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.
Structure and composition of woody vegetation around permanent-artificial and ephemeral-natural water points in northern Gonarezhou National Park, Zimbabwe
Gandiwa, E. ; Tupulu, N. ; Zisadza-Gandiwa, P. ; Muvengwi, J. - \ 2012
Tropical Ecology 53 (2012)2. - ISSN 0564-3295 - p. 169 - 175.
herbaceous vegetation - african savanna - elephants - impact - provision - botswana - cover - piosphere - landscape - gradients
The main objective of this study was to compare woody vegetation structure and composition along a distance gradient from permanent-artificial and ephemeral-natural water points in northern Gonarezhou National Park (GNP), Zimbabwe. Woody plants were sampled in May 2010 using a stratified systematic design with plots systematically placed at 100, 500, 1000 and 2000 m from four selected water points. A total of 912 woody plants were assessed in 32 sampling plots and 63 woody plant species were recorded. There were no significant differences in mean height, number of stems per plant, density and diversity with distance from water points. Significant differences in basal areas were only recorded at 2000 m. Only one ephemeral-natural water pan showed a decrease in plant density with increase in distance from the water pan. Our results suggest that there has been some slight degradation of woody vegetation around water points in northern GNP.
Diversity and production of Ethiopian dry woodlands explained by climate- and soil- stress gradients
Eshete, A. ; Sterck, F.J. ; Bongers, F. - \ 2011
Forest Ecology and Management 261 (2011)9. - ISSN 0378-1127 - p. 1499 - 1509.
species-diversity - altitudinal gradients - boswellia-papyrifera - deciduous forest - african savanna - rain-forest - costa-rica - frankincense - regeneration - communities
Dry woodlands cover about 14% of the total African land surface and represent about 25% of the natural vegetation. They are characterized by a seasonal climate, with a dry season of 4–7 months. Large parts of these ecosystems are degrading due to grazing, fire or exploitation by people. We studied species richness and productivity patterns of dry woodlands in Ethiopia. For such ecosystems, classic productivity and diversity hypotheses predict that species richness and productivity increase as the wet season length increases, and decrease when soil conditions create water stress. We inventoried and measured trees in 18 2-ha plots distributed in two sites, one higher altitude site with a shorter wet season than the lower altitude site. We found that the stand volume per hectare was lower in the site with a shorter wet season. Across all 18 plots we observed that stand volume decreased with soil water stress (estimated from texture and depth). This was in line with the prediction. The species richness was lower in the short-wet-season woodlands, but was unaffected by variation in soil conditions. This suggests that climate driven constraints (wet season length) set the limits to species richness, and not soil conditions. As far as we know, this study is one of the first studies that evaluated these productivity and diversity hypotheses for dry African woodlands. --------------------------------------------------------------------------------
Structure and composition of Androstachys johnsonii woodland across various strata in Gonarezhou National Park, southeast Zimbabwe
Gandiwa, E. ; Chikorowondo, G. ; Zisadza-Gandiwa, P. ; Muvengwi, J. - \ 2011
Tropical conservation science 4 (2011)2. - ISSN 1940-0829 - p. 218 - 229.
african savanna - elephants - acacia - fire - regeneration - dynamics - trees - responses
A study on the structure and composition of Androstachys johnsonii Prain (Euphorbiaceae) woodland across three strata was conducted in Gonarezhou National Park (GNP), southeast Zimbabwe. Specifically, the objectives of the study were: (i) to determine the spatial structure and composition of A. johnsonii woodland in GNP and (ii) to determine factors that influence the structure and composition of A. johnsonii woodland in GNP. This study was based on a stratified random design with three major soil groups, and 30 plots were sampled in May 2010. The three soil strata were comprised of soils derived from (i) rhyolite, (ii) malvernia and (iii) granophyre bedrocks. A total of 1258 woody plants were assessed and 41 woody species were recorded. There were significant differences in mean tree heights, tree densities, basal area and species diversity in A. johnsonii woodland across the three soil strata. In contrast, there were no significant differences in the mean number of dead plants per ha in the three study strata in the GNP. Our study findings suggest that A. johnsonii woodland in GNP is being degraded. GNP management should develop a monitoring program through establishing monitoring plots in A. johnsonii woodland, and further studies need to be carried out, particularly on recruitment of A. johnsonii in the GNP.
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
Changes in soil nutrients, vegetation structure and herbaceous biomass in response to grazing in a semi-arid savanna of Ethiopia
Tessema, Z.K. ; Boer, W.F. de; Baars, R.M.T. ; Prins, H.H.T. - \ 2011
Journal of Arid Environments 75 (2011)7. - ISSN 0140-1963 - p. 662 - 670.
middle awash valley - african savanna - south-africa - long-term - nitrogen - redistribution - grasslands - mineralization - phosphorus - rangelands
The effect of grazing was studied on vegetation structure, herbaceous biomass, basal and bare ground covers, together with soil nutrient concentrations in two locations in an Ethiopian semi-arid savanna. The lightly grazed sites had significantly higher herbaceous diversity, total abundance, basal cover and aboveground biomass, and a lower percentage of bare ground compared with the heavy grazed sites. Grazing pressure had no effect on the density and number of woody species as well as on the proportion of encroaching woody species. The light grazing sites had higher organic carbon, phosphorus and exchangeable bases, and therefore a higher pH and higher electrical conductance, indicating an improved soil nutrient status compared with heavy grazing sites, mainly attributed to the higher basal cover and standing biomass at light grazed sites, and the export of nutrients through grazing and dung collection from the heavily grazed sites. There were significantly higher soil nutrients, species diversity, aboveground biomass and basal cover in the light grazing sites compared with heavy grazing sites. We concluded that changes in herbaceous vegetation, standing biomass and soil compositions are caused by interactions between grazing, soil and vegetation, and these interactions determine the transitions of semi-arid savannas.
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
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.
Interacting effects of grass height and herbivores on the establishment of an encroaching savanna shrub
Hagenah, N. ; Munkert, H. ; Gerhardt, K. ; Olff, H. - \ 2009
Plant Ecology 201 (2009)2. - ISSN 1385-0237 - p. 553 - 566.
seedling establishment - african savanna - south-africa - tropical savannas - bruchid beetles - dynamics - fire - woodland - survival - forest
Shrub encroachment is a widely observed problem in Southern African savannas. Although the effects of herbivory and grass height on woody species recruitment have been studied individually, little information exists about how these factors interact. In this study seeds and seedlings of the encroaching shrub Dichrostachys cinerea were planted in clipped and unclipped grass plots, with and without large herbivores present. Seed germination, seedling survival and seedling predation were monitored for 8 months. Germination started earlier in plots where herbivores were excluded. Overall, the earlier the seeds germinated, the longer the seedlings survived. Clipping positively affected the number of germinated seeds, seedling growth and survival but effects varied among herbivore exclusion treatments and sites. Invertebrates caused the majority of the seedling damage. We conclude the recruitment of D. cinerea is influenced by the interplay of grass height and herbivory. In this study, the presence of large herbivores early in the wet season, and the absence of simulated grazing later on, affected the regeneration of D. cinerea negatively. However, differences in effects among sites suggest that the mechanisms found here may work differently in other habitats
Dynamics of grazing lawn formation: An experimental test of the role of scale-dependent processes
Cromsigt, J.P.G.M. ; Olff, H. - \ 2008
Oikos 117 (2008)10. - ISSN 0030-1299 - p. 1444 - 1452.
plant-herbivore interactions - mixed-grass prairie - community structure - growth-responses - african savanna - salt-marsh - serengeti - vegetation - heterogeneity - grazers
Grazing lawns are characteristic for African savanna grasslands, standing out as intensely grazed patches of stoloniferous grazing-tolerant grass species. Grazing lawn development has been associated with grazing and increased nutrient input by large migratory herds. However, we argue that in systems without mass migrations disturbances, other than direct grazing, drive lawn development. Such disturbances, e.g. termite activity or megaherbivore middens, also increase nutrient input and keep the bunch vegetation down for a prolonged time period. However, field observations show that not all such disturbances lead to grazing lawns. We hypothesize that the initial disturbance has to be of a minimal threshold spatial scale, for grazing intensity to be high enough to induce lawn formation. We experimentally tested this idea in natural tall savanna grassland. We mowed different-sized plots to simulate initial disturbances of different scales (six times during one year) and applied fertilizer to half of the plots during two years to simulate increased nutrient input by herbivores or termite activity. Allowing grazing by naturally occurring herbivores, we followed the vegetation development over more than three years. Grazing kept bunch grass short in coarser, fertilized plots, while grasses grew out toward their initial height in fine-scale and unfertilized plots. Moreover, lawn grasses strongly increased in cover in plots with an increased nutrient input but only after coarser scale disturbance. These results support our hypothesis that an increased nutrient input in combination with grazing indeed induces grazing lawn formation, but only above a threshold scale of the initial disturbance. Our results provide an alternative mechanism for the development of grazing lawns in systems that lack mass migrating herds. Moreover, it gives a new spatial dimension to the processes behind grazing lawn development, and hence help to understand how herbivores might create and maintain spatial heterogeneity in grassland systems.
Herbivores as architects of savannas: inducing and modifying spatial vegetation patterning
Knegt, H.J. de; Groen, T.A. ; Vijver, C.A.D.M. van de; Prins, H.H.T. ; Langevelde, F. van - \ 2008
Oikos 117 (2008). - ISSN 0030-1299 - p. 543 - 554.
tree-grass coexistence - african savanna - semiarid savannas - simulation-model - arid ecosystems - grazing systems - dynamics - fire - heterogeneity - stability
In this paper, we address the question whether and through which mechanisms herbivores can induce spatial patterning in savanna vegetation, and how the role of herbivory as a determinant of vegetation patterning changes with herbivore density and the pre-existing pattern of vegetation. We thereto developed a spatially explicit simulation model, including growth of grasses and trees, vertical zonation of browseable biomass, and spatially explicit foraging by grazers and browsers. We show that herbivores can induce vegetation patterning when two key assumptions are fulfilled. First, herbivores have to increase the attractiveness of a site while foraging so that they will revisit this site, e.g. through an increased availability or quality of forage. Second, foraging should be spatially explicit, e.g. when foraging at a site influences vegetation at larger spatial scales or when vegetation at larger spatial scales influences the selection and utilisation of a site. The interaction between these two assumptions proved to be crucial for herbivores to produce spatial vegetation patterns, but then only at low to intermediate herbivore densities. High herbivore densities result in homogenisation of vegetation. Furthermore, our model shows that the pre-existing spatial pattern in vegetation influences the process of vegetation patterning through herbivory. However, this influence decreases when the heterogeneity and dominant scale of the initial vegetation decreases. Hence, the level of adherence of the herbivores to forage in pre-existing patches increases when these pre-existing patches increase in size and when the level of vegetation heterogeneity increases. The findings presented in this paper, and critical experimentation of their ecological validity, will increase our understanding of vegetation patterning in savanna ecosystems, and the role of plant¿herbivore interactions therein.
Savanna fires govern community structure of ungulates in Bénoué National Park, Cameroon
Klop, L.F. ; Goethem, J. - \ 2008
Journal of Tropical Ecology 24 (2008)1. - ISSN 0266-4674 - p. 39 - 47.
species cooccurrence patterns - herbivore assemblages - resource competition - african savanna - ant assemblages - desert rodents - santa-rosalia - south-africa - null model - body-size
We studied the effects of savanna fires on the structure of local ungulate communities in a West African woodland savanna. The distribution of 11 ungulate species over 9¿15 burned sites (the number of which increased as burning activity continued during the dry season) and 7¿13 unburned sites was compared with a variety of null models or randomized `virtual communities¿. Five different parameters of community structure were examined: body mass distribution, co-occurrence patterns, species richness, species density and guild dominance. Overall, ungulate species were not randomly distributed over burned and unburned sites. The regular spacing of body masses in the set of species recorded on burned and unburned sites indicated competition, since species similar in body mass are more likely to compete than species of different size. However, co-occurrence patterns on burned sites were random, indicating absence of competition at fine spatial scales due to differential habitat use within the burned landscape. Although the attractiveness of the regrowth on burned sites resulted in higher numbers of ungulates compared with unburned sites, species richness was not different. Grazers were the dominant guild on burned sites, but there were no differences in species richness or species density between grazers and browsers on unburned sites.
Shifts in native ungulate communities on a former cattle ranch in Tanzania
Treydte, A.C. ; Edwards, P.J. ; Suter, W. - \ 2005
African Journal of Ecology 43 (2005)4. - ISSN 0141-6707 - p. 302 - 311.
african savanna - domestic herbivores - sympatric wild
When an area is brought under protection, current animal populations and their habitat preferences need to be assessed to predict population trends and future habitat availability. Using data from walking transects, we estimated the size of native ungulate populations on an abandoned cattle ranch in a coastal savannah in Tanzania, now included in the new Saadani National Park. Data were analysed with distance sampling and conventional strip transect techniques and were compared with results of previous wildlife counts. Few individuals of mainly browsing species were present in former cattle grazing areas exhibiting high bush-encroachment while a ten times higher biomass of browsers and grazers was found in the cattle-unmodified savannah. Population sizes of some species increased twofold between 1991 and 2001 within the entire area but neither population size nor species richness increased in the abandoned rangeland during our 3-year study period from 2001 to 2003. We conclude that the former ranch has potential for future recolonization by wild ungulates. Resettlement will take place gradually with 'pioneer-species' facilitating the entry of more demanding species. Habitat restoration through wildlife can be observed and quantified on Mkwaja Ranch which will be of importance for future management of native ungulates reclaiming abandoned rangeland.
Changes in woody plant composition of three vegetation types exposed to a similar fire regime for over 46 years
Nangendo, G. ; Stein, A. ; Steege, H. ter; Bongers, F.J.J.M. - \ 2005
Forest Ecology and Management 217 (2005)2-3. - ISSN 0378-1127 - p. 351 - 364.
falls-national-park - long-term exclusion - african savanna - disturbance gradient - species composition - northern botswana - large herbivores - burkea-africana - south-africa - seed banks
The effect of regular burning of woodland vegetation in Africa over an extended period (46 years) was studied in the Murchison Falls National Park, Uganda. Areas identified as having different vegetation types in 1958 were revisited to analyze vegetation changes and to test the prediction that convergence would occur as a result of the regular fire. In each of the three vegetation cover types, a transect was analysed. Results show that the vegetation cover types have changed and there is evidence of vegetation convergence. A detrended correspondence analysis (DCA) and a Morisita similarity index analysis showed a good separation between the 1958 and 2004 transects for each of the vegetation cover types. The DCA also showed that the three 2004 transects were ompositionally closer to each other than the 1958 transects. The 2004 transects had also shifted away from the 1958 transects. Whereas in 1958 compositional similarity was highest between the Terminalia glaucescens conversion transect (T2) and the wooded grassland conversion transect (T3) (0.86), in 2004 it was most similar between the transects closest to Wairingo river (T1 and T2, correlation coefficient of 0.80). T1 was referred to as the Terminalia woodland transect. Comparing the 2004 transects to the 1958 transects, a low compositional similarity was observed. The highest was between T3-1958 and T3-2004 (0.62). Additionally, fewer indicator species (species specific for one transect) were identified in 2004 than in 1958 and, the transects shared more species in 2004 than in 1958. All the big trees (DBH _ 30 cm) that existed along T1 in 1958 died off. Clustering of individual woody plants, a protective mechanism used by plants in presence of fire, was identified in each of the plots analysed. To counterbalance the unifying effect of fire for the vegetation in the area and to maintain diverse vegetation, a variety of fire management regimes are needed.
The influence of savanna trees on nutrient, water and light availability and the understorey vegetation
Ludwig, F. ; Kroon, H. de; Berendse, F. ; Prins, H.H.T. - \ 2004
Plant Ecology 170 (2004)1. - ISSN 1385-0237 - p. 93 - 105.
semi-arid savanna - grass interactions - african savanna - hydraulic lift - plants - kenya - shade - productivity - environments - herbivores
In an East African savanna herbaceous layer productivity and species composition were studied around Acacia tortilis trees of three different age classes, as well as around dead trees and in open grassland patches. The effects of trees on nutrient, light and water availability were measured to obtain an insight into which resources determine changes in productivity and composition of the herbaceous layer. Soil nutrient availability increased with tree age and size and was lowest in open grassland and highest under dead trees. The lower N:P ratios of grasses from open grassland compared to grasses from under trees suggested that productivity in open grassland was limited by nitrogen, while under trees the limiting nutrient was probably P. N:P ratios of grasses growing under bushes and small trees were intermediate between large trees and open grassland indicating that the understorey of Acacia trees seemed to change gradually from a N-limited to a P-limited vegetation. Soil moisture contents were lower under than those outside of canopies of large Acacia trees suggesting that water competition between trees and grasses was important. Species composition of the herbaceous layer under Acacia trees was completely different from the vegetation in open grassland. Also the vegetation under bushes of Acacia tortilis was different from both open grassland and the understorey of large trees. The main factor causing differences in species composition was probably nutrient availability because species compositions were similar for stands of similar soil nutrient concentrations even when light and water availability was different. Changes in species composition did not result in differences in above-ground biomass, which was remarkably similar under different sized trees and in open grassland. The only exception was around dead trees where herbaceous plant production was 60% higher than under living trees. The results suggest that herbaceous layer productivity did not increase under trees by a higher soil nutrient availability, probably because grass production was limited by competition for water. This was consistent with the high plant production around dead trees because when trees die, water competition disappears but the high soil nutrient availability remains. Hence, in addition to tree soil nutrient enrichment, below-ground competition for water appears to be an important process regulating tree-grass interactions in semi-arid savanna
Effects of fire and herbivory on the stability of savanna ecosystems
Langevelde, F. van; Vijver, C.A.D.M. van de; Kumar, L. ; Koppel, J. van de; Ridder, N. de; Andel, J. van; Skidmore, A.K. ; Hearne, J.W. ; Stroosnijder, L. ; Bond, W.J. ; Prins, H.H.T. ; Rietkerk, M. - \ 2003
Ecology 84 (2003)2. - ISSN 0012-9658 - p. 337 - 350.
semiarid grazing systems - african savanna - simulation-model - coexistence - vegetation - dynamics - ecology - catastrophe - management - woodlands
Savanna ecosystems are characterized by the co-occurrence of trees and grasses. In this paper, we argue that the balance between trees and grasses is, to a large extent,determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel, load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous changes, such as bush encroachment, on sandy soils. Such changes, however, are frequently observed. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.