3,780 research outputs found
Nutrients in an African Savanna: the consequences of supply heterogeneity for plants and animals
In savannas, trees and grasses co-exist and share resources such as water and nutrients. The ratio between the tree and grass components (i.e., vegetation structure) importantly controls productivity, animal assemblages and earth-atmosphere feedbacks. As the structure of savanna vegetation is inherently unstable and easily disturbed, finding out how the vegetation structure is controlled is of great importance for rangeland management and conservation. Currently four factors are believed to control the vegetation structure in savannas; namely, water, nutrients, herbivory and fire. While the water and fire factors have been intensely studied, the roles of nutrients and herbivores as factors are less well known. Improving our understanding of the role of nutrients in savannas is relevant, because it is increasingly realised that global change also alter the availability of nutrients, e.g., increased nitrogen deposition. How savanna systems respond to nutrient perturbations is uncertain. Changes in soil nutrient availability are also likely to feed back to changes in forage quality, which may influence large herbivore use and vegetation impact patterns, thus indirectly influencing vegetation structure. Moreover, it is increasingly realised that not only changes in the availability of nutrients influence plants and subsequently herbivores, but also how nutrients become spatially available. In fact, new ideas suggest that differences in the scale of spatial resource heterogeneity may control how resources are partitioned between co-existing species differing in size, e.g., large trees coexisting with small grasses in savanna systems. To test how changes in nutrient availability and spatial configurations influence savanna systems, several field experiments were conducted in a semi-arid savanna in South Africa. In the study area we found that nutrient (N, P and K) availability negatively affected tree (Colophospermum mopane) seedling establishment in fertilization experiments. Also, trees failed to re-colonize nutrient-rich kraal sites that were abandoned almost half a century ago. In dry savannas, it is currently believed that the success of tree seedling establishment exercises large control over the relative dominance of trees, thus an increase in nutrient availability may feed back to a structurally more open vegetation state. Different explanations may account for constrained tree seedling establishment under fertile soil conditions. We tested the hypothesis that the intensification of grass competition reduces tree seedling recruitment in fertile environments. In controlled competition experiments it was shown that negative nutrient effects on tree seedlings only occur when seedlings were competing with grasses in mixtures. Furthermore, we found that adding both water and nutrients to tree seedling-grass plant mixtures cancelled the negative effects of added nutrients on tree seedlings. Thus the suppressing effect of increased nutrient availability on seedlings appears to operate indirectly through the pre-empting of soil water resources by vigorous herbaceous growth under fertile conditions. Since woody seedlings are vulnerable to water stress, increased nutrient-induced water stress translates into higher mortality rates and suppressed growth of tree seedlings in fertile areas. In support, transplanted tree seedling mortality increased during a mid-season drought as local fertilizer concentration increased. In conclusion, intensified herbaceous competition under fertile soil conditions appears to be a viable mechanism explaining poor tree seedling recruitment under fertile soil conditions. Savannas and nutrients While establishing tree seedlings suffer under fertile conditions, our data suggest that established (mature) tree growth benefitted from an increase in nutrient availability, especially following an increase in N availability. With regards to increased atmospheric N deposition, we predict that tree cover may initially expand following nutrient enrichment in dry savannas, although tree cover responses may be insensitive to current levels of N deposition. However, in the long-term tree cover is expected to decline, because of constrained tree recruitment that appears to be more sensitive to small increases in N availability. The relative availability of nutrients such as N and P may also influence how resources are partitioned between co-existing trees and grasses. In a field experiment we found evidence that trees were relatively more limited by N than P availability. An East African study has shown that grasses underneath trees are more P than N limited and we found evidence that the competiveness of sub-canopy grasses in our study increased when only P was supplied. The relative availability of N vs. P may therefore offer an additional axis governing resource partitioning between trees (non N-fixing) and grasses in savanna systems. This supports the idea that organisms with a high growth potential, apparently grasses in savannas, have relatively high P requirements to sustain rapid protein synthesis, while slower growing organisms such as trees are more limited by the availability of N. The availability of soil nutrients strongly mediated where large herbivores concentrated their impact in the landscape. Both grazers and browsers responded positively to fertilization, apparently via the control that soil nutrient availability has on forage quality. Tree and grass leaf N and P concentrations increased and condensed tannin concentrations in trees decreased following fertilization. Under high local soil nutrient concentrations the vegetation biomass was in some instances reduced below control biomass by herbivores, indicating that top-down herbivore effects potentially override bottom-up nutrient effects under fertile conditions. In ecology, it is increasingly realized that it is not only the availability of nutrients, but also how nutrients become spatially available that matters. Data from a large field experiment where a gradient in the scale of nutrient patchiness (i.e., patch grains sizes 2 x 2 m, 10 x 10 m or 50 x 50 m) was created, suggested that the scale of nutrient patchiness controlled the partitioning of resources between co-existing trees and grasses. For the same local fertilizer concentration, tree leaf quality was unresponsive to fertilization in small patches, but responded in the larger patches. Grass leaf quality increased with local fertilizer concentration regardless of patch size. The differential responses of trees and grasses to scale differences subsequently modulated the responses of the browser and grazer guilds. For the same high local fertilizer concentration, grazers responded to both fine and coarse scales of nutrient patchiness, while browsers responded only to the coarse scale of nutrient patchiness. In turn, the selective grazing in the fine scale of nutrient patchiness treatment, apparently stimulated tree growth. In the coarser scale of patchiness treatment both browsers and grazer impact intensified locally. Thus the scale of nutrient patchiness controlled nutrient partitioning between trees and grasses, which was apparently closely tracked by the large herbivore assemblage, resulting in differential local impacts on the tree and grass layers. Apart from local effects, the scale of nutrient patchiness may also regulate the use and quality of forage resources at larger (e.g., landscape) scales. In the large field fertilizer experiment, calculations suggest that the total herbaceous off-take by grazers peaked where the same fertilizer amount (15 kg N plot-1) was spread over the whole plot surface area rather than concentrated in 10 x 10 m patches. Thus, how nutrients are distributed in an area controls secondary productivity and where herbivores concentrate their impact. The scale related patterns observed in the large fertilizer experiment may have been reinforced by plastic responses (e.g., fine root proliferation in nutrient-rich soil) to heterogeneous nutrient supplies of trees in the coarse scale treatments and grasses in the fine scale treatments, respectively. To test this, the same nutrient amount was supplied at two different scales of patchiness to focal trees with their associated grasses. Two years after fertilization, large-scale fertilized trees showed increased shoot growth and increased leaf N concentrations compared to small-scale fertilized trees receiving the same N amount. Conversely, trees in a small-scale configuration fertilized with P showed negative responses compared to large-scale counterparts. These results suggest that differences in the local scale of nutrient patchiness also influence how nutrients are partitioned between co-existing trees and grasses. Herbivores not only respond to nutrient heterogeneity, but may also create spatial heterogeneity in nutrient availability, which, in turn, may influence the vegetation structure of savannas. We studied the current soil nutrient status, tree and grass biomass patterns and large herbivore use of nine former livestock holding pen areas (kraals) in a semi-arid, nutrient poor savanna. These were contrasted with nearby control sites located in the surrounding landscape. The kraals, formerly enriched by livestock dung and urine, were abandoned around 1970 and since then wildlife replaced livestock in these parts. We found that around 40 years later, kraal soil had elevated concentrations of inorganic N, extractable P, K, Ca and Mg compared to control sites, which resulted in high quality forage in kraal sites. Trees also failed to invade these sites, thus kraals remained as structurally open patches in the otherwise dense savanna. Evidently, wild large herbivores maintain the high nutrient status of kraal sites, probably by importing nutrients into these sites and by accelerating local nutrient cycling. In turn, the increased local nutrient availability prevents tree seedlings from establishing under these fertile conditions. Finally, this study provided evidence that changes in the availability of nutrients influenced the success of woody seedling establishment, which may feed back to changes in the relative proportions of trees and grasses in dry savannas. Furthermore, this study supports the new idea that the scale of resource heterogeneity influences how resources are partitioned between co-existing trees and grasses, which, in turn, modulated browser vs. grazer use and impact patterns on the vegetation. In conclusion, this study provides new information on nutrient-plant-herbivore interactions in a dry savanna with potentially important implications for the management of dry savannas in general. <br/
’n Algemene geloof ?
In die feesbundel wat verskyn het by die sestigste verjaarsdag van prof. De Vleeschauwer (Universiteit van Suid-Afrika) vind ons ook ’n artikel van prof. Stoker Een en ander oor menslike vryheid.Dit is nie ons bedoeling om hierdieartikel te bespreek nie; ons ag ons ook nie hiertoe bevoeg nie. Maar ons wil wel ’n enkele opmerking maak oor wat ons leesinsake geloof
Local impact of community renewable energy:A case study of an Orcadian community-led wind scheme
Local benefits of community renewable energy (CRE) –ranging from an increase in social cohesion, jobs, services, knowledge and skills - are widely assumed in academia and among policy makers. However, there are both a lack of evidence on these impacts and a lack of formal impact assessment methodologies to assess them. This research explores change mapping, an impact assessment methodology, to contribute to evaluative frameworks for assessment of the effects of CRE projects on their host communities. With this methodology, the local impacts of a 900 kW community wind project on the Scottish island Shapinsay are assessed, using an exploratory survey, interviews, and two focus group sessions. When it comes to the local impacts, the biggest changes residents experience are either direct or indirect effects of the RE revenues. Modest but ostensible effects are visible on local economic development, social cohesion, and knowledge and skill development. From the case study can be concluded that community-ownership of energy technologies can indeed be empowering, but also creates vulnerability. Stable energy policies and support during the operational phase are needed to keep up the stream of RE income that functions as a source of independent income
A clinical diagnosis of oral leukoplakia : a guide for dentists
In view of the many white or predominantly white lesions of the oral mucosa it is a challenge for dentists to clinically identify a leukoplakia, being a potentially (pre)malignant lesion. Based on the available literature and experience of the authors the parameters of a clinical diagnosis of oral leukoplakia have been studied. A guide has been presented that should help dentists to establish a clinical diagnosis of leukoplakia as accurate as possible. Probably in most parts of the world dentists will need the help of a specialist for confirmation or exclusion of the clinical diagnosis of oral leukoplakia and for further management of the patient, including patient information
The role of the dentist in the diagnosis and management of patients with oral mucosal diseases
Based on a few case reports of oral mucosal diseases a number of questions is raised about the role of dentists-general practitioners in the diagnostic procedure and management of patients with such diseases. For instance, are dentists prepared to prescribe topical corticosteroids and should dentists be taught how and when to take a biopsy? And how about palpation of the neck? A strong recommendation is made to take clinical pictures for proper documentation and, if needed, for telediagnostic procedures. Another issue relates to the communication between dentists and dental specialists when dealing with patients with oral diseases. In case of a patient suffering from burning mouth syndrome or any other type of chronic orofacial pain, the question is raised whether dentists-general practitioners are prepared to manage such patients. Furthermore, there is a call for structuring the collaboration between dentists-general practitioners and dental specialists, including oral and maxillofacial surgeons
Insecticide Efficacy Against Trimen’s False Tiger Moth, Agoma trimenii (Lepidoptera: Agaristidae)
Trimen’s false tiger moth, Agoma trimenii (Lepidoptera: Agaristidae), has developed pest status in vineyardsin the Northern Cape and Groblersdal areas of South Africa, and an integrated pest management system isrequired. The objective of this study was to test the susceptibility of A. trimenii larvae to three commercialproducts (Delegate®WG, Steward®150 EC and three doses of DiPel® DF). Bioassay tests using all threeproducts, and semi-field trials to test the potential of DiPel® DF against A. trimenii larvae, applied at differentwater volumes (50 g/1 000 L/ha and 50 g/1 430 L/ha) were performed. The residual activity of DiPel® DF,when applied at different water volumes, was investigated daily. Delegate® WG, Steward®150 EC and therecommended dose of DiPel® DF showed 100% larval mortality within seven days. Delegate® WG and therecommended dose of DiPel® DF proved to be the fastest acting products. The product label recommendeddose of DiPel® DF (0.25 g/500 mL distilled water) proved the most effective dose (in comparison to halvedand doubled dosages) and showed 100% mortality five days after application. Increasing the water volumeper ha of a spray application of DiPel® DF for the same application area, showed no significant increasein larval mortality. A reduction in insecticidal activity for DiPel® DF applied at both water volumes wasseen between leaves picked four days after spraying and leaves picked five days after spraying, and nomortality was observed after day 6. To help improve efficacy, attention should be given to increasing spraycoverage and residual activity of DiPel® DF, as well as using all tested products within an integrated pestmanagement system
Proliferative verrucous leukoplakia: a critical appraisal of the diagnostic criteria
Since its introduction in the literature in 1985, the term proliferative verrucous leukoplakia (PVL) has been the
subject of an ongoing discussion with regard to its definition. Widespread or multifocal occurrence of oral leukoplakia is not just synonymous to PVL. In the present treatise the proposal is made to require the involvement of
more than two oral oral subsites, a total added seizeof the leukoplakic areas of at least 3 centimeters, and a well
documented period of at least five years of disease evolution being characterized by spreading and the occurrence
of one or more recurrences in a previously treated area
Social attention biases in juvenile wild vervet monkeys: implications for socialisation and social learning processes.
The concept of directed social learning predicts that social learning opportunities for an individual will depend on social dynamics, context and demonstrator identity. However, few empirical studies have examined social attention biases in animal groups. Sex-based and kinship-based biases in social learning and social attention towards females have been shown in a despotic and female philopatric primate: the vervet monkey (Chlorocebus pygerythrus). The present study examined social attention during the juvenile period. Social attention was recorded through 5-min focal observations during periods of natural foraging. Kin emerged as the most important focus of social attention in juveniles, intensified by biased spatial proximity towards matrilineal related members. The highest-ranking conspecifics were more frequently observed by juveniles than low-ranking ones. Additionally, younger and orphaned juveniles showed higher levels of social attention overall, compared to other age categories. No effect of the juvenile's hierarchical rank was detected, suggesting that the variation in social attention recorded reflects different biases and stages of social learning and socialisation, rather than social anxiety. Juvenile females tended to exhibit a dominance-based bias more strongly than did males. This might be explained by a greater emphasis on attaining social knowledge during juvenile socialisation in the philopatric sex. Moreover, despite a preferred association between juveniles, social attention was more often directed to adults, suggesting that adults may still be more often chosen as a target of attention independent of their dominance rank
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