Transcript Determinants of the composition and distribution of wildlife

Determinants of the composition
and distribution of wildlife
communities in African Savannas
Biomass and type of herbivores
ECOLOGICAL DETERMINANTS
• African savannas famous for diversity and abundance of large mammalian herbivores
that they support (Owen-Smith, 1982, du Toit and Cumming, 1999; Illus and O’Connor,
2000)
• Body range from 4kg dik dik to 5000kg elephant
• 44 large herbivores of 29 genera frequent African savannas (Owen-Smith, 1982)
• 20 species grazers (feeds mainly on herbaceous vegetation), 13 browsers ( feeding
mainly on woody vegetation), 10 mixed feeders and 1 omnivore.
• Browser/grazer dichotomy special feature of African Savanna (Owen-Smith,1982; du
Toit and Cumming,1999)
• High faunal diversity and herbivore density –attributed to the high spatial heterogeneity
of african savanna
• Distribution of herbivore biomass increase with rainfall up to about 700mm per year
and then levels off or declines at higher rainfall (Bell, 1982)
• Herbivore biomass on fertile soils is considerably higher than on infertile soils
Hig
h
Soil
Wat
er
Low quality woodland
and forest:
Very low biomass
selective feeders incl
smaller antelopes
and primates
Low quality woodland
Low biomass of tolerant
herbivores i.e
elephants, buffalo, zebra, etc
High quality woodland
and forest
High biomass of mixed range
of herbivores
elephants, rhinos, giraffe,
kudu,nyala, buffalo, zebra,
hartebeest, etc
High quality fine-leafed
woodland
and short grassland:
High biomass of selective
browsers and
grazers i.e giraffe, impala,
kudu, gazelle,
Wildebeest, springbok
Low quality grassland
Low biomass of
Low
medium tolerant grazers & mixed
feeders: eland,zebra, roan,
reedbuck
Low
Soil Nutrients
High
Distribution of large herbivore biomass in relation to soil moisture
and rainfall. (source:Bell,1986)
ECOLOGICAL DETERMINANTS
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For natural communities of indigenous large herbivores primary ecological
determinates of community composition are:
– Soil, rainfall, and vegetation characteristics of a particular region
Data from game reserves in arid and semi arid savannas across eastern and southern
Africa-total biomass 9kgkm-2) is positively related to mean annual rainfall (Coe et.al.
1976)
Applies as well to greater ungulate species (East,1984)
Inference is that herbivore biomass is governed by net annual above ground primary
production which in turn is governed by annual rainfall
Herbivores are not only depended on food quantity but food quality which in turn is
influenced by mean annual rainfall and soil fertility
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Plant quality-is function of ratio of digestible(cell content) and indigestable component(cell wall)
High rainfall and/or soil water infiltration rates reduce cell content/cell-wall ratio (i.e reduce forage
digestibility), while high soil-nutrient levels increase (Bell, 1986)
Savannas herbivores functionally grouped two guilds: grazers and browsers-mixed
feeders oscillate between guilds, seasonally, belonging more to the grazing guild in the
wet season and browsing guild in the dry season
–
presence of plant secondary metabolites, more prevalent in browse (foliage in dicotyledonous plants) than
in grass
ECOLOGICAL DETERMINANTS
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Due to rainfall patterns, underlying geology and leaching African savannas are
divided in to two broad categories:
Moist dystrophic savannas- receive more than 650mm annual rainfall, woody
vegetation dominated by Brachystegia, Julbernadia,Burkea and Ochna species
Arid eutrophic savannas-less than 650mm rainfall per annum, vegetation
dominated by Acacia, Commiphora, and Colophospermum species
– Moist dystrophic savanna poor quality feed compared to arid eutrophic savanna
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All herbivores prefer feeding on the best available foods, however due to the
nature of the relationship between mass specific metabolic demands and gut
capacity in herbivores(Demment et al, 1985)the largest herbivores can
tolerate poorest diets
This dietary tolerance/body size relationship is the basis for Bell’s hypothesis
that the herbivore biomass in moist dystrophic savanna should be dominated
by large species with a wide dietary tolerance (i.e. elephant, buffalo, zebra)
While smaller selective feeders should predominate in arid dystrophic
savannas (i.e. gazelle, impala, kudu, wildebeest).
ECOLOGICAL DETERMINANTS
• Bell’s hypothesis is supported by East’s division of herbivores into two
groups on the basis whether their peak biomass densities occur in moistdystrophic or arid eutrophic
• Moist dystrophic savanna support relatively low biomass densities in
which mean annual rainfall exceeds 1000mm, these larger species
(elephant, hippo, black rhino, buffalo, eland)- have much wider
disribution than smaller ungulates that are more closely associated with
arid dystrophic savannas,Owen-Smith,1988
ECOLOGICAL DETERMINANTS
• Mega herbivores (animal exceeding 1000kg in adult body mass i.e
elephant, black rhino, white rhino, hippo and giraffe) make up 50-70% of
herbivore biomass in most savannas
• There are four exceptions to this general rule (Bell, 1986) as follows
– Areas where man has eliminated some or all larger herbivores (mega
herbivores)
– The East African volcanic short grass plains
– High rain fall low – nutrient woodland and forest (i.e northern Angola,
central Democratic Republic of Congo ) but not in the high nutrient
volcanic forests
Variation within ecosytems
• Much spatial heterogenity within savanna compared with nothern
temparate and boreal (McNaughton and Georgiadis,1986; Brown and
Gibson, 1983)
• Spatial heterogenity (or Beta diversity) results in spatial variation in palnt
available moisture and soil nutrients which in turn creates patches in
quality and quantity of vegetation used by ungulates as food
– Spatial variation-important factor undrlying the structure of their large
mammal communities
• Patchness in savannas associated with major adaptive radiations within
mammalian fauna which have resulted in the high diversity of species
found today (Maglio and Cook, 1978)
• Variation in the diversity of habitat used by herbivores is realted to the
body size/ diatary tolerance relationship (du toit and Owen-Smith, 1986)
Variation within ecosytems
• Larger species, wider feeding tolerance, are able to feed in a wide range
of habitats than smaller species hence evenly spead across the ecosystem
• Smaller species (klipsringer, steenbok, duiker, bushback, etc), habiat
specialist and populations are clumped in suitable (and usually separate )
habitat types.
• A result of habitat specialisation is allopatric speciation- this explain why
smaller ungulate size classes inlude so many species
• Temporal variation in in the distribution of soil moisture within extensive
and relatively uniform vegetation- dominated by seasonally migrating
grazers
• Distribution of plant available moisture close to the soil surface creates
spatially discrete distributions of green grass-causingmigrations on a local
scale in response to catenary effects (buffalo and zebra-serengett:
Bell,1970), or floodplain dynamics lechwe, Kafue flats, Rees, 1978.
• Or temporal-spatial variation in rainfall-wildebeest,zebra, and Thomson’s
gazelle in serengeti,Maddock, 1975/ wildebeest,eland, and hartebeestKalahari
Human factors
Land transformation
• Large wildlife reseves proclaimed in Southern Afica for protecting large
mammals in hot and disease riiden country
• Percentage area allocation in SADC for wildlife reserves approx 10%
• Park planners made no provision of corriodord between protected areas,
as a resulted indigenous ecosystem of the region has become fragmented
and isolated to a greater or lesser degree by human land use
• ‘Ecological islands’- too smal to allow large mammal esp elepahnts
populations to self regulate, so that internal management and edge
effects (e.g predator-livestock conflicts, poaching, etc.) have shifted
composition and dynamics of large mammal communities away from their
natural states
Disease and veterinary controls
• Rinderpest introduced with infected cattle in 1889
– Caused 95% mortality of ruminants across Africa,Sinclair, 1978
– Caused anomalies in distribution patterns e.g absence of giraffe and
wildebeest in Zambezi valley
– Created
• EU Beef exports have a profound effect cattle -wildlife interface
– Foot and mouth disease-buffalo are a resevoir of infection and vector of trnsmission for
the virus
– In Zim, buffalo have been exterminated from most beef ranches and extensive fencing
have been erected to prevent any contact between buffalo and export beef herd
• Control of trypanosomiasis have a negative effect on wildlife communities
– Tsetse flies (Glossina spp) transmit trypanosome parasites from indigenous tsetse (and
resistant cattle)-so early control mechanisms aimed at exterminating wildlife
populations from prescribed areas.
– This was followed by hunting only species prefferd by Tsetse and ultimately control
hunting in a cordon between game and cattle fences
Disease and veterinary controls
– Ordour baited insecticides are being used and are
effective
– Use of fence have been abondened
– Wildlife seldom recover I these areas, because
areas now occupied by cattle farmers
Wildlife economics
• Wildlife conservation-expensive business and it has to generate income to
for their own conservation or they will be lost
• Bell and Clarke (1986) using 1981 data from 17 countries
– Adequate level of conservation and law enforcement effort requires a
minimum expenditure of usd200 per km2
– Only RSA, have exceeded this figure, and possibly Nam and Kenya in
some parks
– Below this level, profits from poaching outweigh risks of being
apprended
– As a result both black and white rhino have been exetrminated in all
countries other than RSA???? and Nam
– Flow of ivory in world markets from 1950 to 1987-steady decline in
African elephant population (Caughley, et al, 1990)
Wildlife economics
• CBNRM and private conservancies offsetting deficiency of national parks
– Game rancher seldom allow large predator populations to recover
• Artificial water supplies available and predation and disease are not
limiting large herbivore attain biomass densities that are significantly
higher than would occur under natural conditions in the same ecosystems
– Water supplies extends foraging area available to water dependant species (mainly
grazers)
– But spatial patchiness in forage abundance becomes reduced as a direct result
– During drought (becoming more frequent and intense) reduced reserves of forage
become depleted quickly and ensuing crashes in herbivores populations are steeper and
deeper than in unmanaged systems (Walker et al, 1987)
– Hence fro ecological perspective game ranches are no substitute for national parks