Carbon based secondary metabolites in African savanna woody species in relation to anti-herbivore defense

Includes bibliographical references.Chemical defense in African savanna woody species have been inferred from foliar condensed tannin and total phenolic concentrations. However these measures are controversial since condensed tannin chemical heterogeneity compromises its simple quantification and total phenolics, which is a measure of general foliar phenolics, includes many different low molecular weight phenolics. Some of these low molecular weight phenolics have been shown to deter herbivory individually or in combination with others. The growth differentiation balance hypothesis predicts a trade-off between costs of chemical defenses or carbon based secondary metabolites relative to the demand for photosynthate by growth. However, this hypothesis was developed in northern boreal and temperate forests and the extent to which it applies to African savanna woody species remains unclear. In this study we: 1) Compared the use of Quebrachowith Sorghumtannin as standards for condensed tannin quantification, relative to absolute condensed tannin concentration in selected species; 2) Determined low molecular weight phenolic profiles of 6 species with different life history, morphological and functional traits; and 3) Investigated whether the growth differentiation balance hypothesis explains allocation to carbon based secondary metabolites in Combretum apiculatumalong a 6-level N gradient; and following 100% simulated herbivory along the N gradient.Condensed tannins were extracted from 5 tree species and purified. Condensed tannin concentrations were determined using the acid-butanol assay and expressed as Quebracho, Sorghumand absolute tannin concentrations. Chemical composition of different tannin polymers were determined by thiolysis using HPLC and HPLC-MS. Photosynthesis, growth variables andphenotypic phytochemical responses to the different N and N x herbivory treatments were measured in C. apiculatum. Low 7molecular weight phenolics were determined and identified using HPLC and HPLC-MS respectively.Results from this study suggest: that thecontinued use of Quebrachotannin as condensed tannin standard in the acid-butanol assay is unjustified; that generalizations based on life history, morphological and functional traits may be misleading since foliar low molecular weight phenolics are not necessarily related to these traits; and that the growth differentiation balance hypothesis does not explain allocation to carbon based secondary metabolites in C. apiculatum

Can Armstrong cope with Libet’s challenge?

Magister Artium - MAAccording to our ordinary conception of voluntary action, our actions are the causal result of conscious intentions. To take a very basic example: I wish to take a sip of coffee, and I therefore reach out and take hold of the mug. However, studies performed by Libet challenge this ordinary conception. What Libet found in his experiments was that the brain initiates voluntary actions and the person becomes consciously aware of an intention to act only some 400 msec after the brain’s initiation; for instance, my brain has already initiated the process of causing my arm to reach out and take hold of the mug some 400 msec before I am aware that I wish to take a sip of coffee. That is, conscious intention doesn’t appear to precede voluntary action at all – it actually follows it (or follows its initiation, at least), and thus Libet’s studies present a serious challenge to our ordinary conception of voluntary action. This project will investigate whether a particular theory of mind – namely, Armstrong’s Central State Materialism – can cope with the challenge posed by Libet’s studies and salvage our ordinary conception of voluntary action. Armstrong’s theory appears promising in this regard because his account of consciousness and introspection as higher-order states seems to allow room that we will become aware of our willings only after those willings are already initiated

A quality Assurance framework for digital household survey processes in South Africa

Philosophiae Doctor - PhDOfficial household-based survey statistics is predominantly collected using the paper-and-pen data collection (PAPDC) method. In recent times, the world has seen a global rise in the use of digital technology, especially the use of mobile handheld devices for the collection of survey data in various fields of statistical collection. Various sectors in the population require data for a multitude of purposes, from planning, monitoring and during the evaluation of projects and programmes. The pressure of attaining the data often requires data or information producers to gather more data or information more frequently with improved quality, efficiency, and accuracy. The quality of data or information collected remains uncertain as more surveys enter the global arena. The overall survey quality needs to improve continuously. The data used may not be trustworthy and users should be aware of this. There should be a continuous holistic assessment of the validity and reliability of data before these are used (T. Chen, Raeside, & Khan, 2014). Digital data collection (DDC) offers national statistical organisations (NSOs) in Africa possible, albeit partial, solutions to several current quality, performance, and cost-efficiency concerns. Potential benefits found in the literature for DDC methods over PAPDC methods include, inter alia: increased speed of data collection, increased data accuracy, timeous availability of data, higher data quality, effective data security and lower costs for data-collection processes. Most NSOs in Africa, including South Africa, currently rely on manual, paper-based data collection methods for continuous official household survey collection. Paper-based methods tend to be slower, to rely on manual reporting and involve more survey-intensive resources. With the rise of handheld mobile Global Positioning Systems (GPS) enabled devices, official household surveys are able to monitor surveys spatially, and in real-time. The information could be securely synchronised to a central secure database, to allow for immediate post-processing and data analysis

A thorny issue: Woody plant defence and growth in an East African savanna

Recent work suggests that savanna woody plant species utilise two different strategies based on their defences against herbivory; a low nutrient/high chemical defence strategy and a nutrition paired with mostly architectural defences strategy. The concept that chemical and structural defences can augment each other and do not necessarily trade-off has emanated from this work. In this study, we examine woody plant defence strategies, how these respond to herbivore removal and how they affect plant growth in an East African savanna. At three paired long-term exclosure sites with high browser and mixed-feeder densities at Mpala Ranch, Kenya, we investigated: (a) whether defences employed by the dominant fine- and broad-leaved woody savanna species form defence strategies and if these align with previously proposed strategies, (b) how nine key plant defence traits respond to herbivore removal and (c) how effective the different defence strategies are at protecting against intense herbivory (by measuring plant growth with and without herbivores present). We identified three defence strategies. We found a group (a) with high N, short spines and high N-free secondary metabolites, a group (b) with high N, long spines and low N-free secondary metabolites and a group (c) with moderate N, no spines and low N-free secondary metabolites (most likely defended by unmeasured chemical defences). Structural defences (spine length, branching) were generally found to be induced by herbivory, leaf available N increased or did not respond, and N-free secondary metabolites decreased or did not respond to herbivory. Species with long spines combined with increased “caginess” (dense canopy architecture arising from complex arrangement of numerous woody and spiny axis categories) of branches, maintained the highest growth under intense browsing, compared to species with short spines and high N-free secondary metabolites and species with no spines and low N-free secondary metabolites. Synthesis. At our study site, structural traits (i.e. spines, increased caginess) were the most inducible and effective defences against intense mammalian herbivory. We propose that high levels of variability in the way that nutrient and defence traits combine may contribute to the coexistence of closely related species comprising savanna woody communities

Feeding choices and impacts of extralimital giraffe on two keystone tree species in the Kgalagadi National Park

In this article we determine the effect of an extralimital megaherbivore on the reproductive potential and vegetation structure of two keystone tree species in the Auob River in the south western Kalahari Desert of southern Africa. Using spoor and dung counts we establish the presence of giraffe in three predetermined density zones by walking 50 transects across the river in each zone. We also photographed six trees from each species in each zone and use these photographs to determine browse impact on reproductive potential, canopy volume as well as the percentage dieback on the extremities of the canopy. We then perform stable carbon and nitrogen isotope analysis on the leaves of the trees and compare these relative to the isotope ratios of giraffe dung to ascertain dietary preference. Crude protein was determined as a guide to nutritive value. Finally, we determine both chemical and physical defences for the two species. Our results show a significant negative impact of giraffe browse on tree canopies, no significant differences in recruitment and a noticeable decrease in flowers and pods at the giraffe browse height of 2 m – 5 m. No significant differences in crude protein or condensed tannins were found but significant differences in spinescence. Giraffe are not endemic to the Auob River and our study shows that the introduction of these animals is having a negative impact on the canopies of Vachellia haematoxylon. While there are, as yet, no significant impacts on reproductive potential we speculate that this will happen with time. Conservation implications: Our study shows that giraffe are significantly impacting the canopies of two common tree species in the Auob River in the arid Kgalagadi Transfrontier Park. Without management intervention an increasing population of giraffe will result in substantial changes to the plant community vegetation structure of the river

Intraspecific host preferences of mopane moths (Imbrasia belina) in mopane (Colophospermum mopane) woodland

Colophospermum mopane trees are a common larval food plant of the mopane moth (Imbrasia belina), which commonly reaches outbreak proportions. Our study investigated factors determining host tree choice by ovipositing mopane moths within a single host species (C. mopane). Tree size was the primary determinant of oviposition at both the habitat (tall riverine-, medium height woodland- and short-shrub mopane) and individual tree scale, with an increase in the number of egg masses with increasing tree size (estimated by height and canopy volume). Preference for larger trees was only evident at the habitat scale, however, as individual trees were utilized as expected according to the availability of their canopy size class. More detailed tree characteristics, such as leaf size, shoot size, stem number and even leaf chemistry (protein : tannin ratio and total polyphenols) had no influence on host choice. Host choice was based on the most obvious measure of resource abundance, namely tree size, as expected for an outbreak species, since resource availability rather than nutritional quality is likely to be the primary determinant of larval survival

Intraspecific host preferences of mopane moths (Imbrasia belina) in mopane (Colophospermum mopane) woodland

Colophospermum mopane trees are a common larval food plant of the mopane moth (Imbrasia belina), which commonly reaches outbreak proportions. Our study  investigated factors determining host tree choice by ovipositing mopane moths within a single host species (C. mopane). Tree size was the primary determinant of oviposition at both the habitat (tall riverine-, medium height woodland- and short-shrub mopane) and individual tree scale, with an increase in the number of egg masses with increasing tree size (estimated by height and canopy volume). Preference for larger trees was only evident at the habitat scale, however, as individual trees were utilized as expected according to the availability of their canopy size class. More detailed tree characteristics, such as leaf size, shoot size, stem number and even leaf chemistry (protein:tannin ratio and total polyphenols) had no influence on host choice. Host choice was based on the most obvious measure of resource abundance, namely tree size, as expected for an outbreak species, since resource availability rather than nutritional quality is likely to be the primary determinant of larval survival.Key words: host choice, resource abundance, tree size, outbreak, oviposition

Differential effects of defoliation by mopane caterpillars and pruning by African elephants on the regrowth of Colophospermum mopane foliage

Plant responses to herbivory vary depending on herbivory type, yet the comparative effects of defoliation (e.g. by insects) and pruning (e.g. by large mammals) on a single tree species are poorly documented. We investigated this in the Northern Province of South Africa by comparing the regrowth of Colophospermum mopane trees previously defoliated by caterpillars or pruned by elephants, the two main browsers of C. mopane foliage. Shoots were up to 160% and 125% longer after natural (elephant) and simulated pruning and leaves ~25% longer in regrowth after natural pruning (n = 13–15 trees per treatment). Shoot density and chemical defences in leaves (tannin:protein ratio and total polyphenolic concentration) were, however, no different from control trees. Simulated defoliation resulted in statistically insignificant changes to regrowth in terms of leaf and shoot size (both slightly decreased) and shoot density (slightly increased). Natural (caterpillar) defoliation, however, resulted in regrowth with significantly decreased shoot and leaf size (about 50% and 20% of control lengths, respectively), as well as decreased leaf chemical defence. Shoot and leaf length were longer on trees flushing for the first time after pruning and late-season defoliation had a greater negative impact than mid-season defoliation. Despite the differences in regrowth characteristics after pruning and defoliation, mopane plants showed no apparent trade-off in investment between tolerance and resistance after either herbivory type, as neither regrowth nor chemical defence occurred at the expense of the other