Time filter

Source Type

Murray-Hudson M.,University of Florida | Murray-Hudson M.,Okavango Research Institute | Lane C.R.,U.S. Environmental Protection Agency | North S.,U.S. Environmental Protection Agency | Brown M.T.,University of Florida
Wetlands | Year: 2012

This study examined the distribution of wetland plants used in macrophyte-based index of biotic integrity (IBI) metrics to determine the effectiveness of zone sampling in assessing wetland condition. Using sampling data from a previous study of 74 emergent isolated wetlands, macrophyte taxonomic data and resulting IBI metrics were analyzed for various wetland zones and compared against the original site metrics. Zones were defined by parsing each sampling transect (from wetland edge to center) into thirds-creating an outer, intermediate, and inner zone-and constructing two additional zones representing two-thirds of the original sampling area-an outer plus intermediate zone and an intermediate plus inner zone. While a highly-significant decrease in mean species richness was observed from the wetland exterior to interior, there were no significant differences between metric values derived fromsite data and those derived from zone data. Linear correlations between metrics from the outer zone and site data were highly significant (R 2>0.94), but linear correlation strength decreased with the intermediate and inner zones, respectively. Thus, macrophyte-based indicators of biological integrity can be efficiently assessed by sampling the outer third of a wetland-a 66% reduction in the area and effort required to monitor isolated wetland condition. © US Government 2012. Source

Tedder M.,University of KwaZulu - Natal | Morris C.,University of KwaZulu - Natal | Fynn R.,Okavango Research Institute | Kirkman K.,University of KwaZulu - Natal
Grassland Science | Year: 2012

The structure and composition of savanna vegetation is influenced by resource availability and disturbance. Grasses, a major component of savanna systems, influence the tree-grass balance by competing with trees for light, water and soil nutrients as well as providing fuel for fires. Overgrazing, and the ensuing decreased grass competition and fire effects resulting from diminished grass vigor and cover, are thought to be major drivers of bush encroachment. To investigate impacts of competition from grass on saplings of Acacia karroo Hayne. and Acacia nilotica (L.). Willd. ex. Del. under high and low soil fertility, six grass species, Aristida junciformis (Trin & Rupr.), Eragrostis capensis (Thunb.) Trin., Hyparrhenia hirta Stapf., Panicum maximum Jacq., Sporobolus africanus (Poir.) Robyns & Tournay and Themeda triandra Forssk., were planted in pots with a tree sapling as the central test species. The major competitive effects experienced by tree saplings were dependent on nutrient level and the identity of the competing grass species (P < 0.001). Aboveground A. karroo was more strongly affected by grass competition under high nutrient conditions, whereas an increase in nutrient level had little effect on the competition between A. nilotica and the grasses (P < 0.001). Acacia karroo was also strongly affected by grass competition belowground; however, the opposite trend was observed for A. nilotica (P < 0.001). Eragrostis capensis was one of the most competitive grass species, reducing sapling aboveground biomass by 85% and 65% under high and low nutrient conditions, respectively, and sapling belowground biomass by 71% on average. Increasing nutrient availability resulted in an increase in competition exerted on A. karroo by all grasses but little to no change for A. nilotica. Changes in savanna composition and structure are thus likely to be influenced by grass species composition and soil nutrient status. © 2012 The Authors Grassland Science © 2012 Japanese Society of Grassland Science. Source

Eby S.,Colorado State University | Burkepile D.E.,Florida International University | Fynn R.W.S.,Okavango Research Institute | Govender N.,Scientific Services | And 10 more authors.
Oecologia | Year: 2014

Large herbivore grazing is a widespread disturbance in mesic savanna grasslands which increases herbaceous plant community richness and diversity. However, humans are modifying the impacts of grazing on these ecosystems by removing grazers. A more general understanding of how grazer loss will impact these ecosystems is hampered by differences in the diversity of large herbivore assemblages among savanna grasslands, which can affect the way that grazing influences plant communities. To avoid this we used two unique enclosures each containing a single, functionally similar large herbivore species. Specifically, we studied a bison (Bos bison) enclosure at Konza Prairie Biological Station, USA and an African buffalo (Syncerus caffer) enclosure in Kruger National Park, South Africa. Within these enclosures we erected exclosures in annually burned and unburned sites to determine how grazer loss would impact herbaceous plant communities, while controlling for potential fire-grazing interactions. At both sites, removal of the only grazer decreased grass and forb richness, evenness and diversity, over time. However, in Kruger these changes only occurred with burning. At both sites, changes in plant communities were driven by increased dominance with herbivore exclusion. At Konza, this was caused by increased abundance of one grass species, Andropogon gerardii, while at Kruger, three grasses, Themeda triandra, Panicum coloratum, and Digitaria eriantha increased in abundance. © 2014 Springer-Verlag Berlin Heidelberg. Source

Thole B.,Ngurdoto Defluoridation Research Station | Mtalo F.,University of Dar es Salaam | Masamba W.,Okavango Research Institute
Clean - Soil, Air, Water | Year: 2012

Breakthrough characteristics, kinetics, and dose-effect in defluoridation with bauxite, gypsum, magnesite, and their composites were determined. The aim was to identify optimum filter and configuration viable for groundwater defluoridation. Bed depth service time (BDST) design model and empty bed residence time (EBRT) optimization model were employed to characterize breakthrough. Higher doses obtained lower loading capacities but higher sorption percentages and breakthrough times. Breakthrough times obtained were 50 400, 32 400, 25 200, and 19 800s for 150, 120, 75, and 45g, respectively. The equation h{stroke}=1.0×10-4 δ2 -0.022 δ + 1.5053 defined the operating line with h{stroke}, adsorbent exhaustion rate, in gL-1 and δ, EBRT, in seconds. A critical bed depth (Zo) of 6.56cm was obtained. Second order kinetic rate constants were 0.73, 1.17, and 1.81gmg-1s-1 for magnesite, gypsum, and bauxite, respectively. The composite, gypsum and bauxite decreased water pH but magnesite increased pH in water defluoridation. Experimental data did not fit the two-parameter logistics model; model values were significantly different from experimental values. Optimum defluoridation characteristics were obtained in fixed bed. Despite high residual sulphates and apparent color, fixed-bed defluoridation with raw composites of these materials, treated in this manner, is viable. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Source

Rutina L.P.,Okavango Research Institute | Moe S.R.,Norwegian University of Life Sciences
Ecosystems | Year: 2014

The substantial increase in elephant populations across many areas in southern Africa over past decades is prompting concerns about the effects on biodiversity. We investigated the outcomes of elephant disturbance on tree-species presence, density, and richness, and on alpha and beta diversity within riparian woodland in Chobe National Park, Botswana. We enumerated all tree species occurring in 32 plots (0.06 ha) along the Chobe riverfront. Plots were stratified by soil type (nutrient-rich alluvium vs. nutrient-poor Kalahari sand covering alluvium) and elephant impact (high vs. low impact on both soil types). We tested four predictions: elephants reduce tree density, richness, and alpha diversity; beta diversity is greater in vegetation subjected to high elephant impact; elephant impact on tree-species composition is greater on nutrient-poor than on nutrient-rich soil; and the loss or decline of abundant tree species on heavily disturbed sites is offset by an increase in abundance of functionally similar species, ones that are minor on lightly disturbed sites. Elephant browsing substantially affected tree-species composition, reducing density, species richness, evenness, and alpha diversity but had no effect on beta diversity. The dominant species on relatively undisturbed areas were partly replaced by functionally similar species on heavily disturbed sites. Soil type influenced species composition on lightly disturbed sites but was less important at higher elephant densities. Our findings are important for areas with extreme dry-season densities of elephants but should not be extrapolated to infer purported effects of elephants on tree diversity at lower densities. © 2014, Springer Science+Business Media New York. Source

Discover hidden collaborations