendron. Similarly, there was also an overall increase in plant water status, as shown by the increased water potential in individuals that were subjected to the misting treatment. It was found that control individuals showed a decrease in plant water potential (i.e. lost water) during the day, as can be expected when soil water is not replenished. All species showed significant stomatal conductance, during both night and day. Results indicate that misting events have a significant effect on the overall plant water status in all functional types and the presence of foliar uptake in both ericoids and restioids; thus indicating that cloud events may have an important effect on the vulnerability of these species to drought, under the precepts of global climate change.
- ItemOpen AccessEffects of repeated fire on the savanna / forest boundary(2012) Wills, Cameron; Bond, William JSavanna 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.
- ItemOpen AccessThe general continuum model for structured populations, with two case studies in plant ecology(1994) Laurie, Henri De Guise; Reddy, B Daya; Cowling, Richard MThe broad aim of this thesis is to investigate the formulation and usefulness of a very general model for plant population dynamics. In chapter 1, the goal of generality is discussed, particularly in the light of the lack of interaction between field and experimental population studies on the one hand and theoretical population dynamics on the other hand. A distinction is ma.de between descriptive and axiomatic theories, and it is suggested that they serve different purposes. The advantages of a. rigorous framework are pointed out and the basic elements of the continuum approach are introduced. In chapter 2, the model is proposed, the existence and uniqueness of solutions to its equations is proved, and an algorithm for numerically -approximating transient solutions is discussed. The question of generality is addressed in two places, and it is argued that the basic framework presented here is in principle adequate to model the processes of plant population dynamics in full detail, though the existence proof cannot to accommodate all possible models. In particular, models with time lags are excluded. Further limitations of the existence proof ill terms of constitutive relations are pointed out. In consequence, the theory here presented does not fully exploit the possibilities for generality inherent in the basic equations. In chapter 3, the question of what data would allow identification of factors determining somatic growth and mortality is investigated computationally. It is shown that using only the average size is insufficient. A class of models which includes all possible combinations of three types of size dependence in somatic growth and mortality is formulated. Qualitative parameter estimation for the various models yields size distributions that can be classified into the following biologically meaningful groups: group (i) has no models that use dependence on relative size; group (ii) has all the models in which somatic growth depends on relative size group (iii) has the models where only mortality depends on relative size. Thus it appears that size distribution may be used to distinguish various forms of size dependence in somatic growth and mortality. In chapter 4, a lottery model criterion for coexistence of plants with disjoint generations is developed, which is shown to require relative density dependence. Computer simulations aiming to initiate the use of exploratory calculations in studies of coexisting serotinous proteoids in fynbos indicate that the aspect of plant population dynamics most sensitive to density dependence is seed production, then somatic growth, while mortality is least sensitive to density dependence.
- ItemOpen AccessA hierarchical phytosociology of the Greater Zandvlei area(2006) Szöke, Tim; Hoffmann, TimmThe Zandvlei Nature Reserve is to be expanded to form the Greater Zandvlei Estuary Nature Reserve and will encorporate a much greater area (250 more hectares). The Greater Zandvlei area was classified by means of Zurich Montpellier (Braun Blanquet) procedures. 60 relevés were used to create a hierarchical phytosociology using TWINSPAN and DCA-ordination analyses. 3 major community types were identified, which were then subdivided to reveal 6 community types important to future management: Moist Grassland/Wetland Disturbed Parkland, Dune Asteraceous Fynbos, Thicket/Shrub Mosaic, Rhus-Euclea Dune Thicket and Homogenous Patches. CCA-ordination indicates that the abiotic soil factors sampled (texture composition, pH, carbon content and visible disturbance) explain very little of the variation between the communities (r²=0.002). This phytosociology is provided as a tool for future research in the area, and it is hoped to assist in future management decisions regarding the newly founded GZENR.
- ItemOpen AccessHow successful is grassland restoration after removal of pine plantations on the eastern shores of Lake St Lucia?(2008) Zaloumis, Nicholas Paul; Bond, William J; Hoffmann, TimmThe coastal grasslands that occur along the Eastern Shores of Lake St. Lucia are rich in plant species, including endemics. These grasslands in North KwaZulu-Natal, South Africa, are remnants of a severely threatened vegetation type. Large portions of the Eastern Shores have been utilized for commercial pine plantations over the last 50 years; however these have been removed over the last 17 years. It is therefore important that efforts to restore grasslands that have been previously afforested stands of pines are successful. These grassland systems are fire driven and therefore many plants have adapted to these conditions by having massive storage organs below the ground. Using several response variables I was able to assess the effects of plantations on alpha- and beta-diversity and functional traits of forb and other grassland species and attempted to determine whether these disturbed grassland were returning to their original state. A high level of community heterogeneity was found at both scales for natural vegetation, while more homogenous, species low patterns were seen in post-plantation sites. Most notable was the large number of missing resprouting species in pine sites and that, in contrast to forest rehabilitation studies, no evidence for succession was found. The implication of this is massive as grassland rehabilitation may be a lot harder than was previously thought. It is likely that the fire adapted forb species are bad colonizers and have struggled to return to disturbed sites. These grasslands are therefore susceptible to activities that remove or eliminate the considerable amount of below ground biomass from the soil, and these sorts of disturbances should be avoided. Active rehabilitation methods will be required if any progress is going to be made and until such methods are effective considerable effort should be made in conserving the remaining grasslands that we have left and promoting its pharmaceutical, horticultural and societal value.
- ItemOpen AccessThe impact of land use on biodiversity in xeric succulent thicket, South Africa(1997) Fabricius, Christo, 1956-; Hockey, Phil A RHuman-induced land management practices are key factors which influence the dynamics of landscapes, land elements and biotic communities in Xeric Succulent Thicket, a notoriously fragile and low resilience ecosystem in the Eastern Cape, South Africa. In its natural state this vegetation type is characterized by a continuous mosaic of clumps of diverse woody shrubs in a matrix of grass, hardy dwarf shrubs and bare ground. When indigenous herbivores are replaced by domestic livestock at unsustainable stocking rates, the ratio of clump to interclump areas decreases. This has impacts on biodiversity at the landscape level, and affects the composition and species richness of plant, arthropod and reptile communities and fungal symbionts. The study has two main objectives: 1) to detennine the local-level effects of different types of land use on biodiversity in Xeric Succulent Thicket; and 2) to better understand the factors which affect biodiversity in different taxonomic groups and at different spatial scales, so that the preservation function of protected areas could be enhanced. The key questions which are addressed in the thesis are what does 'biodiversity' mean, and what are its different dimensions in Xeric Succulent Thicket; what roles do protected areas and other lands play in preserving biodiversity, how are different taxonomic groups affected by different types of land use; and how can biodiversity be monitored and measured? . The regional 'conservation landscape' is seen as the protected area plus the land immediately adjoining it, and is viewed as an integrated mosaic of anthropogenic landscapes (land management units), land elements (patches within landscapes). producer communities (assemblages of interacting plant species), consumer communities (assemblages of interacting herbivores and predators) and soil microorganisms. From a process point of view, the research relates the intensity of disturbance, mainly as a result of herbivory, to landscape complexity, the structure of land elements, and the species richness of producers and consumers within land elements. The patterns which emerge are interpreted in the context of ecosystem functioning, from the point of view of a practicing conservation biologist.
- ItemOpen AccessThe impact of megaherbivore grazers on grasshopper communities via grassland conversion in a savannah ecosystem(2003) Currie, Gwen; Picker, Michael D; Bond, William JGrasshoppers are sensitive indicators of the state of disturbance of grassland ecosystems. This study examined the grasshopper communities inside a game reserve, comparing those found on frequently grazed areas with communities inside plots that exclude megaherbivores. The vegetation inside the protected plots was found to differ from the openly grazed areas in terms of grass height and aerial cover, but not in % greenness or richness of forb species. Grass species varied with locality rather than grazing impact. Total numbers of grasshoppers did not differ significantly between the two contrasting areas (100.2 in vs 93.5 out), however grasshopper species richness did, with the outside, short-grass plots having on average 17 different species, and the inside tall-grass plots a higher mean of 24.5. Grasshopper communities responded primarily to grass height and vegetation cover, but not to grass species or greenness of vegetation.