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.
Roads as a threat to the serengeti ecosystem
Fyumagwa, R. ; Gereta, E. ; Hassan, S. ; Kideghesho, J.R. ; Kohi, E. ; Keyyu, J. - \ 2013
Conservation Biology 27 (2013)5. - ISSN 0888-8892 - p. 1122 - 1125.
In an opinion letter¿¿to Nature (September 2010), Dobson et al. (2010) oppose the planned road through northern Tanzania that crosses Serengeti National Park (SNP). They contend that the road will jeopardize the Serengeti ecosystem by interrupting the wildebeest (Connochaetes taurinus) migratory corridor. This opinion is supported by other scientists using mathematical models (Holdo et al. 2011). However, all arguments presented against the project have been questioned (Homewood et al. 2010). As has often been the case in the conservation of African natural resources, some scientists present views that do not account for other key components of conservation: economic growth, reduction of poverty, improvement of quality of life, and social well-being. As scientists working in Serengeti, we believe that the published reports about the Serengeti road mislead the world about its potential effects on the ecosystem.
The bulldozer herbivore: how animals benefit from elephant modifying an African savanna
Kohi, E. - \ 2013
Wageningen University. Promotor(en): Herbert Prins, co-promotor(en): Fred de Boer. - S.l. : s.n. - ISBN 9789461735355 - 170
herbivoren - loxodonta africana - afgrazen - voedingsgewoonten - habitats - diergedrag - dierecologie - savannen - afrika - herbivores - loxodonta africana - browsing - feeding habits - habitats - animal behaviour - animal ecology - savannas - africa
Herbivore-vegetation interactions are important structuring forces in savanna that modify the availability and quality of forage resources. Elephant for example, are known for their ability to change the vegetation structure through toppling trees, uprooting, snapping, debarking and breaking branches. Controlling the number of elephant is a common response of wildlife managers who think that the increase of elephant will further destroy the habitat and hence cause loss of biodiversity. However, our knowledge on how elephant feeding habits affect other large herbivore species in habit use is limited. Therefore, the question in this thesis is: What is the impact of elephant feeding habits on species diversity of large herbivores in African savanna? To answer this question, it is important to understand the responses of trees when impacted by elephant. What proportion of the browsed biomass is made available after a tree is pushed over or snapped by elephant? How is the forage quality affected? Is the seasonal and intensity of browsing affecting forage availability? Field experiments and field surveys were used to investigate the tree’s response and herbivore species responses to elephant impact. These experiments were (1) simulation of timing and intensity of browsing (hand defoliation) and (2) manipulation of vegetation i.e. simulated pushed over trees, uprooted trees (tree removal) and snapped trees (tree cut at the stem). The field survey involved measuring impacted trees by elephant. Leaf biomass and quality of pushed over, snapped and uprooted trees were measured. The defoliation experiment was conducted in the roan antelope enclosure in Kruger National Park South Africa, and the vegetation manipulation experiment was conducted in the Umbabati Private Nature Reserve, South Africa.
The results indicate that elephant foraging habits change the distribution of forage, increasing the forage availability at lower feeding heights, which means that accessibility of forage to medium and small herbivores increases. Elephant browsing also improved forage quality and availability in the dry season, which is very important to browsing animals. A high intensity of browsing by elephant in the wet season increased the dry season forage, because the amount of new regrowth (leave compensation) is proportional to the amount of leaves that was removed. Elephant therefore initiate inter and intra-species facilitation processes. Inter-species facilitation occurs when other herbivores species utilize the regrowth stimulated by elephant, whereas intra-species facilitation occurs when the browse resource is exploited by other elephants. The herbivore responses to elephant browsing clearly indicated that facilitation effects occur, especially for certain guilds when selecting their habitat. For example, small predation-sensitive herbivore species (steenbok, impala and common duiker) preferred completely opened up areas, whereas large herbivores were less affected in their habitat preference by elephant impact. Greater kudu selected pushed over and control plots and rarely visited opened up areas. These differential response of herbivores species to elephant impact resulted in a high species richness of large mammals in elephant impacted areas. In conclusion, elephant feeding habits play a major role in structuring the herbivore assemblage/community through modifying the vegetation. Resource heterogeneity increased under the influence of elephant feeding, in particular through increasing the accessibility of leaf biomass at lower feeding heights, increasing green leaf availability in the dry season, and improving the nutrient content in re-growth. Moreover, habitat selection, especially that of small herbivores was positively influenced by elephant impact. With these findings, this study contributes to a better understanding of the role of elephant feeding habits and its cascading effects to other herbivore species.
Differentiation of plant age in grasses using remote sensing
Knox, N. ; Skidmore, A.K. ; Werff, H.M.A. van der; Groen, T.A. ; Boer, W.F. de; Prins, H.H.T. ; Kohi, E. ; Peel, M. - \ 2013
International Journal of applied Earth Observation and Geoinformation 24 (2013)10. - ISSN 0303-2434 - p. 54 - 62.
difference water index - monitoring vegetation - nitrogen concentration - imaging spectroscopy - hyperspectral data - boreal regions - time-series - green-up - phenology - reflectance
Phenological or plant age classification across a landscape allows for examination of micro-topographical effects on plant growth, improvement in the accuracy of species discrimination, and will improve our understanding of the spatial variation in plant growth. In this paper six vegetation indices used in phenological studies (including the newly proposed PhIX index) were analysed for their ability to statistically differentiate grasses of different ages in the sequence of their development. Spectra of grasses of different ages were collected from a greenhouse study. These were used to determine if NDVI, NDWI, CAI, EVI, EVI2 and the newly proposed PhIX index could sequentially discriminate grasses of different ages, and subsequently classify grasses into their respective age category. The PhIX index was defined as: (An VNIR+ log(An SWIR2))/(An VNIR- log(An SWIR2)), where An VNIRand An SWIR2are the respective nor- malised areas under the continuum removed reflectance curve within the VNIR (500-800 nm) and SWIR2 (2000-2210 nm) regions. The PhIX index was found to produce the highest phenological classification accuracy (Overall Accuracy: 79%, and Kappa Accuracy: 75%) and similar to the NDVI, EVI and EVI2 indices it statistically sequentially separates out the developmental age classes. Discrimination between seedling and dormant age classes and the adult and flowering classes was problematic for most of the tested indices. Combining information from the visible near infrared (VNIR) and shortwave infrared region (SWIR) region into a single phenological index captures the phenological changes associated with plant pigments and the ligno-cellulose absorption feature, providing a robust method to discriminate the age classes of grasses. This work provides a valuable contribution into mapping spatial variation and monitoring plant growth across savanna and grassland 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.
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.
|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.
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
Effects of simulated browsing on growth and leaf chemical properties in Colophospermum mopane saplings
Kohi, E. ; Boer, W.F. de; Slot, M. ; Wieren, S.E. van; Ferwerda, J.G. ; Grant, R.C. ; Heitkonig, I.M.A. ; Knegt, H.J. de; Knox, N. ; Langevelde, F. van; Peel, M.J.S. ; Slotow, R. ; Waal, C. van der; Prins, H.H.T. - \ 2010
African Journal of Ecology 48 (2010)1. - ISSN 0141-6707 - p. 190 - 196.
african savanna - responses - acacia - tannin - herbivores - benefits - defense - biomass - plants - damage
Browsing intensity influences a plant's response to herbivory. Plants face a trade-off between investment in the production of secondary compounds and investment in growth. To elucidate this trade-off, we simulated four browsing intensities (0%, 50%, 75% and 100%) on mopane saplings, Colophospermum mopane (J.Kirk ex Benth.) J.Léonard, in a greenhouse experiment. This showed that, with increasing defoliation intensity, plants change their investment strategy. At intermediate levels of defoliation (50%), mopane saplings increased the synthesis of condensed tannins, so that tannin concentrations followed a hump-shaped relation with defoliation intensity, with significantly higher tannin concentration at intermediate defoliation levels. When defoliated heavily (75% and 100%), tannin concentrations dropped, and plants were carbon stressed as indicated by a reduced growth rate of the stem diameter, and leaf production and mean individual leaf mass were reduced. This suggests that, at intermediate defoliation intensity, the strategy of the plants is towards induced chemical defences. With increasing defoliation, the relative costs of the secondary metabolite synthesis become too high, and therefore, the plants change their growing strategy. Hence, browsers should be able to benefit from earlier browsing by either adopting a low or a relatively high browsing pressure
|Large herbivore responses to nutrient heterogeneity in an African savanna
Pretorius, Y. ; Boer, W.F. de; Coughenour, M.B. ; Knegt, H.J. de; Kroon, H. de; Grant, C.C. ; Heitkonig, I.M.A. ; Kohi, E. ; Mwakiwa, E. ; Peel, M.J.S. ; Skidmore, A.K. ; Slotow, R. ; Waal, C. van der; Langevelde, F. van; Wieren, S.E. van - \ 2009
In: Satisfying Giant Appetites / Pretorius, Y., Wageningen : Wageningen University - ISBN 9789085854784 - p. 39 - 56.
|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, C.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. - \ 2009
In: Satisfying Giant Appetites / Pretorius, Y., Wageningen : Wageningen University - ISBN 9789085854784 - p. 57 - 76.
Positive Aspects of Elephants ... an Experiment
Boer, W.F. de; Kohi, E. - \ 2009
Kruger Park Times (2009). - p. 6 - 6.
Water and nutrients alter herbaceous competitive effects on tree seedlings in a semi-arid savanna
Waal, C. van der; Kroon, H. de; Boer, W.F. de; Heitkonig, I.M.A. ; Skidmore, A.K. ; Knegt, H.J. de; Langevelde, F. van; Wieren, S.E. van; Grant, R.C. ; Page, B.R. ; Slotow, R. ; Kohi, E. ; Mwakiwa, E. ; Prins, H.H.T. - \ 2009
Journal of Ecology 97 (2009)3. - ISSN 0022-0477 - p. 430 - 439.
african savannas - colophospermum-mopane - grass competition - soil-moisture - south-africa - woody cover - fire - establishment - vegetation - acacia
1. Globally, both climatic patterns and nitrogen deposition rates show directional changes over time. It is uncertain how woody seedlings, which coexist with herbaceous plants in savannas, respond to concurrent changes in water and nutrient availability. 2. We investigated competition effects between herbaceous vegetation and tree seedlings (Colophospermum mopane) under changed water and nutrient (fertilized) conditions in a garden experiment situated in a semi-arid savanna. 3. Herbaceous competition significantly suppressed woody seedling growth. The effect of herbaceous competition on woody seedling growth remained constant with both increasing water and nutrient availability. However, during a wet-season drought, herbaceous competition apparently caused premature leaf senescence in non-irrigated treatments. Fertilization exacerbated negative competition effects during the drought, while irrigation prevented leaf loss of tree seedlings in spite of herbaceous competition and fertilization. 4. Based on a conceptual model, we propose that the vigorous response of herbaceous plants to increased nutrient availability leads to faster depletion of soil water, which increasingly causes water stress in woody seedlings if the interval between watering events is prolonged, e.g. during wet-season droughts. 5. Synthesis. Our data support the notion that changes in drought frequency are of greater importance to woody recruitment success than changes in annual rainfall amount. Based on the water and nutrient interactions observed in our experiment, we suggest that the effect of increased nitrogen deposition on woody seedling recruitment is contingent on water availability.