Field observations in the Serengeti National Park, Tanzania have revealed several modes of grazing system, including migration, sustained-yield grazing and grazing succession which contribute on the one hand to ecological separation of herbivores by habitat choice, but on the other hand to positive association of species on wet season pastures known as hot spots. Drawing from results of experimental field studies, we consider what is known about resource competition and facilitation, the two key ecological processes underlying these grazing systems, and specify the shape of the relationship between species richness and pasture height that is predicted depending on which process is dominating in the grazing system. We test between these alternatives by analysing for the first time data on herbivore distributions and vegetation condition from a series of 33 survey flights over the entire Serengeti ecosystem. Our preliminary results strongly support the importance of facilitation as a process structuring the herbivore community of the Serengeti Plains in the wet season. Resource competition may limit use of the shortest swards

Baird, D. R.

Murray, M. G.

English

University of Kentucky

University of Kentucky UKnowledge International Grassland Congress Proceedings XIX International Grassland Congress The Influence of Large Animal Diversity in Grazed Ecosystems M. G. Murray University of Edinburgh, UK D. R. Baird University of Edinburgh, UK Follow this and additional works at: https://uknowledge.uky.edu/igc  Part of the Plant Sciences Commons, and the Soil Science Commons This document is available at https://uknowledge.uky.edu/igc/19/23/22 This collection is currently under construction. The XIX International Grassland Congress took place in São Pedro, São Paulo, Brazil from February 11 through February 21, 2001. Proceedings published by Fundacao de Estudos Agrarios Luiz de Queiroz This Event is brought to you for free and open access by the Plant and Soil Sciences at UKnowledge. It has been accepted for inclusion in International Grassland Congress Proceedings by an authorized administrator of UKnowledge. For more information, please contact UKnowledge@lsv.uky.edu. THE INFLUENCE OF LARGE ANIMAL DIVERSITY IN GRAZED ECOSYSTEMSM.G. Murray and D.R. BairdInstitute of Cell, Animal and Population Biology, University of Edinburgh, West Mains Road,Edinburgh EH9 3JT, UKAbstractField observations in the Serengeti National Park, Tanzania have revealed several modesof grazing system, including migration, sustained-yield grazing and grazing succession whichcontribute on the one hand to ecological separation of herbivores by habitat choice, but on theother hand to positive association of species on wet season pastures known as hot spots.  Drawingfrom results of experimental field studies, we consider what is known about resource competitionand facilitation, the two key ecological processes underlying these grazing systems, and specifythe shape of the relationship between species richness and pasture height that is predicteddepending on which process is dominating in the grazing system. We test between thesealternatives by analysing for the first time data on herbivore distributions and vegetation conditionfrom a series of 33 survey flights over the entire Serengeti ecosystem.  Our preliminary resultsstrongly support the importance of facilitation as a process structuring the herbivore communityof the Serengeti Plains in the wet season.  Resource competition may limit use of the shortestswards.Keywords: Grazing ecosystem, species richness, herbivore biomass, resource competition,facilitationIntroductionOn surveying large herbivores in African savanna parks, ecologists have observed thateach species may prefer certain habitats over others, giving rise to a degree of ecologicalseparation (Lamprey 1963, Sinclair 1977); but conversely, a variety of species will sometimesconcentrate in the same habitats giving rise to hot spots of species richness (McNaughton andBanyikwa 1995). Longer-term observation has revealed that these same herbivores may take partin extensive grazing patterns.  Most notable are the long distance migrations of wildebeest, zebraand gazelle in the Serengeti-Mara ecosystem and of kob and tiang in southern Sudan (Bell 1971;Fryxell and Sinclair 1988; Howell et al 1989).  Other examples include the grazing succession atthe end of the wet season, in which the larger herbivores are first to move down the local catenafrom the short grass community at the top to the tall grasses at the bottom (Vesey-Fitzgerald1965), and the occupation of new habitat by grazing ungulates following removal of the dominantgrazer (Eltringham 1974). It is the aim of ecologists that study these diverse assemblages of largemammals to reveal the primary grazing processes. The question being addressed today is whetheran underlying explanation, a kind of unified theory for grazing, can be developed to explainextensive grazing patterns and to predict the distributions of large herbivores across grasslandhabitats.Much progress has been made in recent years towards an understanding of how the localbiomass and structure of pastures influences the short-term or daily intake of herbivores ofdifferent species (Hodgson & Illius 1996), but relatively little is understood about the processesoccurring in the grazing community at a larger scale.  Research in the Serengeti National Park hasshown how an integral part of these large-scale processes, is the modification of vegetation by theherbivores themselves. Which species remain on a particular site depends on how the vegetationhas been modified (McNaughton 1976; Murray & Illius 2000).  In combination with the stochasticnature of rainfall, disease and other external events, the modification of vegetation by herbivorescontributes to the creation of a more dynamic grazing ecosystem.   Grazing processes in the serengeti ecosystemPrincipal ecological features of the Serengeti ecosystem in Tanzania are the open grassplains covering an area of approximately 7,500 km2 in the south and southeast, where rainfall islow (300-700mm per annum), and the open scrub and woodlands, together with smaller grasslandplains in the remaining moister areas (up to 1200 mm rainfall), covering some 17,500 km2 (mapin Murray 1995). The gradient in rainfall and in growing conditions is exploited by the migratorywildebeest, which spend the wet season, nominally from November to May on the short grasslandsto the southeast and the dry season in the woodlands.  They share the ecosystem with some 14other indigenous grazing ungulates, as well as cattle and other domestic species. The herbivoresexploit a great variety of grass species of which more than 120 have been collected in SerengetiNational Park.  Important forage species are the mat-forming grasses of the southeast, includingAndropogon, Digitaria and Sporobolus species and some of the common woodland grasses witherect growth form such as species of  Bothriochloa, Digitaria, Panicum, Pennisetum  andThemeda.A number of effects of grazing can be discerned immediately following the passage of aherd. The height and biomass of swards are reduced and should grazing pressure be sufficientlyintense, it may give rise to a ‘grazing lawn’ (Figure 1). If the herbivores specialise in selectivefeeding, there may be a significant reduction in the content of green leaf in the sward leaving apoor quality, stemmy pasture or ‘grazing stubble’ (Figure 2; Murray & Illius 2000). Over a periodof a few days to several weeks, intensive grazing can also give rise to secondary effects in theproduction of new vegetative growth (McNaughton 1976).  But whilst it may be obvious thatselective grazing will modify the vegetation in these ways, it has not been known whether thismodification would affect the intake of ungulates or whether it would have importance instructuring the ungulate community.Two resource-based hypotheses have emerged from the studies of grazing ungulates inSerengeti to explain their community dynamics: one is based on competition and the other onfacilitation. According to the competition hypothesis, low biomass swards such as grazing lawnswill only support herbivores of small body size and/or wide mouths. So the consequence ofgrazing down a tall sward will be a reduction in the number of herbivore species utilising thatsward due to grazing competition (Figure 3; Murray & Illius 2000). According to the facilitationhypothesis, heavy grazing of high biomass swards stimulates new production of more digestiblefood, creating forage that can be utilised by the smaller herbivore species, and so the overallnumber of herbivore species will increase (Figure 4).In order to test these two hypotheses we have undertaken a preliminary analysis of adataset of large mammal distributions compiled by the Serengeti Ecological MonitoringProgramme (SEMP) from monthly aerial surveys of the Serengeti-Mara ecosystem over theperiod, July 1969 to August 1972. Each month several planes and crews took part in the surveyof the entire ecosystem, recording information against a 5 X 5 km grid. Observations of the morecommon large animals (wildebeest, zebra, Thomson’s gazelle, Grant’s gazelle, topi, hartebeest,giraffe, rhino, elephant, buffalo, impala, eland and cattle) were recorded according to one of fourabundance classes: 1-25; 26-250; 251-2500; 2501+. Observations of rare or low density species(roan, oryx, warthog, ostrich, lion, cheetah, leopard, reedbuck, kudu, waterbuck and bushbuck)were also noted, usually with an actual count of numbers seen. In the case of burnt ground, thestate of grass regrowth was classified as bare ground, or in one of five height classes. Thegreenness of regrowth was assigned to one of five classes, varying from dry and parched to lush.On unburnt ground, the height and greenness of unburnt grass were also each assigned to one offive classes. In all, data from 33 reconnaisance flights were entered into the database by SEMP.The combined biomass of large herbivores on the Serengeti Plains during the wet seasonwas highest at an intermediate height of vegetation, indicating that food may have been limitingon the shortest grass swards, but quality may have been limiting on the tallest grass swards (Figure5).  However, species richness was maintained on the shorter swards, and only declined on thetallest pastures (Figure 6).  This result suggests that facilitation of grazing may be enabling morespecies to utilise short pastures.  In order to investigate this possibility further, an index ofvegetation quality (q) was defined as: q = greenness class / height class. Thus, the highest valuesof quality were assigned to short green swards and the lowest to tall non-green swards. Thequality index (q) predicted mean herbivore biomass with surprising accuracy (Figure 7). Speciesrichness was also closely correlated with vegetation quality (Figure 8).  These results furtherindicate that ungulates tend to congregate in large numbers on the high quality swards in theSerengeti Plains during the wet season, and that the overall interaction between herbivores andgrasslands facilitates the formation of higher species richness in the herbivores.   ConclusionAlthough our analysis of the 1969-1972 survey data is preliminary, these first resultsemphasise the importance of grazing pressure by large herds in creating pastures that can beutilised by many species, so confirming the early observations of Vesey-Fitzgerald (1965) andMcNaughton (1976).  The findings here relate solely to the Plains grasslands in the wet seasons,but the occurrence of hot spots of species richness in the woodlands area suggests that facilitationmay be of widespread importance in the ecosystem (McNaughton 1988).  In addition to thisecological process, the reduction in herbivore biomass that was evident on the shortest swardssuggests that there are conditions on the Plains in which intake by herbivores may be constrained. This could lead to differential displacement of herbivore species because of differences in thethreshold at which sward height and biomass begin to limit food intake (Murray & Illius 2000).The survey data reveal a low abundance of buffalo (the heaviest ruminant grazer) on the Plains,which may indicate the effect of resource competition. This would suggest that the overall patternof community dynamics amongst grazing herbivores in the Serengeti ecosystem is being influencedby both competition and facilitation processes. There is a need for greater resolution in themethods used to study competition and facilitation so as to clarify their influence on large animaldiversity.AcknowledgementsThe survey data were collected by scientists working at the Serengeti Research Institute(now SWRC) from 1969-1972 as part of the activities of SEMP. The database used in this analysiswas supplied by K. Campbell who managed the re-entry of raw data into digital form.ReferencesBell, R.H.V. (1971). A grazing ecosystem in the Serengeti. Scientific American 224:86-93Eltringham, S.K. (1974). Changes in the large mammal community of Mweya Peninsula,Rwenzori National Park, Uganda, following removal of hippopotamus. Journal of AppliedEcology 11: 855-865.Fryxell, J.M. and Sinclair A.R.E. (1988). Seasonal migration by white-eared kob in relation toresources. African Journal of Ecology 26:17-31.Hodgson, J. and Illius A.W. (eds.) (1996). The ecology amd management of grazing systems.CAB International.Howell, P., Lock J.M. and Cobb S. (eds.) (1989). The Jonglei canal: local impact andopportunity. Cambridge University Press, Cambridge.Lamprey, H.F. (1963). Ecological separation of the large mammal species in the Tarangire GameReserve, Tanganyika. East African Wildlife Journal 1:63-92.McNaughton, S.J. (1976). Serengeti migratory wildebeest: facilitation of energy flow by grazing.Science 191:92-94.McNaughton, S.J. (1988). Mineral nutrition and spatial concentrations of African ungulates.Nature 334:343-345.McNaughton, S.J. and Banyikwa F.F. (1995). Plant communities and herbivory. Pages 49-70in A.R.E. Sinclair & P. Arcese, editors. Serengeti II: Dynamics, Management and Conservationof an Ecosystem.  University of Chicago Press, Chicago, USA.Murray, M.G. (1995). Specific nutrient requirements and migration of wildebeest. Pages 231-256in A.R.E. Sinclair & P. Arcese, editors. Serengeti II: Dynamics, Management and Conservationof an Ecosystem.  University of Chicago Press, Chicago, USA.Murray, M.G. and Illius A.W. (1996). Multispecies grazing in the Serengeti. Pages 247-272 inJ. Hodgson & A.W. Illius, editors. The Ecology and Management of Grazing. CAB International,Wallingford, UK.Sinclair, A.R.E. (1977). The African Buffalo. University of Chicago Press, Chicago, USA.Vesey-Fitzgerald, D.F. (1965). The utilisation of natural pastures by wild animals in the RukwaValley, Tanganyika. East African Wildlife Journal 3:38-48.Figure 1 - Evidence for resource depletion during periods of foraging by topi and wildebeest: mean height of green leaf in the sward with standard errors are shown for experimental plotsbefore and after grazing trials.Grass plots1 2 3 4 5 6 7 8 9 10 11Sward height (cm)0510152025303540Prior to grazingAfter grazingFigure 2 - Evidence for depletion of resource quality during periods of foraging by topi andwildebeest:  mean proportion of green leaf in the sward with standard errors as measured inexperimental plots before and after grazing.Grass plots1 2 3 4 5 6 7 8 9 10 11Green leaf (%)0510152025303540Prior to grazingAfter grazing01002003004005006007000 2 4 6 8Leaf Height (cm)Intake (mg.kg-0.75.min-1)Figure 3 - Intake rate of yearling wildebeest (filled squares) and topi (filled diamonds) feeding ongrass swards in an early vegetative growth stage within the Serengeti National Park.Figure 4 - The relationship between herbivore species richness and the height of grass insavanna habitats, as predicted by a) a classical model of resource competition and b) a modelof resource facilitation (see text for further details).Grass height0 1 2 3 4 5Species richness01234a) Competition modelGrass height0 1 2 3 4 5Species richness01234b) Facilitation modelFigure 5 -  Observed relationship between herbivore biomass (combined for all species) andgrass height in the Serengeti Plains during the wet season months, January to May, based onsurvey flights from 1969-1972. Mean herbivore biomass is indicated by the boxes containingthe sample size (i.e. the number of  5 X 5 km survey quadrats of that vegetation height); barsindicate the standard deviation.                      vegetation height0 1 2 3 4 5herbivore biomass (tonnes per 25km2 )-200-100010020030040050060031666504636Figure 6 - Observed relationship between herbivore species richness and grass height in theSerengeti Plains during the wet season months, January to May, based on survey flights from1969-1972. Mean species richness is indicated by the boxes containing the sample size (i.e. thenumber of  5 X 5 km survey quadrats of that vegetation height); bars indicate the standarddeviation.vegetation height0 1 2 3 4 5mean species richness01234531 666 504636Figure 7 -  Relationship between herbivore biomass (tonnes per 25 km2) and the vegetationquality on the Serengeti Plains in the wet season (R2  = 0.86, P<0.001).02000004000006000000 1 2 3 4Vegetation qualityMean herbivore biomassFigure 8 - Relationship between species richness of herbivores and the vegetationquality on the Serengeti Plains in the wet season (R2 = 0.80, P<0.001).11.522.533.540 1 2 3 4vegetation qualitymean species richness

The Influence of Large Animal Diversity in Grazed Ecosystems

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The Influence of Large Animal Diversity in Grazed Ecosystems

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