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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    Optimization of net returns from wildlife consumptive and non-consumptive uses by game reserve management
    Mwakiwa, E. ; Hearne, J.W. ; Stigter, J.D. ; Boer, W.F. de; Henley, M. ; Slotow, R. ; Langevelde, F. van; Peel, M. ; Grant, C.C. ; Prins, H.H.T. - \ 2016
    Environmental Conservation 43 (2016)2. - ISSN 0376-8929 - p. 128 - 139.
    Landowners and game reserve managers are often faced with the decision whether to undertake consumptive (such as hunting) and/or non-consumptive (such as tourism) use of wildlife resources on their properties. Here a theoretical model was used to examine cases where the game reserve management allocated the amount of land devoted to hunting (trophy hunting) and tourism, based on three scenarios: (1) hunting is separated from tourism but wildlife is shared; (2) hunting and tourism co-exist; and (3) hunting and tourism are separated by a fence. The consumptive and non-consumptive uses are not mutually exclusive; careful planning is needed to ensure that multiple management objectives can be met. Further, the analysis indicates that the two uses may be undertaken in the same area. Whether they are spatially, or temporally separated depends on the magnitude of the consumptive use. When consumptive use is not dominant, the two are compatible in the same shared area, provided the wildlife population is sufficiently large.
    Why elephant have trunks and giraffe long tongues: how plants shape large herbivore mouth morphology
    Pretorius, Y. ; Boer, W.F. de; Kortekaas, K. ; Wijngaarden, M. van; Grant, R.C. ; Kohi, E.M. ; Mwakiwa, E. ; Slotow, R. ; Prins, H.H.T. - \ 2016
    Acta Zoologica 97 (2016)2. - ISSN 0001-7272 - p. 246 - 254.
    We investigated whether mass and morphological spatial patterns in plants possibly induced the development of enlarged soft mouth parts in especially megaherbivores. We used power functions and geometric principles to explore allometric relationships of both morphological and foraging characteristics of mammalian herbivores in the South African savannah, covering a body size range of more than three orders magnitude. Our results show that, although intradental mouth volume scaled to a power slightly less than one to body mass, actual bite volume, as measured in the field, scaled to body mass with a factor closer to 1.75. However, when including the volume added to intradental mouth volume by soft mouth parts, such as tongue and lips (or trunks in elephant), mouth volume scaled linearly with actual bite volume and in a similar fashion as actual bite volume to body size. Bite mass and bite leaf mass scaled linearly with body size.We conclude that these scaling relationships indicate that large herbivores use their enlarged soft mouth parts to not only increase bite volume and thereby bite mass, but also select soft plant parts and thereby increase the leaf mass fraction per bite.
    Intrinsic and extrinsic factors influencing large African herbivore movements
    Venter, J.A. ; Prins, H.H.T. ; Mashanova, A. ; Boer, W.F. de; Slotow, R. - \ 2015
    Ecological Informatics 30 (2015). - ISSN 1574-9541 - p. 257 - 262.
    Understanding environmental as well as anthropogenic factors that influence large herbivore ecological patterns and processes should underpin their conservation and management. We assessed the influence of intrinsic, extrinsic environmental and extrinsic anthropogenic factors on movement behaviour of eight African large herbivore species. A cumulative odds ordinal logistic regression was used to determine the effect of season, feeding niche, number of vegetation types, home range size, and fences on the number of exponential distributions observed. When animals faced the trade-off between forage quality and quantity during the dry season, they moved further between forage areas and water sources in order to get to better forage, which added to the number of movement scales observed. Elephants had a lower number of movement scales, compared to all the other feeding types, which could be attributed to them being able to switch between browse and graze. The number of movement scales increased in more heterogeneous areas. Animals with larger home ranges, which are also larger species, and animals more restricted by fences, had fewer movement scales. In order for managers to effectively manage protected areas and associated biodiversity they need take cognisance of the different scales animals operate under, and the different factors that may be important for different species.
    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.
    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.
    Forage patch use by grazing herbivores in a South African grazing ecosystem
    Venter, J.A. ; Nabe-Nielsen, J. ; Prins, H.H.T. ; Slotow, R. - \ 2014
    Acta Theriologica 59 (2014)3. - ISSN 0001-7051 - p. 457 - 466.
    predation risk - heterogeneous pastures - antipredator response - distribution patterns - behavioral-responses - habitat selection - seed dispersal - burned patches - wolf predation - elk
    Understanding how different herbivores make forage patch use choices explains how they maintain an adequate nutritional status, which is important for effective conservation management of grazing ecosystems. Using telemetry data, we investigated nonruminant zebra (Equus burchelli) and ruminant red hartebeest (Alcelaphus buselaphus subspecies camaa), use of burnt patches in a landscape mosaic of nutrient-poor, old grassland interspersed with young, recently burnt, nutrient-rich grass patches. The Mkambati Nature Reserve landscape on the east coast of South Africa provided large grazers with a challenge in finding and using appropriate patches in which to forage to meet their nutritional requirements. In Mkambati, grassland fires, mostly ignited by poachers, induce regrowth of young nutrient-rich grass, which subsequently attract grazers. We tested if the study animals foraged more in burnt patches than in the unburned grassland and whether burnt patch use was related to the distance to the previously visited burnt patch, burnt patch size, burnt patch age, and distance to areas with high poaching risk using MANOVA. In general, zebra moved faster than red hartebeest, and both species moved faster in unburnt grassland than in burnt patches. Red hartebeest and zebra patch selection were influenced by interpatch distance, patch age, patch size, and poaching risk. A limited set of intrinsic traits, i.e., body mass, digestion strategy, and muzzle width, yielded different patch use rules for the two species. Large ungulates patch use behaviour varied among species and across conditions and was influenced by anthropogenic impacts such as poaching and changed fire regimes. This could potentially affect biodiversity negatively and needs to be factored into management of conservation areas.
    Reconstructing grazer assemblages for protected area restoration
    Venter, J.A. ; Prins, H.H.T. ; Slotow, R. - \ 2014
    PLoS ONE 9 (2014)3. - ISSN 1932-6203 - 10 p.
    south-african savanna - middle stone-age - conservation management - distribution patterns - foraging behavior - landscape-scale - herbivore - fire - heterogeneity - biodiversity
    Protected area management agencies often struggle to reliably reconstruct grazer assemblages due to a lack of historical distribution data for their regions. Wrong predictions of grazing assemblages could potentially affect biodiversity negatively. The objective of the study was to determine how well grazing herbivores have become established since introduction to the Mkambati Nature Reserve, South Africa, how this was influenced by facilitation and competition, and how indigenous grazer assemblages can best be predicted for effective ecological restoration. Population trends of several grazing species were investigated in in order to determine how well they have become established since introduction. Five different conceivable grazing assemblages reflecting a range of approaches that are commonly encountered during conservation planning and management decision making were assessed. Species packing was used to predict whether facilitation, competition or co-existence were more likely to occur, and the species packing of the different assemblages were assessed using ANCOVA. Reconstructing a species assemblage using biogeographic and biological information provides the opportunity for a grazer assemblage that allows for facilitatory effects, which in turn leads to an ecosystem that is able to maintain its grazer assemblage structure. The strength of this approach lies in the ability to overcome the problem of depauperate grazer assemblages, resulting from a lack of historical data, by using biogeographical and biological processes, to assist in more effectively reconstructing grazer assemblages. Adaptive management of grazer assemblage restoration through reintroduction, using this approach would further mitigate management risks.
    Influence of predators on the spatial distribution and movement of their prey
    Langevelde, F. van; Woersem, A. ; Boer, W.F. de; Bie, S. de; Slotow, R. ; Burger, A. ; Swart, J. ; Prins, H.H.T. - \ 2013
    In: 11th Savanna Science Network Meeting. - Skukuza, South Africa : South African National Parks, Scientific Services - p. 44 - 44.
    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.
    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.
    Diet selection of African elephant over time shows changing optimization currency
    Pretorius, Y. ; Stigter, J.D. ; Boer, W.F. de; Wieren, S.E. van; Jong, C.B. de; Knegt, H.J. de; Grant, R.C. ; Heitkonig, I.M.A. ; Knox, N. ; Kohi, E. ; Mwakiwa, E. ; Peel, M.J.S. ; Skidmore, A.K. ; Slotow, R. ; Waal, C. van der; Langevelde, F. van; Prins, H.H.T. - \ 2012
    Oikos 121 (2012)12. - ISSN 0030-1299 - p. 2110 - 2120.
    mammalian herbivores - geometrical approach - loxodonta-africana - national-park - food - quality - forage - trees - terrestrial - complexity
    Multiple factors determine diet selection of herbivores. However, in many diet studies selection of single nutrients is studied or optimization models are developed using only one currency. In this paper, we use linear programming to explain diet selection by African elephant based on plant availability and nutrient and deterrent content over time. Our results indicate that elephant at our study area maximized intake of phosphorus throughout the year, possibly in response to the deficiency of this nutrient in the region. After adjusting the model to incorporate the effects of this deficiency, elephant were found to maximize nitrogen intake during the wet season and energy during the dry season. We reason that the increased energy requirements during the dry season can be explained by seasonal changes in water availability and forage abundance. As forage abundance decrease into the dry season, elephant struggle to satisfy their large absolute food requirements. Adding to this restriction is the simultaneous decrease in plant and surface water availability, which force the elephant to seek out scarce surface water sources at high energy costs. During the wet season when food becomes more abundant and energy requirements are satisfied easier, elephant aim to maximize nitrogen intake for growth and reproduction. Our study contributes to the emerging theory on understanding foraging for multiple resources
    Seasonal diet changes in elephant and impala in mopane woodland
    Kos, M. ; Hoetmer, A.J. ; Pretorius, Y. ; Boer, W.F. de; Knegt, H.J. de; Grant, C.C. ; Kohi, E. ; Page, B. ; Peel, M. ; Slotow, R. ; Waal, C. van der; Wieren, S.E. van; Prins, H.H.T. ; Langevelde, F. van - \ 2012
    European Journal of Wildlife Research 58 (2012)1. - ISSN 1612-4642 - p. 279 - 287.
    colophospermum-mopane - aepyceros-melampus - foraging behavior - national-park - herbivores - digestion - ecology - goats - trees - serengeti
    Elephant and impala as intermediate feeders, having a mixed diet of grass and browse, respond to seasonal fluctuations of forage quality by changing their diet composition. We tested the hypotheses that (1) the decrease in forage quality is accompanied by a change in diet from more monocots in the wet season to more dicots in the dry season and that that change is more pronounced and faster in impala than in elephant; (2) mopane (Colophospermum mopane), the most abundant dicot species, is the most important species in the elephant diet in mopane woodland, whereas impala feed relatively less on mopane due to the high condensed tannin concentration; and (3) impala on nutrient-rich soils have a diet consisting of more grass and change later to diet of more browse than impala on nutrient-poor soils. The phosphorus content and in vitro digestibility of monocots decreased and the NDF content increased significantly towards the end of the wet season, whereas in dicots no significant trend could be detected. We argue that this decreasing monocot quality caused elephant and impala to consume more dicots in the dry season. Elephant changed their diet gradually over a 16-week period from 70% to 25% monocots, whereas impala changed diets rapidly (2-4 weeks) from 95% to 70% monocots. For both elephants and impala, there was a positive correlation between percentage of monocots and dicots in the diet and the in vitro digestibility of these forage items. Mopane was the most important dicot species in the elephant diet and its contribution to the diet increased significantly in the dry season, whereas impala selected other dicot species. On nutrient-rich gabbroic soils, impala ate significantly more monocots than impala from nutrient-poor granitic soils, which was related to the higher in vitro digestibility of the monocots on gabbroic soil. Digestibility of food items appears to be an important determinant of diet change from the wet to the dry season in impala and elephants.
    Soil nutrient status determines how elephant utilize trees and shape environments
    Pretorius, Y. ; Boer, W.F. de; Waal, C. van der; Knegt, H.J. de; Grant, R.C. ; Knox, N. ; Kohi, E. ; Mwakiwa, E. ; Page, B.R. ; Peel, M.J.S. ; Skidmore, A.K. ; Slotow, R. ; Wieren, S.E. van; Prins, H.H.T. - \ 2011
    Journal of Animal Ecology 80 (2011)4. - ISSN 0021-8790 - p. 875 - 883.
    large herbivores - national-park - habitat selection - african savannas - woody vegetation - patch structure - forage quality - food quality - south-africa - east-africa
    Elucidation of the mechanism determining the spatial scale of patch selection by herbivores has been complicated by the way in which resource availability at a specific scale is measured and by vigilance behaviour of the herbivores themselves. To reduce these complications, we studied patch selection by an animal with negligible predation risk, the African elephant. We introduce the concept of nutrient load as the product of patch size, number of patches and local patch nutrient concentration. Nutrient load provides a novel spatially explicit expression of the total available nutrients a herbivore can select from. We hypothesized that elephant would select nutrient-rich patches, based on the nutrient load per 2500m2 down to the individual plant scale, and that this selection will depend on the nitrogen and phosphorous contents of plants. We predicted that elephant would cause more adverse impact to trees of lower value to them in order to reach plant parts with higher nutrient concentrations such as bark and root. However, elephant should maintain nutrient-rich trees by inducing coppicing of trees through re-utilization of leaves. 5.Elephant patch selection was measured in a homogenous tree species stand by manipulating the spatial distribution of soil nutrients in a large field experiment using NPK fertilizer. Elephant were able to select nutrient-rich patches and utilized Colophospermum mopane trees inside these patches more than outside, at scales ranging from 2500 down to 100m2. Although both nitrogen and phosphorus contents of leaves from C. mopane trees were higher in fertilized and selected patches, patch choice correlated most strongly with nitrogen content. As predicted, stripping of leaves occurred more in nutrient-rich patches, while adverse impact such as uprooting of trees occurred more in nutrient-poor areas. Our results emphasize the necessity of including scale-dependent selectivity in foraging studies and how elephant foraging behaviour can be used as indicators of change in the availability of nutrients.
    Scale of nutrient patchiness mediates resource partitioning between trees and grasses in a semi-arid savanna
    Waal, C. van der; Kroon, H. de; Heitkonig, I.M.A. ; Skidmore, A.K. ; Langevelde, F. van; Boer, W.F. de; Slotow, R. ; Grant, R.C. ; Peel, M.P.S. ; Kohi, E. ; Knegt, H.J. de; Prins, H.H.T. - \ 2011
    Journal of Ecology 99 (2011)5. - ISSN 0022-0477 - p. 1124 - 1133.
    african savanna - south-africa - woody cover - cattle dung - heterogeneity - vegetation - dynamics - nitrogen - water - availability
    1. Scaling theory predicts that organisms respond to different scales of resource patchiness in relation to their own size. We tested the hypothesis that the scale of nutrient patchiness mediates resource partitioning between large trees and small grasses in a semi-arid savanna. 2. In a factorial field experiment, Colophospermum mopane trees and associated grasses were fertilized at either a fine or coarse scale of patchiness with nitrogen (N), phosphorus (P) or N + P. The growth of marked tree shoots, herbaceous biomass and leafNand P concentrations were monitored for 2 years following fertilization. 3. Responses of trees were partly scale dependent. Tree leaf N concentration and shoot length relatively increased with fertilization at a coarse scale. Tree leaf mass decreased when P was supplied at a fine scale of patchiness, suggesting intensified grass competition. 4. Phosphorus fertilization increased leaf P concentrations more in grasses than trees, whereas N fertilization increased leaf N concentration moderately in both trees and grasses. Herbaceous above-ground biomass around focal trees was negatively correlated with tree size when fertilized with N, suggesting intensified tree competition. 5. Synthesis. Our results support the hypothesis that trees benefit more from nutrients supplied at a relatively coarse scale of patchiness. No direct responses of grasses to scale were detected. In trees, the scale effect was surpassed by the effect of sample year, when rainfall varied
    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.
    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.
    The spatial scaling of habitat selection by African elephants
    Knegt, H.J. de; Langevelde, F. van; Skidmore, A.K. ; Delsink, A. ; Slotow, R. ; Henley, S. ; Bucini, G. ; Boer, W.F. de; Coughenour, M.B. ; Grant, C.C. ; Heitkonig, I.M.A. ; Henley, M. ; Knox, N. ; Kohi, E. ; Mwakiwa, E. ; Page, B.R. ; Peel, M. ; Pretorius, Y. ; Wieren, S.E. van; Prins, H.H.T. - \ 2011
    Journal of Animal Ecology 80 (2011)1. - ISSN 0021-8790 - p. 270 - 281.
    niche factor-analysis - ecological-niche - national-park - large herbivores - landscape - movement - patterns - heterogeneity - distributions - availability
    1. Understanding and accurately predicting the spatial patterns of habitat use by organisms is important for ecological research, biodiversity conservation and ecosystem management. However, this understanding is complicated by the effects of spatial scale, because the scale of analysis affects the quantification of species–environment relationships. 2. We therefore assessed the influence of environmental context (i.e. the characteristics of the landscape surrounding a site), varied over a large range of scales (i.e. ambit radii around focal sites), on the analysis and prediction of habitat selection by African elephants in Kruger National Park, South Africa. 3. We focused on the spatial scaling of the elephants’ response to their main resources, forage and water, and found that the quantification of habitat selection strongly depended on the scales at which environmental context was considered. Moreover, the inclusion of environmental context at characteristic scales (i.e. those at which habitat selectivity was maximized) increased the predictive capacity of habitat suitability models. 4. The elephants responded to their environment in a scale-dependent and perhaps hierarchical manner, with forage characteristics driving habitat selection at coarse spatial scales, and surface water at fine spatial scales. 5. Furthermore, the elephants exhibited sexual habitat segregation, mainly in relation to vegetation characteristics. Male elephants preferred areas with high tree cover and low herbaceous biomass, whereas this pattern was reversed for female elephants. 6. We show that the spatial distribution of elephants can be better understood and predicted when scale-dependent species–environment relationships are explicitly considered. This demonstrates the importance of considering the influence of spatial scale on the analysis of spatial patterning in ecological phenomena.
    Visited sites revisited - site fidelity in African elephants
    Knegt, H.J. de; Pretorius, Y. ; Langevelde, F. van; Boer, W.F. de; Gort, G. ; Skidmore, A.K. ; Slotow, R. ; Henley, S. ; Delsink, A. ; Grant, C.C. ; Prins, H.H.T. - \ 2010
    In: Beyond the here and now / de Knegt, H.J., Wageningen : Wageningen University - ISBN 9789085856283 - p. 35 - 46.
    Large herbivores alter vegetation structure through soil nutrient mediation in a semi-arid savanna: Lessons from a natural experiment with abandoned kraals
    Waal, C. van der; Kool, A. ; Meijer, S.S. ; Kroon, H. de; Kohi, E. ; Heitkonig, I.M.A. ; Boer, W.F. de; Langevelde, F. van; Grant, R.C. ; Peel, M.J.S. ; Slotow, R. ; Skidmore, A.K. ; Knegt, H.J. de; Mwakiwa, E. ; Pretorius, Y. ; Wieren, S.E. van; Prins, H.H.T. - \ 2010
    In: Nutrients in African Savanna / van der Waal, C., Wageningen : Wageningen University - ISBN 9789085856740 - p. 83 - 102.
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