In southern Africa, disturbance contributes to the heterogeneity of grassland and savanna ecosystems. Fire and grazing act as the primary disturbances in these systems, and interactions between the two are common. As such, an understanding of the relationship between fire and grazing is essential for the conservation of biotic diversity and the production of high-quality forage for game and livestock. Frequent fires followed by concentrated grazing have been shown to facilitate patches of short, palatable grasses ("grazing lawns") within grassland and savanna landscapes. The effects of grazing lawn management on other aspects of biodiversity have received little attention. Forbs (non-graminoid, herbaceous plants) are an important component of grassland ecosystems, but how they respond to disturbance is largely unknown. This study compared changes in forb communities on and off of firebreaks (an extreme example of a grazing lawn) in a high altitude mesic grassland. Native herbivore biomass was significantly higher on firebreaks than in the surrounding lightly grazed matrix and the firebreaks, while more compacted, did not show significant signs of degradation. There were no significant differences in forb abundance or richness between annually burned and intermittently burned plots, and there was no species turnover associated with the annually burned, heavily grazed treatment. Speciesspecific differences in functional traits on annually burned and intermittently burned plots were likely a function of light limitation, as mammalian palatability scores were nominal. Ultimately, ten years of intensive fire and grazing have had minimal impact on forb communities in a high altitude mesic grassland. The results indicate that in this type of Highveld grassland, heavily grazed and annually burned patches are not detrimental to the forb community and clearly enhance habitat diversity for grazers.

The Arctic areas are warming more rapidly than other parts of the world. Increasing temperatures are predicted to result in shrubification, higher productivity, declining species diversity and new species invasions to the tundra. Changes in species diversity and plant community composition are likely to alter ecosystem functions with potential consequences for human population also at lower latitudes. Thus, in order to better predict the effects of the rapid arctic warming, we need knowledge on how plant communities respond to a warmer climate. Here, I investigate the effects of climate warming on tundra plant communities and focus on the role of mammalian herbivores in mediating these responses. I examined the role of herbivores by incorporating herbivore manipulations to short- and long-term warming experiments as well as along altitudinal gradients. I measured how individual plants and plant communities respond to warming with and without herbivores. Results of my PhD Thesis illustrate several ways how herbivores modify the responses of plants to warming. I found that herbivores (reindeer, hare, voles, lemmings) may prevent lowland forbs from invading open tundra.  Herbivores might also protect small tundra forbs from being outcompeted by taller and denser vegetation under climate warming. Thus, different herbivore pressures may lead to differing plant abundances and distribution shifts in different areas. Furthermore, my results show that high herbivore pressure can reverse the effects of long-term climate warming very rapidly, even in one year. This finding suggests that well-planned targeted reindeer grazing episodes could potentially be used as a conservation tool to keep selected tundra habitats open. Sudden cessation of grazing may initiate rapid changes in plant community, especially if it coincides with warm temperatures. Taken together, I show that herbivores counteract the effects of climate warming by slowing down or preventing vegetation changes in tundra. Therefore, it is important to consider mammalian herbivores when predicting tundra plant community responses to changing climate.

A study to determine some ecosystem level effects of an aquatic invertebrate grazer, Hyalella azteca, was performed in aquatic ix microcosms. Impact of the grazer was assessed in three general areas: 1) inorganic nutrient levels of the microcosm water column, 2) productivity and respiration of the biotic community, and 3) plant community composition in the microcosms. The grazing amphipod caused inorganic phosphorus and nitrogen (except ammonia) levels to be elevated in the microcosms. The increase was due, at least partially, to excretion of nutrients into the water by the amphipod. The presence of H. azteca did not significantly alter levels of gross productivity for the whole system nor for the sediment surface. Productivity to respiration ratios were significantly reduced by the grazing amphipod, indicating the amphipod was inhibiting plant biomass accumulation even though gross productivity was not affected. Systems exposed to amphipod grazing had a twenty-five percent lower plant biomass than controls at the termination of the experiment. Gross productivity to plant biomass ratios were significantly higher in grazed systems, indicating a more actively growing plant community was being maintained by the grazer's activities. Plant community composition was significantly altered by the amphipod. Chara biomass was higher in grazed systems, while filamentous algae, blue-green algal colonies and periphyton had significantly higher biomasses in the control systems. The phosphorus distribution within the grazed microcosms was significantly different from that found in the controls. More phosphorus was incorporated into filamentous algae, blue-green algal colonies and the overall plant compartment in the control, while Chara and the water column contained more phosphorus in the grazed microcosms. The amphipod also caused the percent content of phosphorus to be higher in certain plant categories.

<p>For a complete picture of the lake ecosystem, it is necessary to understand the mechanisms regulating biomass and nutrient status (nutrient limitation, optimal supply, or surplus) of benthic algae, which are important primary producers and a food resource for grazers. This thesis gives an overview of the natural variation of benthic algae at different scales of space and time and on different substrates, and unravels some of the underlying factors. Algal nutrient status was assessed using the C:N:P (carbon:nitrogen:phosphorus) ratio of the entire natural benthic community. A review, observations, and experiments confirmed that a C:N:P ratio of about 158:18:1 (molar basis) represented an optimal nutrient supply, and that substantially higher C:N, N:P, or C:P ratios reflected algal growth limitation caused by an N or P nutrient deficiency. </p><p>Horizontal variation of benthic algal biomass and nutrient status was patchy, of similar amount for all investigated distances, substrates, and lakes, and constituted a dominant proportion of the total variation. For example, patches of nutrient limited algae were found within only 10 m distance from patches with a nutrient surplus. Thus, horizontal variation should not be neglected when sampling benthic algae in lakes. Field observations suggested an impact of wind, nutrients, and grazers on the horizontal variation. Light and nutrients might have caused the observed vertical and temporal variation. Field experiments confirmed a simultaneous control of benthic algal biomass by nutrients and grazing, mediated by light and temperature. Grazing effects were larger than nutrient effects, but the comparison of natural communities in lakes of different trophy suggested that benthic algal biomass was controlled by nutrients in the long run. </p><p>An important nutrient supply was animal excretions, causing a low C:N:P ratio of epizoon on zebra mussels, and algal communities associated with macrograzers. A field experiment revealed that 15N circulated one week longer in epizoon associated with a sessile caddisfly than in surrounding epilithon. In conclusion, the regulation of benthic algal biomass and nutrient status in lakes is complex, and benthic animals should be looked at not only as grazers, but also as a nutrient source.</p>

<p>The aim of this thesis was to identify general structuring mechanisms in benthic food webs within the framework of ecological stoichiometry theory. Ecological stoichiometry is defined as the balance of multiple chemical substances in ecological interactions and explicitly considers the combined dynamics of key elements such as carbon (C), nitrogen (N) and phosphorus (P). Ecological stoichiometry theory was developed for pelagic environments, thus it must be tested whether the same mechanisms are applicable to benthic environments. </p><p>In this thesis, ecological stoichiometry theory was used as a framework to investigate nutrient pathways in benthic littoral ecosystems. I conducted one invertebrate field sampling and six experiments. In the experiments, factors such as grazing, light, nutrients and fish presence were manipulated.</p><p>The results showed that stoichiometric variability in consumers could mostly be explained by taxa. However, there was some stoichiometric variability due to sampling season, site, and nutrient enrichment. </p><p>Grazing mostly increased periphyton N and P content, although nutrient recycling effects were dependent on grazer stoichiometry. Grazing changed benthic algal community composition by increasing the proportion of grazing resistant algae species. Additionally, grazing decreased algal diversity, especially under nutrient poor conditions. The manipulation of fish presence revealed that fish affected primary producer biomass and stoichiometry through nutrient recycling. </p><p>The manipulation of abiotic factors, such as light and nutrient addition could affect periphyton nutrient content, biomass and benthic algal chlorophyll a content. The separate addition of N or P led to an increase of the added nutrient in the periphyton. Increased light intensities led to a decreased cellular chlorophyll a content and increased C:nutrient ratios. </p><p>This thesis arrives at the conclusion that periphyton-grazer-predator interactions in the benthic are bound by stoichiometric constraints. Nutrient recycling by benthic invertebrates and fish are important mechanism in benthic littoral ecosystems.</p>

<p>Agriculture and livestock production were the mainstay of the economies of many sub-Saharan African countries, including of Cameroon, in the 1970s. Things began to change with the discovery of petroleum products and natural minerals, and the push to industrialise. This led to a shift from agricultural production to other more &lsquo / beneficial&rsquo / sectors. In the 1990s there was an &lsquo / imposed&rsquo / liberalisation of the agricultural sector. This liberalisation was marked by a disengagement of most governments in developing countries from assisting agriculture. In Cameroon, disengagement was achieved by the promulgation of law No. 92/006 of 14th August 1992 and its decree of application No. 92/455/PM of 23rd November. This law encouraged the creation of common initiative groups which could independently pool their resources to increase agricultural production. Although there has been a shift to non-agricultural sectors in many sub-Saharan countries, on the whole, however, many rural areas in these nations have remained essentially agro-pastoral. nfortunately some rural areas, like Small Babanki in Cameroon, whose livelihoods are land-based are faced with soil erosion, population pressure and farmer/grazer conflicts which undermine the little economic gains made in these places. Rural-dwellers have resorted to several innovations to circumvent these constraints to agricultural production.</p>

For a complete picture of the lake ecosystem, it is necessary to understand the mechanisms regulating biomass and nutrient status (nutrient limitation, optimal supply, or surplus) of benthic algae, which are important primary producers and a food resource for grazers. This thesis gives an overview of the natural variation of benthic algae at different scales of space and time and on different substrates, and unravels some of the underlying factors. Algal nutrient status was assessed using the C:N:P (carbon:nitrogen:phosphorus) ratio of the entire natural benthic community. A review, observations, and experiments confirmed that a C:N:P ratio of about 158:18:1 (molar basis) represented an optimal nutrient supply, and that substantially higher C:N, N:P, or C:P ratios reflected algal growth limitation caused by an N or P nutrient deficiency. Horizontal variation of benthic algal biomass and nutrient status was patchy, of similar amount for all investigated distances, substrates, and lakes, and constituted a dominant proportion of the total variation. For example, patches of nutrient limited algae were found within only 10 m distance from patches with a nutrient surplus. Thus, horizontal variation should not be neglected when sampling benthic algae in lakes. Field observations suggested an impact of wind, nutrients, and grazers on the horizontal variation. Light and nutrients might have caused the observed vertical and temporal variation. Field experiments confirmed a simultaneous control of benthic algal biomass by nutrients and grazing, mediated by light and temperature. Grazing effects were larger than nutrient effects, but the comparison of natural communities in lakes of different trophy suggested that benthic algal biomass was controlled by nutrients in the long run. An important nutrient supply was animal excretions, causing a low C:N:P ratio of epizoon on zebra mussels, and algal communities associated with macrograzers. A field experiment revealed that 15N circulated one week longer in epizoon associated with a sessile caddisfly than in surrounding epilithon. In conclusion, the regulation of benthic algal biomass and nutrient status in lakes is complex, and benthic animals should be looked at not only as grazers, but also as a nutrient source.

<p>Agriculture and livestock production were the mainstay of the economies of many sub-Saharan African countries, including of Cameroon, in the 1970s. Things began to change with the discovery of petroleum products and natural minerals, and the push to industrialise. This led to a shift from agricultural production to other more &lsquo / beneficial&rsquo / sectors. In the 1990s there was an &lsquo / imposed&rsquo / liberalisation of the agricultural sector. This liberalisation was marked by a disengagement of most governments in developing countries from assisting agriculture. In Cameroon, disengagement was achieved by the promulgation of law No. 92/006 of 14th August 1992 and its decree of application No. 92/455/PM of 23rd November. This law encouraged the creation of common initiative groups which could independently pool their resources to increase agricultural production. Although there has been a shift to non-agricultural sectors in many sub-Saharan countries, on the whole, however, many rural areas in these nations have remained essentially agro-pastoral. nfortunately some rural areas, like Small Babanki in Cameroon, whose livelihoods are land-based are faced with soil erosion, population pressure and farmer/grazer conflicts which undermine the little economic gains made in these places. Rural-dwellers have resorted to several innovations to circumvent these constraints to agricultural production.</p>

The aim of this thesis was to identify general structuring mechanisms in benthic food webs within the framework of ecological stoichiometry theory. Ecological stoichiometry is defined as the balance of multiple chemical substances in ecological interactions and explicitly considers the combined dynamics of key elements such as carbon (C), nitrogen (N) and phosphorus (P). Ecological stoichiometry theory was developed for pelagic environments, thus it must be tested whether the same mechanisms are applicable to benthic environments. In this thesis, ecological stoichiometry theory was used as a framework to investigate nutrient pathways in benthic littoral ecosystems. I conducted one invertebrate field sampling and six experiments. In the experiments, factors such as grazing, light, nutrients and fish presence were manipulated. The results showed that stoichiometric variability in consumers could mostly be explained by taxa. However, there was some stoichiometric variability due to sampling season, site, and nutrient enrichment. Grazing mostly increased periphyton N and P content, although nutrient recycling effects were dependent on grazer stoichiometry. Grazing changed benthic algal community composition by increasing the proportion of grazing resistant algae species. Additionally, grazing decreased algal diversity, especially under nutrient poor conditions. The manipulation of fish presence revealed that fish affected primary producer biomass and stoichiometry through nutrient recycling. The manipulation of abiotic factors, such as light and nutrient addition could affect periphyton nutrient content, biomass and benthic algal chlorophyll a content. The separate addition of N or P led to an increase of the added nutrient in the periphyton. Increased light intensities led to a decreased cellular chlorophyll a content and increased C:nutrient ratios. This thesis arrives at the conclusion that periphyton-grazer-predator interactions in the benthic are bound by stoichiometric constraints. Nutrient recycling by benthic invertebrates and fish are important mechanism in benthic littoral ecosystems.

Magister Philosophiae (Land and Agrarian Studies) - MPhil(LAS) / Agriculture and livestock production were the mainstay of the economies of many sub-Saharan African countries, including of Cameroon, in the 1970s. Things began to change with the discovery of petroleum products and natural minerals, and the push to industrialise. This led to a shift from agricultural production to other more 'beneficial' sectors. In the 1990s there was an 'imposed' liberalisation of the agricultural sector. This liberalisation was marked by a disengagement of most governments in developing countries from assisting agriculture. In Cameroon, disengagement was achieved by the promulgation of law No. 92/006 of 14th August 1992 and its decree of application No. 92/455/PM of 23rd November. This law encouraged the creation of common initiative groups which could independently pool their resources to increase agricultural production. Although there has been a shift to non-agricultural sectors in many sub-Saharan countries, on the whole, however, many rural areas in these nations have remained essentially agro-pastoral. nfortunately some rural areas, like Small Babanki in Cameroon, whose livelihoods are land-based are faced with soil erosion, population pressure and farmer/grazer conflicts which undermine the little economic gains made in these places. Rural-dwellers have resorted to several innovations to circumvent these constraints to agricultural production. / South Africa