What is influencing patchy space-use by wildebeest in western-central Kruger?

K. Yoganand and N. Owen-Smith

Center for African EcologySchool of Animal Plant and Environmental SciencesUniversity of the Witwatersrand

Background

Over 1.4 million wildebeest in Serengeti

Kruger, comparable in area (20,000 vs. 25,000 km2 of Serengeti), has fewer than 12,000 Wildebeest

WB population in central Kruger declined sharply after western-boundary fencing in 1960s & 70s (Whyte 1985)

This sub-population has not recovered in abundance nor resumed migration after fence removal in the 90s

The themes we examine here are:

The habitat and landscape features that govern space-use and thus restrict the distribution of wildebeest

The factors include:The forage conditions (quantity and quality features)

Vegetation cover and visibility features

Landscape-scale features

We placed GPS collars on (breeding) herds of wildebeest, zebra and buffalo in western-central region of Kruger (Orpen gate)

Obtained hourly locations

Measured forage and habitat features at locations/patches of ‘use’and in the surrounding ‘unused’ landscape

Spatial analyses of data

We placed GPS collars on (breeding) herds of wildebeest, zebra and buffalo in western-central region of Kruger (Orpen gate)

Obtained hourly locations

Measured forage and habitat features at locations/patches of ‘use’and in the surrounding ‘unused’ landscape

Spatial analyses of data

•Based on 40,000+ locations obtained at hourly intervals over the last year

•From 10 herds of wildebeest

•Data ranges from 3 to 11 months for each herd

•The collars are still working and we are continuing data collection

• Example of four herds

•Early wet season

• Late wet season

• Dry season

Movement paths covering the convex polygon justifying use of it as a ‘known’ area

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Early-Wet Late-Wet Dry Annual

Area

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Area-knownPatch-used

% of area ‘used’out of area ‘known’: 12.29 12.78 16.67 5.8

Summary of space-use for all (4) wildebeest herds

How did the wildebeest use these patches?

Rate of movement of herds was much lower within patches (indicating foraging) than between patchesMovements between patches were seldom done during nightThese are ‘real’ functional patches and not some artefacts of (auto-correlated) location data or a fractal pattern

69.35 (4.9)

561.31 (47.1)

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movment rate (m/h) within-patch movment rate (m/h) betwn-patches

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night : day movment within patch night : day movment betwn patches

Example from one herd

Are other large herbivores similar in space-use?

The patchy space-use of wildebeests is in contrast with zebra and buffalo herdsZebra and buffalo herds used a large proportion of the area ‘known’ to them

Factors that may influence patchy space-use by wildebeest at different scales

GeologySoilRainfallGrass biomass, structure & qualityPredation risk

Main landscape classes:

Mixed Combretum spp. / Terminaliawoodland on granite (light brown)

Thornveld on Gabbro (darker reddish brown)

Patchy use of grazing lawns in the wet season

Patchy use of seeplines in the dry season

Large-scale factors

The wet season patches were in grazing lawns and sodic sites, and the dry season patches were mainly along seeplines

The processes driving these patterns

What are these patches and surrounding landscape made of?

The two main hypotheses:Forage abundance (grass with favourable characteristics) and nutrient qualityPredation risk

Field sampling

Forage and cover characteristics In class intervals (3, 6, or 7-point scales)Most prevalent class in each plotSummarized by frequency distributions and modes

Sampling in ‘used’ locations and ‘unused’patches

‘Unused’ patches and the transects used to sample those patches

The main ‘forage’ and ‘cover’ variables

Grass species compositionLeaf-table heightAerial and basal cover

Tree coverShrub coverGrass height Distance to hidden space (which could offer cover for predators)

Forage and cover features compared between ‘unused’ patches and ‘used’ locations

Grass species prevalence

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'Used' locations'Unused' patches

Grass leaf-table height

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'unused' patches'used' locations

Grass cover

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11-25% 26-50% 51-75% 76-90%

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Tree canopy cover

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Grass height

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Distance to hidden area / cover for predators

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Features of ‘Used’ locations compared between wet and dry seasons

Stark contrasts between early-wet and late-dry seasons in variables such as;

Prevalent grass species; leaf greenness; distance to hidden area; tree cover; shrub cover; etc.

Remarkably, ‘grass height’ was similar between the two seasons

The key inference: Wildebeest herds are seeking and finding patches on granite in dry season that offer forage comparable in height/structure to the wet season grazing lawns

Grass height in 'used' locations

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Test of the main hypothesesThe statistical models we built with our data (logistic regression models in model-selection framework) show that:

Abiotic, forage (‘bottom-up’ processes) and predation-risk (‘top-down’ process) factors simultaneously influence space-use by wildebeest herds

The ‘forage’ factors alone explain a majority of the variation in the data, but ‘predation risk’ has substantial influence

Plans for further spatial analyses to separate the influence of the two factors

ConclusionsWildebeest herds use space patchily in all seasons

The patches used formed only a fraction of areaavailable to the herds

Short-grass seems to be the most important habitat feature sought after by wildebeest herds

Model results suggest influence of both forage and predation risk factors on wildebeest space-use

However, pending further spatial analysis, we are delaying our key conclusions

Our next step is to determine what factors restrict wildebeest distribution in the landscape

AcknowledgementsAugusto Mabunda – for field assistance and ‘protection’Sanparks Scientific Services

GIS data and support (Sandra MacFadyen and Izak Smit) Thembi Khoza, Patricia Khoza and Adolf Manganyi – for liaisoning

Sanparks Veterinary Services and Helicopter team for collaring and ‘re-collaring’ operations (Peter Buss, Markus Hofmeyr, Johan, Grant Knight, Charlie Thompson)

Sanparks Protection Services – Richard Sowry and KFI field rangers Manyaleti Nature Reserve – Jimmy Thanyani, Mark Bourn and

Dr. Ferreira (Mpumalanga Veterinarian)Timbavati Private Nature ReserveAfrica Wildlife Telemetry – for the collars and data downloadsNRF and University of the Witwatersrand for funding supportAPES, Wits University staff

for administrative supportBarend Erasmus – for discussions on GIS issues