Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position
    Lagendijk, D.D.G. ; Thaker, M. ; Boer, W.F. de; Page, B.R. ; Prins, H.H.T. ; Slotow, R. - \ 2015
    PLoS ONE 10 (2015)6. - ISSN 1932-6203 - 15 p.
    kruger-national-park - loxodonta-africana - woodland regeneration - aepyceros-melampus - foraging behavior - east-africa - dry-season - savanna - herbivores - population
    Elephant are considered major drivers of ecosystems, but their effects within small-scale landscape features and on other herbivores still remain unclear. Elephant impact on vegetation has been widely studied in areas where elephant have been present for many years. We therefore examined the combined effect of short-term elephant presence (<4 years) and hillslope position on tree species assemblages, resource availability, browsing intensity and soil properties. Short-term elephant presence did not affect woody species assemblages, but did affect height distribution, with greater sapling densities in elephant access areas. Overall tree and stem densities were also not affected by elephant. By contrast, slope position affected woody species assemblages, but not height distributions and densities. Variation in species assemblages was statistically best explained by levels of total cations, Zinc, sand and clay. Although elephant and mesoherbivore browsing intensities were unaffected by slope position, we found lower mesoherbivore browsing intensity on crests with high elephant browsing intensity. Thus, elephant appear to indirectly facilitate the survival of saplings, via the displacement of mesoherbivores, providing a window of opportunity for saplings to grow into taller trees. In the short-term, effects of elephant can be minor and in the opposite direction of expectation. In addition, such behavioural displacement promotes recruitment of saplings into larger height classes. The interaction between slope position and elephant effect found here is in contrast with other studies, and illustrates the importance of examining ecosystem complexity as a function of variation in species presence and topography. The absence of a direct effect of elephant on vegetation, but the presence of an effect on mesoherbivore browsing, is relevant for conservation areas especially where both herbivore groups are actively managed.
    Elephant-mediated habitat modifications and changes in herbivore species assemblages in Sabi Sand, South Africa
    Boer, W.F. de; Oort, J.W.A. van; Grover, M. ; Peel, M.J.S. - \ 2015
    European Journal of Wildlife Research 61 (2015)4. - ISSN 1612-4642 - p. 491 - 503.
    kruger-national-park - colophospermum-mopane - savanna vegetation - woodland structure - woody vegetation - competition - population - season - management - dynamics
    Elephant Loxodonta africana conservation might indirectly influence the wider herbivore community structure, as elephants have the ability to significantly modify the savanna habitat. Uncertainty remains as to the consequences of these effects, as elephants might either compete with other species or facilitate foraging especially for grazers and smaller browsing species by increasing the amount of grass or the amount of browse at lower feeding heights. We studied these potential cascading effects of elephants by using 16 years of data (1992–2011) from the Sabi Sand Wildtuin, South Africa, which showed a steady increase in elephant densities from 0.12 to 2.03 elephants/km2 over this period. We demonstrate that tree densities, and browse availability at feeding heights below 2 m, decreased with increasing elephant densities, and that there was no positive effect of elephants on browse availability. The changes in elephant densities were good predictors (R 2 adj¿>¿0.50) in explaining population fluctuations of other herbivore species. The total body mass of grazers increased more than that of the browsers, shifting the community toward a grazer and megaherbivore-dominated community. An increasing density of elephants changes the composition of the herbivore community, as mesobrowsers are unable to benefit from the impact of elephants on trees, but megagrazers show strong positive responses. Hence, changes in elephant densities as a result of poaching or conservation may trigger cascading community effects. These are neglected but important consequences of (negative or positive) human impacts on elephant numbers, especially in restricted areas such as reserves and national parks.
    Modeling elephant-mediated cascading effects of water point closure
    Hilbers, J.P. ; Langevelde, F. van; Prins, H.H.T. ; Grant, C.C. ; Peel, M. ; Coughenour, M.B. ; Knegt, H.J. de; Slotow, R. ; Smit, I. ; Kiker, G.A. ; Boer, W.F. de - \ 2015
    Ecological Applications 25 (2015)2. - ISSN 1051-0761 - p. 402 - 415.
    kruger-national-park - african savanna - south-africa - distribution patterns - wildlife management - sexual segregation - large herbivores - habitat use - landscape - systems
    Wildlife management to reduce the impact of wildlife on their habitat can be done in several ways, among which removing animals (by either culling or translocation) is most often used. There are however alternative ways to control wildlife densities, such as opening or closing water points. The effects of these alternatives are poorly studied. In this paper, we focus on manipulating large herbivores through the closure of water points (WPs). Removal of artificial WPs has been suggested to change the distribution of African elephants, which occur in high densities in national parks in Southern Africa and are thought to have a destructive effect on the vegetation. Here, we modeled the long-term effects of different scenarios of WP closure on the spatial distribution of elephants, and consequential effects on the vegetation and other herbivores in Kruger National Park, South Africa. Using a dynamic ecosystem model, SAVANNA, scenarios were evaluated that varied in (1) availability of artificial WPs, (2) levels of natural water, and (3) elephant densities. Our modeling results showed that elephants can indirectly negatively affect the distributions of mesomixed feeders, mesobrowsers and some mesograzers under wet conditions. The closure of artificial WPs hardly had any effect during these natural wet conditions. Only under dry conditions the spatial distribution of both elephant bulls and cows changed when the availability of artificial water was severely reduced in the model. These changes in spatial distribution triggered changes in the spatial availability of woody biomass over the simulation period of 80 years and this led to changes in the rest of the herbivore community, resulting in increased densities of all herbivores, except for giraffe and steenbok, in areas close to rivers. The spatial distributions of elephant bulls and cows showed to be less affected by the closure of WPs than most of the other herbivore species. Our study contributes to ecologically informed decisions in wildlife management. The results from this modeling exercise imply that long-term effects of this intervention strategy should always be investigated at an ecosystem scale.
    Towards establishing a rhinoceros-specific interferon-gamma (IFN-y) assay for diagnosis of tuberculosis
    Morar, D. ; Schreuder, J. ; Meny, M. ; Kooten, P.J. ; Tijhaar, E. ; Michel, A.L. ; Rutten, V.P.M.G. - \ 2013
    Transboundary and Emerging Diseases 60 (2013)s1. - ISSN 1865-1674 - p. 60 - 66.
    mycobacterium-bovis bcg - kruger-national-park - african buffalo - cattle - infection - esat-6 - tests - cocktails - antigens - cfp-10
    Mycobacterium bovis is the causal agent of bovine tuberculosis (BTB), with a diverse host range, extending from livestock to domestic and captive wild animals as well as free-ranging wildlife species. In South Africa, BTB is endemic in the Kruger National Park (KNP) and the Hluluwe iMfolozi National Park (HiP), where the high prevalence of M. bovis infections in buffalo herds has led to infection of a number of wildlife species. This has raised concerns about the spillover into the rhinoceros population, a species known to be susceptible to both M. bovis and Mycobacterium tuberculosis, jeopardizing breeding and relocation projects that serve to conserve and protect this species. In view of the advantages of the interferon-gamma (IFN-¿) assay in the diagnosis of BTB in a variety of species worldwide, such an assay has been developed for rhinoceroses by Morar and co-workers in 2007. In this study, this assay was optimized using recombinant eukaryotic rhinoceros IFN-¿ and the lower detection limit was calculated to be 0.5 ng/ml. Subsequently, assessing the detection of native rhinoceros IFN-¿ protein in whole-blood samples revealed stimulation with each of the mitogens: pokeweed (PWM), phytohaemagglutinin (PHA) & phorbol 12-myristate 13-acetate and calcium ionophore (PMA/CaI), though most prominently with the latter two. In addition, samples collected from 52 clinically healthy rhinoceroses, of presumed negative BTB status, from two different areas in South Africa were used to determine the cut-off value for a negative test result. This was calculated to be 0.10 (OD490 nm) and as determined in this study is a preliminary recommendation based on IFN-¿ responses observed in samples from BTB-free rhinoceroses only.
    Herbaceous forage and selection patterns by ungulates across varying herbivore assemblages in a South African savanna
    Treydte, A.C. ; Baumgartner, S. ; Heitkonig, I.M.A. ; Grant, C.C. ; Getz, W.M. - \ 2013
    PLoS ONE 8 (2013)12. - ISSN 1932-6203
    kruger-national-park - large generalist herbivores - spatial heterogeneity - semiarid savanna - diet selection - dry season - vegetation - cattle - landscape - grassland
    Herbivores generally have strong structural and compositional effects on vegetation, which in turn determines the plant forage species available. We investigated how selected large mammalian herbivore assemblages use and alter herbaceous vegetation structure and composition in a southern African savanna in and adjacent to the Kruger National Park, South Africa. We compared mixed and mono-specific herbivore assemblages of varying density and investigated similarities in vegetation patterns under wildlife and livestock herbivory. Grass species composition differed significantly, standing biomass and grass height were almost twice as high at sites of low density compared to high density mixed wildlife species. Selection of various grass species by herbivores was positively correlated with greenness, nutrient content and palatability. Nutrient-rich Urochloa mosambicensis Hack. and Panicum maximum Jacq. grasses were preferred forage species, which significantly differed in abundance across sites of varying grazing pressure. Green grasses growing beneath trees were grazed more frequently than dry grasses growing in the open. Our results indicate that grazing herbivores appear to base their grass species preferences on nutrient content cues and that a characteristic grass species abundance and herb layer structure can be matched with mammalian herbivory types.
    Non-linear partial least square regression increases the estimation accuracy of grass nitrogen and phosphorus using in situ hyperspectral and environmental data
    Ramoelo, A. ; Skidmore, A.K. ; Cho, M.A. ; Mathieu, R. ; Heitkonig, I.M.A. ; Dudeni-Tlhone, N. ; Schlerf, M. ; Prins, H.H.T. - \ 2013
    ISPRS Journal of Photogrammetry and Remote Sensing 82 (2013). - ISSN 0924-2716 - p. 27 - 40.
    kruger-national-park - multiple linear-regression - band-depth analysis - vegetation indexes - south-africa - chlorophyll estimation - imaging spectroscopy - absorption features - biochemical content - mineral-nutrition
    Grass nitrogen (N) and phosphorus (P) concentrations are direct indicators of rangeland quality and provide imperative information for sound management of wildlife and livestock. It is challenging to estimate grass N and P concentrations using remote sensing in the savanna ecosystems. These areas are diverse and heterogeneous in soil and plant moisture, soil nutrients, grazing pressures, and human activities. The objective of the study is to test the performance of non-linear partial least squares regression (PLSR) for predicting grass N and P concentrations through integrating in situ hyperspectral remote sensing and environmental variables (climatic, edaphic and topographic). Data were collected along a land use gradient in the greater Kruger National Park region. The data consisted of: (i) in situ-measured hyperspectral spectra, (ii) environmental variables and measured grass N and P concentrations. The hyperspectral variables included published starch, N and protein spectral absorption features, red edge position, narrow-band indices such as simple ratio (SR) and normalized difference vegetation index (NDVI). The results of the non-linear PLSR were compared to those of conventional linear PLSR. Using non-linear PLSR, integrating in situ hyperspectral and environmental variables yielded the highest grass N and P estimation accuracy (R2 = 0.81, root mean square error (RMSE) = 0.08, and R2 = 0.80, RMSE = 0.03, respectively) as compared to using remote sensing variables only, and conventional PLSR. The study demonstrates the importance of an integrated modeling approach for estimating grass quality which is a crucial effort towards effective management and planning of protected and communal savanna ecosystems.
    Optimization of wildlife management in a large game reserve through waterpoints manipulation: a bio-economic analysis
    Mwakiwa, E. ; Boer, W.F. de; Hearne, J.W. ; Slotow, R. ; Langevelde, F. van; Peel, M. ; Grant, C.C. ; Pretorius, Y. ; Stigter, J.D. ; Skidmore, A.K. ; Heitkonig, I.M.A. ; Knegt, H.J. de; Kohi, E. ; Knox, N. ; Prins, H.H.T. - \ 2013
    Journal of Environmental Management 114 (2013). - ISSN 0301-4797 - p. 352 - 361.
    kruger-national-park - surface-water availability - south-africa - herbaceous vegetation - elephants - provision - impact - conservation - biodiversity - systems
    Surface water is one of the constraining resources for herbivore populations in semi-arid regions. Artificial waterpoints are constructed by wildlife managers to supplement natural water supplies, to support herbivore populations. The aim of this paper is to analyse how a landowner may realize his ecological and economic goals by manipulating waterpoints for the management of an elephant population, a water-dependent species in the presence of water-independent species. We develop a theoretical bio-economic framework to analyse the optimization of wildlife management objectives (in this case revenue generation from both consumptive and non-consumptive use and biodiversity conservation), using waterpoint construction as a control variable. The model provides a bio-economic framework for analysing optimization problems where a control has direct effects on one herbivore species but indirect effects on the other. A landowner may be interested only in maximization of profits either from elephant offtake and/or tourism revenue, ignoring the negative effects that could be brought about by elephants to biodiversity. If the landowner does not take the indirect effects of waterpoints into consideration, then the game reserve management, as the authority entrusted with the sustainable management of the game reserve, might use economic instruments such as subsidies or taxes to the landowners to enforce sound waterpoint management.
    A proposed framework for short-, medium- and long-term responses by range and consumer States to curb poaching for African rhino horn
    Ferreira, S.M. ; Ouma, B.O. - \ 2012
    Pachyderm 51 (2012). - ISSN 1026-2881 - p. 52 - 59.
    kruger-national-park - southern africa - elephants - conservation - population - management - science - impact
    African rhinos are suffering a new poaching onslaught for their high-priced horns. Despite intensified anti- poaching activities, the number of rhinos poached per day has continued to increase since 2008. Between 2010 and 2011 more than 1.5% of the African rhino population was poached each year: a higher percentage is projected for 2012. This trend in increased poaching will reverse overall positive rhino population growth in the long term. In response, a rhino emergency summit comprising representatives of rhino range States, the private sector, government officials and non-governmental organizations met in Nairobi during April 2012. Following this meeting, we propose an integrated framework directed at reducing the demand-and-supply ratio associated with the use of rhino horn. The framework is envisaged to guide short- as well as medium- to long-term responses by range States directed at reducing the incentives for poaching and ensuring the persistence of rhinos.
    Remote sensing of forage nutrients: Combining ecological and spectral absorption feature data
    Knox, N. ; Skidmore, A.K. ; Prins, H.H.T. ; Heitkonig, I.M.A. ; Slotow, R. ; Waal, C. van der; Boer, W.F. de - \ 2012
    ISPRS Journal of Photogrammetry and Remote Sensing 72 (2012). - ISSN 0924-2716 - p. 27 - 35.
    south-african savanna - multiple linear-regression - kruger-national-park - mineral-nutrition - leaf biochemistry - hyperspectral reflectance - nitrogen concentration - imaging spectroscopy - grass - quality
    Forage quality in grassland-savanna ecosystems support high biomass of both wild ungulates and domestic livestock. Forage quality is however variable in both space and time. In this study findings from ecological and laboratory studies, focused on assessing forage quality, are combined to evaluate the feasibility of a remote sensing approach for predicting the spatial and temporal variations in forage quality. Spatially available ecological findings (ancillary data), and physically linked spectral data (absorption data) are evaluated in this study and combined to create models which predict forage quality (nitrogen, phosphorus and fibre concentrations) of grasses collected in the Kruger National Park, South Africa, and analysed in both dry and wet seasons. Models were developed using best subsets regression modelling. Ancillary data alone, could predict forage components, with a higher goodness of fit and predictive capability, than absorption data (Ancillary: R2 adj ¼ 0:42—0:74 compared with absorption: R2 adj ¼ 0:11—0:51, and lower RMSE values for each nutrient produced by the ancillary models). Plant species and soil classes were found to be ecological variables most frequently included in prediction models of ancillary data. Models in which both ancillary and absorption variables were included, had the highest predictive capabilities ( R2 adj ¼ 0:49—0:74 and lowest RMSE values) compared to models where data sources were derived from only one of the two groups. This research provides an important step in the process of creating biochemical models for mapping forage nutrients in savanna systems that can be generalised seasonally over large areas.
    Water-removed spectra increase the retrieval accuracy when estimating savanna grass nitrogen and phosphorus concentrations
    Ramoelo, A. ; Skidmore, A.K. ; Schlerf, M. ; Mathieu, R. ; Heitkonig, I.M.A. - \ 2011
    ISPRS Journal of Photogrammetry and Remote Sensing 66 (2011)4. - ISSN 0924-2716 - p. 408 - 417.
    least-squares regression - multiple linear-regression - kruger-national-park - band-depth analysis - red edge position - reflectance spectra - biochemical concentration - chlorophyll estimation - hyperspectral imagery - diffuse-reflectance
    Information about the distribution of grass foliar nitrogen (N) and phosphorus (P) is important for understanding rangeland vitality and for facilitating the effective management of wildlife and livestock. Water absorption effects in the near-infrared (NIR) and shortwave-infrared (SWIR) regions pose a challenge for nutrient estimation using remote sensing. The aim of this study was to test the utility of water-removed (WR) spectra in combination with partial least-squares regression (PLSR) and stepwise multiple linear regression (SMLR) to estimate foliar N and P, compared to spectral transformation techniques such as first derivative, continuum removal and log-transformed (Log(1/R)) spectra. The study was based on a greenhouse experiment with a savanna grass species (Digitaria eriantha). Spectral measurements were made using a spectrometer. The D. eriantha was cut, dried and chemically analyzed for foliar N and P concentrations. WR spectra were determined by calculating the residual from the modelled leaf water spectra using a nonlinear spectral matching technique and observed leaf spectra. Results indicated that the WR spectra yielded a higher N retrieval accuracy than a traditional first derivative transformation (R2=0.84, RMSE = 0.28) compared to R2=0.59, RMSE = 0.45 for PLSR. Similar trends were observed for SMLR. The highest P retrieval accuracy was derived from WR spectra using SMLR (R2=0.64, RMSE = 0.067), while the traditional first derivative and continuum removal resulted in lower accuracy. Only when using PLSR did the first derivative result in a higher P retrieval accuracy (R2=0.47, RMSE = 0.07) than the WR spectra (R2=0.43, RMSE = 0.070). It was concluded that the water removal technique is a promising technique to minimize the perturbing effect of foliar water content when estimating grass nutrient concentrations
    African elephants (Loxodonta africana) amplify browse heterogeneity in African savanna
    Kohi, E. ; Boer, W.F. de; Peel, M. ; Slotow, R. ; Waal, C. van der; Heitkonig, I.M.A. ; Skidmore, A.K. ; Prins, H.H.T. - \ 2011
    Biotropica 43 (2011)6. - ISSN 0006-3606 - p. 711 - 721.
    kruger-national-park - colophospermum-mopane - semiarid savanna - rumen fermentation - quebracho tannins - western zimbabwe - trees - growth - facilitation - vegetation
    There is a growing concern that the feeding habits of the African elephant, which include pushing over, uprooting and snapping trees, may have a negative impact on other herbivores. Browsed trees are known to respond by either increasing production (shoots and leaves) or defence (secondary compounds). It is not clear, however, what proportion of the browsed biomass can be made available at lower feeding heights after a tree is pushed over or snapped; thus, it is also unclear how the forage quality is affected. In a field survey in Kruger National Park, South Africa, 708 Mopane trees were measured over four elephant utilization categories: snapped trees, pushed-over trees, uprooted trees and control trees. The elephants' impact on the leaf biomass distribution was quantified, and the forage quality (Ca, P, K and Mg, N, digestibility and condensed tannin [CT] concentrations) were analyzed. Pushed-over and uprooted trees had the maximum leaf biomass at lower heights (2 m). In all three utilization categories, the minimum leaf biomass was seven times higher than it was for control trees at a height of below 1 m. Leaf nitrogen content increased in all three categories and was significantly higher in snapped trees. CT concentrations increased slightly in all trees that were utilized by elephants, especially on granitic soils in the dry season. The results provide the insight that elephants facilitate the redistribution and availability of browse and improve the quality, which may positively affect small browsing herbivores.
    Frequent burning promotes invasions of alien plants into a mesic African savanna
    Masocha, M. ; Skidmore, A.K. ; Poshiwa, X. ; Prins, H.H.T. - \ 2011
    Biological Invasions 13 (2011)7. - ISSN 1387-3547 - p. 1641 - 1648.
    kruger-national-park - fire-management - propagule pressure - south-africa - biological invasions - ecology - disturbance - ecosystems - vegetation - evolution
    Fire is both inevitable and necessary for maintaining the structure and functioning of mesic savannas. Without disturbances such as fire and herbivory, tree cover can increase at the expense of grass cover and over time dominate mesic savannas. Consequently, repeated burning is widely used to suppress tree recruitment and control bush encroachment. However, the effect of regular burning on invasion by alien plant species is little understood. Here, vegetation data from a long-term fire experiment, which began in 1953 in a mesic Zimbabwean savanna, were used to test whether the frequency of burning promoted alien plant invasion. The fire treatments consisted of late season fires, lit at 1-, 2-, 3-, and 4-year intervals, and these regularly burnt plots were compared with unburnt plots. Results show that over half a century of frequent burning promoted the invasion by alien plants relative to areas where fire was excluded. More alien plant species became established in plots that had a higher frequency of burning. The proportion of alien species in the species assemblage was highest in the annually burnt plots followed by plots burnt biennially. Alien plant invasion was lowest in plots protected from fire but did not differ significantly between plots burnt triennially and quadrennially. Further, the abundance of five alien forbs increased significantly as the interval (in years) between fires became shorter. On average, the density of these alien forbs in annually burnt plots was at least ten times as high as the density of unburnt plots. Plant diversity was also altered by long-term burning. Total plant species richness was significantly lower in the unburnt plots compared to regularly burnt plots. These findings suggest that frequent burning of mesic savannas enhances invasion by alien plants, with short intervals between fires favouring alien forbs. Therefore, reducing the frequency of burning may be a key to minimising the risk of alien plant spread into mesic savannas, which is important because invasive plants pose a threat to native biodiversity and may alter savanna functioning.
    Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation
    Waal, C. van der; Kool, A. ; Meijer, S.S. ; Kohi, E. ; Heitkonig, I.M.A. ; Boer, W.F. de; Langevelde, F. van; Grant, R.C. ; Peel, M.J.S. ; Slotow, R. ; Knegt, H.J. de; Prins, H.H.T. ; Kroon, H. de - \ 2011
    Oecologia 165 (2011)4. - ISSN 0029-8549 - p. 1095 - 1107.
    south-african savanna - kruger-national-park - tree recruitment - woody cover - bottom-up - fire - plant - nitrogen - grass - ecosystems
    In savannas, the tree-grass balance is governed by water, nutrients, fire and herbivory, and their interactions. We studied the hypothesis that herbivores indirectly affect vegetation structure by changing the availability of soil nutrients, which, in turn, alters the competition between trees and grasses. Nine abandoned livestock holding-pen areas (kraals), enriched by dung and urine, were contrasted with nearby control sites in a semi-arid savanna. About 40 years after abandonment, kraal sites still showed high soil concentrations of inorganic N, extractable P, K, Ca and Mg compared to controls. Kraals also had a high plant production potential and offered high quality forage. The intense grazing and high herbivore dung and urine deposition rates in kraals fit the accelerated nutrient cycling model described for fertile systems elsewhere. Data of a concurrent experiment also showed that bush-cleared patches resulted in an increase in impala dung deposition, probably because impala preferred open sites to avoid predation. Kraal sites had very low tree densities compared to control sites, thus the high impala dung deposition rates here may be in part driven by the open structure of kraal sites, which may explain the persistence of nutrients in kraals. Experiments indicated that tree seedlings were increasingly constrained when competing with grasses under fertile conditions, which might explain the low tree recruitment observed in kraals. In conclusion, large herbivores may indirectly keep existing nutrient hotspots such as abandoned kraals structurally open by maintaining a high local soil fertility, which, in turn, constrains woody recruitment in a negative feedback loop. The maintenance of nutrient hotspots such as abandoned kraals by herbivores contributes to the structural heterogeneity of nutrient-poor savanna vegetation.
    Dry season mapping of savanna forage quality, using the hyperspectral Carnegie
    Knox, N. ; Skidmore, A.K. ; Prins, H.H.T. ; Asner, P. ; Werff, H.M.A. van der; Boer, W.F. de; Waal, C. van der; Knegt, H.J. de; Kohi, E. ; Slotow, R. ; Grant, R.C. - \ 2011
    Remote Sensing of Environment 115 (2011)6. - ISSN 0034-4257 - p. 1478 - 1488.
    kruger-national-park - african savannas - neural-networks - south-africa - absorption features - leaf biochemistry - mineral-nutrition - grass quality - nitrogen - reflectance
    Forage quality within an African savanna depends upon limiting nutrients (nitrogen and phosphorus) and nutrients that constrain the intake rates (non-digestible fibre) of herbivores. These forage quality nutrients are particularly crucial in the dry season when concentrations of limiting nutrients decline and non-digestible fibres increase. Using artificial neural networks we test the ability of a new imaging spectrometer (CAO Alpha sensor), both alone and in combination with ancillary data, to map quantities of grass forage nutrients in the early dry season within an African savanna. Respectively 65%, 57% and 41%, of the variance in fibre, phosphorus and nitrogen concentrations were explained. We found that all grass forage nutrients show response to fire and soil. Principal component analysis, not only reduced image dimensionality, but was a useful method for removing cross-track illumination effects in the CAO imagery. To further improve the mapping of forage nutrients in the dry season we suggest that spectra within the shortwave infrared (SWIR) region, or additional relevant ancillary data, are required.
    Reflectance spectroscopy of biochemical components as indicators of tea, Camellia Sinensis, quality
    Bian, B.M. ; Skidmore, A.K. ; Schlerf, M. ; Fei, T. ; Liu, Y.F. ; Wang, T. - \ 2010
    Photogrammetric Engineering and Remote Sensing 76 (2010)12. - ISSN 0099-1112 - p. 1385 - 1392.
    kruger-national-park - green tea - hyperspectral measurements - spectral characteristics - pattern-recognition - field spectrometry - total polyphenols - vegetation - chemistry - imagery
    The potential of reflectance spectroscopy to estimate the concentration of biochemical compounds related to tea (Camellia sinensis (L.)) quality (total tea polyphenols and free amino acids) is demonstrated. Partial least squares regression (PLSR) was performed to establish the relationship between reflectance and biochemicals for leaf powders as well as fresh leaves. Highest accuracy was found for tea powders with a cross-validated r2 of 0.97 for tea polyphenols and 0.99 for free amino acids, and the root mean square error of cross validations (RMSECVS) are 8.36 mg g-1 and 1.01 mg g-1 for the two chemicals. The accuracy achieved at leaf level was slightly lower, with results yielding cross-validated r2 of 0.91 and 0.93 with RMSECVS of 13.74 mg g-1 and 2.32 mg g-1 for tea polyphenols and free amino acids, respectively. Important wavelengths for prediction of the two biochemicals from powder and leaf spectra were identified using the PLSR bcoefficients as indicators. Wavelengths of 1,131 nm, 1,654 nm, 1,666 nm, 1,738 nm and 1,752 nm were identified as bands related to absorption by total tea polyphenols, while 1,492 nm represented the absorption feature of free amino acids. The results obtained using fresh leaves indicate that hyperspectral remote sensing may be useful for routine monitoring of tea chemistry at landscape scale.
    Enhanced use of beneath-canopy vegetation by grazing ungulates in African savannahs
    Treydte, A.C. ; Riginos, C. ; Jeltsch, F. - \ 2010
    Journal of Arid Environments 74 (2010)12. - ISSN 0140-1963 - p. 1597 - 1603.
    kruger-national-park - large herbivores - forage quality - large trees - habitat use - grass - cattle - productivity - nutrients - tanzania
    The cover of large trees in African savannahs is rapidly declining, mainly due to human land-use practices. Trees improve grass nutrient quality and contribute to species and structural diversity of savannah vegetation. However, the response of herbivores to trees as habitat features is unknown. We quantified the habitat use of wild and domestic ungulates in two eastern and southern African savannahs. We assessed grazing intensities and quantified dung depositions beneath and around canopies of different sized trees. Grasses were eaten and dung was deposited twice as frequently beneath large (ca. 5 m in height) and very large trees (7-10 m) than in open grasslands. Small trees (
    Spatial distribution of lion kills determined by the water dependency of prey species
    Boer, W.F. de; Vis, M.J.P. ; Knegt, H.J. de; Rowles, C. ; Kohi, E. ; Langevelde, F. van; Peel, M.J.S. ; Pretorius, Y. ; Skidmore, A.K. ; Slotow, R. ; Wieren, S.E. van; Prins, H.H.T. - \ 2010
    Journal of Mammalogy 91 (2010)5. - ISSN 0022-2372 - p. 1280 - 1286.
    kruger-national-park - panthera-leo - african herbivores - habitat selection - population-dynamics - hunting success - predation risk - abundance - serengeti - behavior
    Predation risk from lions (Panthera leo) has been linked to habitat characteristics and availability and traits of prey. We separated the effects of vegetation density and the presence of drinking water by analyzing locations of lion kills in relation to rivers with dense vegetation, which offer good lion stalking opportunities, and artificial water points with low vegetation density. The spatial distribution of lion kills was studied at the Klaserie Private Nature Reserve, South Africa. The distance between 215 lion kills and the nearest water source was analyzed using generalized linear models. Lions selected medium-sized prey species. Lion kills were closer to rivers and to artificial water points than expected by random distribution of the kills. Water that attracted prey, and not the vegetation density in riverine areas, increased predation risk, with kills of buffalo (Syncerus caffer), kudu (Tragelaphus strepsiceros), and wildebeest (Connochaetes taurinus) as water-dependent prey species. Traits of prey species, including feeding type (food habits), digestion type (ruminant or nonruminant), or body size, did not explain locations of lion kills, and no seasonal patterns in lion kills were apparent. We argue that the cascading impact of lions on local mammal assemblages is spatially heterogeneous
    Tree cover and biomass increase in a southern African savanna despite growing elephant population
    Kalwij, J.M. ; Boer, W.F. de; Mucina, L. ; Prins, H.H.T. ; Skarpe, C. ; Winterbach, C. - \ 2010
    Ecological Applications 20 (2010)1. - ISSN 1051-0761 - p. 222 - 233.
    kruger-national-park - northern botswana - sexual segregation - large herbivores - watering points - colophospermum-mopane - mammalian herbivores - woody vegetation - fire - woodlands
    The growing elephant populations in many parts of southern Africa raise concerns of a detrimental loss of trees, resulting in overall reduction of biodiversity and ecosystem functioning. Elephant distribution and density can be steered through artificial waterpoints (AWPs). However, this leaves resident vegetation no relief during dry seasons. We studied how the introduction of eight AWPs in 1996 affected the spatiotemporal tree-structure dynamics in central Chobe National Park, an unfenced savanna area in northern Botswana with a dry-season elephant density of ~3.34 individuals per square kilometer. We hypothesized that the impact of these AWPs amplified over time and expanded in space, resulting in a decrease in average tree density, tree height, and canopy volume. We measured height and canopy dimensions of all woody plants around eight artificial and two seasonal waterpoints for 172 plots in 1997, 2000, and 2008. Plots, consisting of 50 × 2 m transects for small trees (0.20–3.00 m tall) nested within 50 × 20 m transects for large trees (=3.0 m tall), were located at 100, 500, 1000, 2000, and 5000 m distance classes. A repeated-measures mixed-effect model showed that tree density, cover, and volume had increased over time throughout the area, caused by a combination of an increase of trees in lower size classes and a decrease in larger size classes. Our results indicate that the decrease of large trees can be attributed to a growing elephant population. Decrease or loss of particular tree size classes may have been caused by a loss of browser-preferred species while facilitating the competitiveness of less-preferred species. In spite of 12 years of artificial water supply and an annual elephant population growth of 6%, we found no evidence that the eight AWPs had a negative effect on tree biomass or tree structure. The decreasing large-tree component could be a remainder of a depleted but currently restoring elephant population
    Forage quality of savannas - Simultaneously mapping foliar protein and polyphenols for trees and grass using hyperspectral imagery
    Skidmore, A.K. ; Ferwerda, J.G. ; Mutanga, O. ; Wieren, S.E. van; Peel, M.J.S. ; Grant, R.C. ; Prins, H.H.T. ; Balcik, F. ; Venus, V. - \ 2010
    Remote Sensing of Environment 114 (2010)1. - ISSN 0034-4257 - p. 64 - 72.
    kruger-national-park - african savanna - south-africa - imaging spectroscopy - mammalian herbivores - mineral-nutrition - diet selection - neural-network - nitrogen - vegetation
    Savanna covers about two-thirds of Africa, with forage quantity and quality being important factors determining the distribution and density of wildlife and domestic stock. Testing hypotheses about the distribution of herbivores is hampered by the absence of reliable methods for measuring the variability of vegetation quality (e.g. biochemical composition) across the landscape. It is demonstrated that hyperspectral remote sensing fills this gap by revealing simultaneously the spatial variation of foliar nitrogen (crude protein) as well as the total amount of polyphenols, in grasses and trees. For the first time, the pattern of resources important for feeding preferences in herbivores (polyphenols and nitrogen) is mapped across an extensive landscape and the modeled foliar concentrations are shown to fit with ecological knowledge of the area. We explain how estimates of nitrogen (crude protein) and polyphenols may be scaled up from point-based observations to reveal their spatial pattern, and how the variation in forage quality can influence the management of savannas, including farms, communal grazing areas, and conservation areas. It provides a glimpse of the choices herbivores must face in selecting food resources of different qualities.
    Tree size and herbivory determine below-canopy grass quality and species composition in savannahs
    Treydte, A.C. ; Grant, C.C. ; Jeltsch, F. - \ 2009
    Biodiversity and Conservation 18 (2009)14. - ISSN 0960-3115 - p. 3989 - 4002.
    kruger-national-park - african savannas - spatial heterogeneity - forage quality - vegetation - productivity - nutrients - impact - soil - availability
    Large single-standing trees are rapidly declining in savannahs, ecosystems supporting a high diversity of large herbivorous mammals. Savannah trees are important as they support both a unique flora and fauna. The herbaceous layer in particular responds to the structural and functional properties of a tree. As shrubland expands stem thickening occurs and large trees are replaced by smaller trees. Here we examine whether small trees are as effective in providing advantages for grasses growing beneath their crowns as large trees are. The role of herbivory in this positive tree-grass interaction is also investigated. We assessed soil and grass nutrient content, structural properties, and herbaceous species composition beneath trees of three size classes and under two grazing regimes in a South African savannah. We found that grass leaf content (N and P) beneath the crowns of particularly large (ca. 3. 5 m) and very large trees (ca. 9 m) was as much as 40% greater than the same grass species not growing under a tree canopy, whereas nutrient contents of grasses did not differ beneath small trees (
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