Seasonal selection of soil types and grass swards by roan antelope in a South African savanna

Afr. J. Ecol. 1998, Volume 36, pages 57–70

Seasonal selection of soil types and grass swards by roanantelope in a South African savanna

IGNAS M. A. HEITKÖNIG* and NORMAN OWEN-SMITH

Department of Zoology and Department of Botany, University of the Witwatersrand,PO Wits 2050, Republic of South Africa

SummaryRoan antelope are distributed mainly in regions characterized by infertile soils,offering food of low quality. We hypothesized that roan may select localities withhigher soil nutrient levels and/or grass swards with more favourable properties interms of food abundance or quality than generally available in those regions.Roan antelope were observed in a savanna region in South Africa where soils ofwidely varying nutrient status occurred. Roan favoured open grassland overwooded savanna areas. During the wet season, roan preferred sites withfelsite-derived soil of intermediate soil nutrient status. Grasslands growing onnutrient enriched alluvial soils were preferred outside of the early wet season,although most of the favourable sward characteristics were present in otherlandscape units. Food quantity, rather than quality appeared to attract roan toforaging sites in the late wet and early dry seasons. Food quality appeared moreimportant in the early wet and late dry seasons. The higher degree of clusteringof leafy material within foraging swards seemed to be an additional discriminat-ing factor. The factors governing the selection of foraging sites by roan did notseem notably different from those influencing other species of grazing ruminant,but roan nevertheless seemed tolerant of stemmy grasslands growing on nutrientricher substrates.

Key words: forage biomass, forage quality, landscape, ungulates

Résumé

Les antilopes roannes sont distribuees principalement dans des regions caracteri-sees par un sol non fertile, offrant une nourriture de mauvaise qualite. Nousavons emis l’hypothese que les roannes pouvaient choisir des endroits ou les tauxde nutriments dans le sol sont plus eleves et/ou le gazon herbeux a des proprietesplus avantageuses en terme d’abondance ou de qualite que ce qui existe engeneral dans la region. On a observe des antilopes roannes dans une region desavane en Afrique du Sud ou le statut des nutriments du sol varie fortement. Lesroannes privilegiaient les prairies ouvertes plutot que les savanes arborees. Ensaison des pluies, les roannes preferaient les sites ou le sol derive du felsite avait

*Corresponding author and present address: Department of Terrestrial Ecology and Nature Conser-vation, Wageningen Agricultural University, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands.E-mail: [email protected], Fax: +31/317-484845.

? 1998 East African Wild Life Society.

un statut nutritif intermediaire. Les prairies croissant sur des sols enrichis par lesalluvions etaient preferees sauf au debut de la saison des pluies, bien que laplupart des caracteristiques favorables du gazon se soient trouvees dans lesautres unites du paysage. La quantite plutot que la qualite de la nourrituresemblait attirer les roannes aux endroits ou elles se nourrissaient a la fin de lasaison des pluies et en debut de saison seche. La qualite de la nourriture sembleplus importante au debut de la saison des pluies et en fin de saison seche. Le tauxplus eleve d’agglomeration des feuilles sur les gazons de nourrissage semblait etreun facteur supplementaire de discrimination. Les facteurs regissant le choix dessites de nourrissage par les roannes ne semblent pas tres differents de ceux quiinfluencent les autres especes de ruminants, mais les roannes semblent tolerer lesterrains herbeux poussant sur des substrats plus riches en nutriments.

Introduction

Roan antelope (Hippotragus equinus Desmarest) are unusual among Africangrazing ruminants in that they reach their highest abundance in moist/dystrophicsavannas where soils are predominantly infertile (Anderson & Talbot 1965;Jarman 1972; Duncan 1975; East 1984). Infertile soils generally support grass-lands of low quality (Bell 1984). In order to survive within such regions and tosatisfy their nutrient requirements, roan could selectively occupy localities withinlandscapes where soil nutrients are concentrated (Seagle & McNaughton 1992),or where the grass sward remains favourable for feeding longer into the dryseason.

Roan antelope are predominantly grazers and reportedly favour opensavanna or open grassland, with medium to tall grass (Joubert 1976; Smithers1983; Bell 1984; Heitkönig 1994). We consider two levels of habitat selection thatmay affect the relative success of roan antelope in nutrient-poor savannas: (i)selection for regions of particular soil types with elevated nutrient levels withinlandscapes, and (ii) selection for grass swards offering particularly favourablefeatures of food abundance or quality for feeding.

Materials and methodsStudy areaObservations were made on a population of about 35 roan antelope whichoccupied a 2300 ha section of the Nylsvley Nature Reserve in Northern Province,South Africa (24)40*S, 28)40*E). This area included a section of the floodplain ofthe Nyl River and adjoining savanna, and had a mean elevation of 1100 m. Thelow plateau north and west of the floodplain was underlain by felsite or rhyolitelavas (henceforth called felsite) of the Rooiberg Group. These soils weregenerally shallow with low to intermediate nutrient levels. To the south and eastsandstone and conglomerates (henceforth called sandstone) of the WaterbergGroup formed a low plateau with rocky outcrops, where generally deep, sandyand nutrient-poor soils dominated. The nutrient-rich floodplain was underlainpartially by turf soils derived from basalts of the Karoo system and partly byfelsite and sandstone. Soil and other characteristics of landscape units (LSU) aresummarized in Table 1.

58 I. M. A. Heitkönig and N. Owen-Smith

? East African Wild Life Society, Afr. J. Ecol., 36, 57–70

Table1.Landscape

unitsandtheirrelativesizeintheNylsvleyNatureReserve,SouthAfrica,characterizedby

soilorigin,soilvariables,andgeneralphysiognom

y.Adaptedfrom

vegetation

unitsdescribedby

Coetzeeetal.(1976);soildatafrom

Harmse(1977)andFrost(1987)

Landscapeunit

Code

Studyarea

cover(%)

Soilorigin

Soil

nutrientlevel

Soil

drainage

Physiognomy

Plant

communities4

Alluvialgrassland

AlG

23·4

Karoo

sedimentsandbasalt(alluvium

)high

1poor

grassland&marsh

(seasonally

flooded)

marsh,3a

Alluvialsavanna

AlS

21·2

Karoo

sediments(alluvium

)high

1poor

microphyllousandmixed

broad-leavedsavanna&thicket

2a,3b

Felsitegrassland

FeG

5·9

RooibergGroup

lavas(felsite)

medium2

poor

grassland

1.3.2.1

Felsitesavanna

FeS

21·4

RooibergGroup

lavas(felsite)

medium2

good

broad-leavedsavanna

1.3.1.1,1.3.1.2

Sandstonegrassland

SdG

8·2

WaterbergGroup

conglometates(sandstone)

low3

poor

grassland

1.3.2.2

Sandstonesavanna

SdS

19·9

WaterbergGroup

conglomerates(sandstone)

low3

good

broad-leavedsavanna

1.1,1.2

1Deepsoils,claycontent18–55%,pH

5·4–9·1,S-value5–23me/100g,phosphorus7–20ppm.

2Shallowtomoderatelydeepsoils,claycontent<10–35%,pH

4·8–5·5,S-value

<5,phosphorus3–10ppm.

3Shallowtodeepsoils,claycontent<8%,pH

4·7–6·1,S-value

<1me/100g,phosphorus2ppm.

4Plant

communitiesaccordingto

Coetzee

etal.(1976):1.1,Eragrostispallens–Burkeasavanna;1.2,Barleriabremekampii–Diplorynchussavanna;1.3.1.1,Rhus

leptodictya–Combretum

apiculatum

variationoftheEragrostisracemosa–Combretum

apiculatum

savanna(1.3.1);1.3.1.2,Cymbopogonplurinodis–Combretum

apiculatum

variationof

theEragrostisracemosa–Combretum

apiculatum

savanna(1.3.1);1.3.2.1,

Eragrostisnindensis–Digitaria

monodactyla

variationof

theEragrostis

racemosa–Digitariamonodactylagrassland(1.3.2);1.3.2.2,Tristachyarehmannii–DigitariamonodactylavariationoftheEragrostisracemosa–Digitariamonodactyla

grassland(1.3.2);2a,Sporobolusioclados–Acaciatortilissavanna(2.1)andEucleaundulata–Acaciatortilissavanna&thicket(2.2);3a,Acaciabipartita–Setariawoodii

grassland;3b,Acaciabipartita–Setariawoodiisavanna;marsh,(notfurtherdescribedby

theauthors).

Foraging site selection by roan antelope 59


Vegetation on the floodplain consisted of open grassland as well as Acaciaspp. savanna (Coetzee et al., 1976). Mixed broadleaf savanna occurred on theplateaus, with Burkea africana dominant on the sandstone and Combretumspp. predominant on the felsite. Plant species associations identified usingBraun–Blanquet analysis (Coetzee et al., 1976) are listed in the footnote toTable 1. Rainfall averaged 620 mm with >80% falling between October andMarch.

Observation procedureAt the start of an observation, the herd or a large herd fragment was locatedradio-telemetrically through one or both of the two radio-collared females. Theanimals were followed during daylight hours from a vehicle.

We used feeding spells as the time unit for our investigations. A feeding spellwas defined as the time period during which at least 20% of the individuals in theherd were observed feeding during at least six consecutive herd scans. A herdscan was carried out every five minutes during the day, when the activity of eachvisible animal was classed as feeding, moving, standing, socially interacting, lyingdown, ruminating or drinking. Only feeding activities are considered in thispaper. The position of the group was also recorded during the herd scans andallocated to the corresponding landscape unit. In our definition of feeding spells,the lower cut-off point of 20% of feeding individuals was arbitrarily chosen tofunction as a buffer against frequent temporary disruptions in feeding activities.Feeding spells lasted at least 30 min and generally 1–2 h.

Foraging sites (FS) were defined as localities where feeding spells occurred.Boundaries of foraging sites were drawn by linking the locations of the animalsoccupying the outer herd positions during a feeding spell. Foraging sitesincluded, but were not exclusively made up of feeding stations sensu Novellie(1978). Non-foraging sites (NFS) were defined as localities adjacent to foragingsites with vegetation that appeared different either in physiognomy or in thebotanical composition of the grass layer. NFSs were typically not more than100 m removed from FSs. Foraging sites were tested for independence. All timelapses in between successive feeding spells were ranked from shortest to longestduration. It was noted whether the next feeding spell took place in a different FSfrom where the previous feeding spell occurred. If foraging sites were indepen-dent, a change in FS would occur with similar probability, regardless of the timelapse between successive feeding spells. Since the observed changes in FSsshowed no significant difference from the hypothetical data distribution underindependence of foraging sites (P>0·50, Kolmogorov–Smirnov one-sample test),foraging sites were considered to be independent.

Observations were conducted over up to 3 days every 4–6 weeks between late1983 and late 1985. Observation days comprising less than 8 h of continuousobservations were excluded from data analysis.

Grass sward analysisOne to three days after herd observations were conducted, the detailed structureof the grass layer in FSs and NFSs was analysed using a horizontal point frame(HPF; Grimsdell & Bell 1975). Two vertical aluminium poles, 1·5 m tall and 1 m



apart, were aligned north–south. The poles contained holes every 30 mm fromtop to bottom, ending at 50 mm above ground level. A 2 mm diameter, straightmetal wire was first pushed through the hole in one vertical pole coinciding withthe top of the grass sward, and then horizontally guided to the correspondinghole in the other pole. The wire was partitioned into 25 sections of 40 mm each.All grass parts touching the wire were classed as green or dry leaf, sheath or stem(including inflorescence), and counted. The wire was then repeatedly lowered30 mm, and the same procedure was followed at each level. For over half of allhorizontal point frame analyses, the number of partitions touched by plant partsat 110, 200 and 320 mm above ground level was recorded, to index the extent towhich plant parts were clustered.

All species of grass growing within 1 m of the horizontal point frame werelisted. The top level of the sward was recorded by means of a measuring tape.The height of the grass sward leaf table was defined as the height above groundlevel below which 95% of all leaf recordings occurred; this height was determineda posteriori from the HPF records. The number of sample stations ranged fromthree to 35 per season and per LSU. In all, 237 horizontal point frame analyseswere carried out, resulting in ~67,000 plant part records.

The factors recorded were grouped into three categories. ‘Forage quantity’encompassed leaf density and green leaf density. ‘Forage quality’ includedleaf:stem ratio and green leaf:old leaf ratio. ‘Feeding deterrents’ included stemdensity. Since more than 95% of all feeding records for roan occurred below300 mm above ground level (Heitkönig, unpubl.), records made above that levelwere ignored and the remaining levels were grouped into two height classes. The‘lower sward’ comprised the levels from 50 to 140 mm above ground level, whilethe ‘upper sward’ comprised the levels from 170 to 290 mm above ground level.Leaf density was significantly higher in the lower sward than in the upper sward(P<0·001, Kolmogorov–Smirnov test).

Aggregate values for each point frame measurement were calculated asfollows. Leaf or stem density was calculated as the sum of densities (m"1)recorded at each level, divided by the number of levels, for each sward layer. Theratio of green:old leaf was calculated as (green leaf density at lower swardlevel+green leaf density at upper sward level)/(old leaf density at lower swardlevel+old leaf density at upper sward level). A similar procedure was followedfor other ratios.

Aggregate sward measures were transformed, averaged, and their standarderror calculated for foraging sites and nonforaging sites by landscape unit andseason.

Seasons

Seasons were demarcated using changes in grass phenology. The early wet seasonspanned the period between rain-initiated grass growth and grass flowering(October–December). During the late wet season, grass remained green, mature,and postflowering (January–March). The early dry season was characterizedby progressive drying out and browning of grass (April–June). By the latedry season grass was mostly dry and brown and green leaves were sparse(July–September).



Selection for landscape units

For each foraging site, the associated landscape unit was listed and the listingsfor each LSU were summed by season. The seasonal sums were compared to theexpected sums based on the proportional area of each LSU relative to the totalarea of the Reserve available to roan, to test the null hypothesis of no selectionamong LSUs. LSU occupation was tested for independence, analogous to thetest of independence on foraging sites. It was noted whether the next feedingspell took place in a different LSU from where the previous feeding spelloccurred. Since the observed changes in LSUs showed no significant differencefrom the hypothetical data distribution under independence of LSUs (P>0·20,Kolmogorov–Smirnov one-sample test), LSUs visited by roan were consideredto be independent.

Statistical analysis

Chi-squared goodness-of-fit tests were applied to the seasonal frequencies ofLSU occupation by roan. Our data met the chi-squared restraints suggested byRoscoe & Byars (1971) down to the analysis of the wet vs. dry season contrast,but not on a more detailed seasonal level. To determine selection among LSUs,a multiple range comparison was carried out by applying the Bonferroni zstatistic to establish confidence limits around the true proportion of utilizationfor each LSU at 95% confidence level (Neu et al., 1974; Byers et al., 1984).

Grass sward density measures were square root transformed and ratios log(x+1) transformed to normalize the data (Duncan 1983) and to stabilize andequalize the variances (Zar 1984). Averages and standard errors were calculatedfor each variable by foraging site and by nonforaging site, and by season forthose landscape units that were frequently used. Two-tailed t-tests were appliedto these averages.

ResultsSelection for soil types and landscape units

On a year-round basis, roan preferred grassland above savanna landscapes(P<0·001, chi-squared, 5 d.f.) and avoided sandstone substrate (P<0·01, chi-squared, 5 d.f.) (Table 2). Selection among grassland types did not dependstrongly on soil type (P>0·15, chi-squared, 2 d.f.). Grassland on felsite waspreferred in the wet season (P<0·05, Bonferroni conf. int.) while grasslandunderlain by alluvium was preferred in the dry season (P<0·05, Bonferroni conf.int.) (Table 2). Savanna on sandstone was avoided throughout the year (P<0·05,Bonferroni conf. int.) (Table 2).

Grass sward characteristics in foraging sites

In Table 3 sward characteristics for preferred LSUs (use:availability ratio >1)are listed, with seasonally most favourable sward characteristics underlined.Early wet season alluvial grassland, the only LSU listed in Table 3 witha use:availability ratio <1 (rank 5), did not apparently offer particularlyfavourable properties then. In general, the seasonally most favourable sward



characteristics were found outside alluvial grasslands (underlined in Table 3).LSUs with rank 1 were consistently characterized by the highest density of greenleaf in the upper sward layer. Green leaf density peaked in the late wet season(15 m"1 at the lower sward level) on felsite savanna. Old leaf density peakedin the late dry season (8 m"1 at the higher sward level) in alluvial grassland.Sward quality (green:old leaf ratio and leaf:stem ratio) reached the highestvalues on felsite and sandstone substrate in almost all cases, and showed agradual decline throughout the year. Felsite savanna ranked only third amongpreferred LSUs in the late wet and early dry seasons, but offered favourablesward quality (e.g. leaf:stem ratio near 6:1) and deterrent properties. In particu-lar, stem density values near 1 m"1 were about half those of other LSUs then.The lowest stem density values tended to occur in FSs other than alluvialgrassland throughout the year. The leaf table and sward top layer varied littleamong LSUs (Table 3).

Contrasts between FSs and NFSs within landscape units

Contrasts between FSs and NFSs were more pronounced in favouredLSUs other than alluvial grasslands for quantity, quality, deterrent and height

Table 2. Simultaneous confidence intervals using the Bonferroni approach for utilization of landscapeunits by roan antelope in the Nylsvley N.R., South Africa, based on feeding spell records. The Bonferroniconfidence interval around the true observed proportion of usage is compared to the expected proportionof usage

LSU Season

No. offeedingspells

observed

Prop. offeedingspells

observed

Areasize(ha)

Expectedprop. ofusage

Bonferroni conf. int.on proportion of use

(95% familyconfidence coefficient)

Sign.level

Alluvial grassland Wet 17 0·3207 538 0·234 0·1518<pAlG<0·4898 NSDry 26 0·5531 0·3621<pAlG<0·7443 *Year 43 0·4300 0·2995<pAlG<0·5605 *

Alluvial savanna Wet 8 0·1509 488 0·212 0·0213<pAlS<0·2805 NSDry 6 0·1276 0·0000<pAlS<0·2560 NSYear 14 0·1400 0·0486<pAlS<0·2314 NS

Felsite grassland Wet 11 0·2075 136 0·059 0·0607<pFeG<0·3543 *Dry 3 0·0638 0·0000<pFeG<0·1577 NSYear 14 0·1400 0·0486<pFeG<0·2314 NS

Felsite savanna Wet 10 0·1886 492 0·214 0·0471<pFeS<0·3303 NSDry 7 0·1489 0·0121<pFeS<0·2857 NSYear 17 0·1700 0·0710<pFeS<0·2690 NS

Sandstone grassland Wet 5 0·0943 189 0·082 0·0000<pSdG<0·2001 NSDry 4 0·0851 0·0000<pSdG<0·1923 NSYear 9 0·0900 0·0146<pSdG<0·1654 NS

Sandstone savanna Wet 2 0·0377 458 0·199 0·0000<pSdS<0·1066 *Dry 1 0·0212 0·0000<pSdS<0·0768 *Year 3 0·0300 0·0000<pSdS<0·0749 *

Total Wet 53 2300Dry 47Year 100



Table3.Seasonalcharacteristicsofgrassswardstructureinroan

foraging

sitesinseveralimportantlandscapeunits,NylsvleyN.R.,SouthAfrica,basedon

average

seasonalrecordsfrom

HorizontalPointFrame(HPF)records.SE

values(inbrackets)arederivedfrom

squarerootorlogtransformed,normalizeddatavalues,and

areindicativeoftrueSE

valuesonly.LSU

swererankedbytheirseasonaluse/availabilityratio(leftcolumn).Boldfiguresindicateseasonallyhighestswardquantityor

qualityvalues,orlowestswarddeterrentvaluesinLSU

slisted

LSUSeason,rank

Foragequantity

Foragequality

Foragedeterrent

Swardheight

No.ofHPF

records

Greenleaf

density

(m"1)‘low’1

Greenleaf

density

(m"1)‘high’2

Oldleaf

density

(m"1)‘high’

Green:oldleaf

ratio

Leaf:stem

ratio

Stem

density

(m"1)‘low’

Stem

density

(m"1)‘high’

Leaf

table

(mm)

Top

layer

(mm)

Alluvialgrassland

Earlywet,5

7·3

0·1

0·1

3·6

3·1

1·9

0·1

118

205

25(0·41)

(0·01)

(0·00)

(0·32)

(0·20)

(0·17)

(0·00)

(3)

(8)

Latewet,1

10·4

9·8

3·2

1·9

2·1

10·3

5·5

561

1096

10(1·24)

(0·94)

(0·34)

(0·36)

(0·17)

(0·96)

(0·40)

(36)

(52)

Earlydry,2

5·0

3·0

5·2

0·6

2·4

7·5

3·0

460

1020

27(0·37)

(0·28)

(0·34)

(0·09)

(0·11)

(0·41)

(0·20)

(16)

(27)

Latedry,1

1·0

0·2

8·3

0·0

4·8

3·5

2·2

325

845

9(0·10)

(0·02)

(0·57)

(0·01)

(0·41)

(0·21)

(0·27)

(11)

(21)

Felsitegrassland

Earlywet,1

8·4

0·8

1·6

0·4

8·5

2·1

0·8

219

789

5(0·56)

(0·05)

(0·25)

(0·07)

(0·88)

(0·39)

(0·15)

(5)

(15)

Latewet,2

12·1

9·2

3·4

1·3

3·4

7·2

4·3

394

1251

9(0·63)

(0·36)

(0·38)

(0·24)

(0·26)

(0·53)

(0·36)

(20)

(61)

Earlydry,1

11·5

4·7

2·9

1·2

5·9

2·2

2·1

291

1594

6(0·91)

(0·39)

(0·15)

(0·13)

(0·57)

(0·30)

(0·23)

(7)

(61)

Latedry,6

——

——

——

——

—0



Table3.Continued.

LSUSeason,rank

Foragequantity

Foragequality

Foragedeterrent

Swardheight

No.ofHPF

records

Greenleaf

density

(m"1)‘low’1

Greenleaf

density

(m"1)‘high’2

Oldleaf

density

(m"1)‘high’

Green:oldleaf

ratio

Leaf:stem

ratio

Stem

density

(m"1)‘low’

Stem

density

(m"1)‘high’

Leaf

table

(mm)

Top

layer

(mm)

Sandstonegrassland

Earlywet,2

11·2

0·2

0·0

5·9

6·1

1·0

0·0

130

144

6(0·23)

(0·02)

(0·00)

(0·19)

(0·16)

(0·03)

(0·00)

(3)

(5)

Latewet,5

——

——

——

——

—0

Earlydry,5

——

——

——

——

—0

Latedry,2

3·4

0·7

5·1

0·2

3·8

4·9

1·4

211

516

6(0·37)

(0·14)

(1·10)

(0·05)

(0·45)

(0·74)

(0·24)

(19)

(47)

Felsitesavanna

Earlywet,6

——

——

——

——

—0

Latewet,3

14·7

7·8

1·3

3·1

6·4

2·7

1·4

366

1403

35(0·70)

(0·25)

(0·10)

(0·22)

(0·27)

(0·13)

(0·08)

(9)

(35)

Earlydry,3

13·3

4·0

1·3

2·3

5·9

2·9

0·7

257

1318

9(0·78)

(0·37)

(0·23)

(0·39)

(0·32)

(0·22)

(0·08)

(10)

(89)

Latedry,5

——

——

——

——

—0

1‘low’referstothelowergrasslayer(<150mm).

2‘high’referstotheupperswardlayer(150–300mm).



parameters of swards (Table 4). Old leaf density in the upper sward layer wasoften significantly lower in FSs than in NFSs in alluvial and sandstone grassland,and in felsite savanna. Green:old leaf ratio was usually significantly higher in FSsthan in NFSs on alluvial and sandstone grassland, but similar or lower in felsitegrassland. Alluvial grassland failed to show a higher ratio of leaf:stem in FSsthan in NFSs, in contrast to other grasslands and felsite savanna. Stem densitieswere significantly lower in FSs than in NFSs, except in alluvial and felsitegrassland in the late wet and early dry seasons. Grass height in FSs tended to besimilar to or lower than in NFSs. Plant parts were significantly more clustered inFSs compared to NFSs in most LSUs for part of the year, except on felsitegrassland in the early dry season.

Discussion

General methodological considerations

It is conceivable that the grass sward structure, in particular the green ediblebiomass, was altered by roan feeding prior to the analyses being conducted. Thispotentially resulted in lower densities of green leaves and sheaths, lowergreen:old leaf ratios and lower leaf:stem ratios in FSs when compared to theprefeeding situation. Yet, we believe that the effect was relatively small, since thesward analyses were conducted in foraging sites, rather than in actual foragingstations.

Selection for soil types and landscape units

The preference that roan showed for grassland relative to wooded savanna forfeeding confirms the observations of Joubert (1976). Among African grazingungulates a similar preference for relatively open savanna is shown by wildebeest(Connochaetes gnou Zimmerman), zebra (Equus burchelli Gray), warthog(Phacochoerus aethiopicus (Pallas)) (Hirst 1975), reedbuck (Redunca arundinum(Boddaert)) (Sheppe & Osborne 1971), topi (Damaliscus lunatus (Burchell))(Duncan 1975), hartebeest (Alcelaphus buselaphus (Pallas)) (Lamprey 1963), andsome populations of buffalo (Syncerus caffer (Sparrman)) (Sheppe & Osborne1971; Sinclair 1977). In contrast sable antelope (Hippotragus niger (Harris))(Jarman 1972; Grobler 1981) and waterbuck (Kobus ellipsiprymnus (Ogilby))(Lamprey 1963; Tomlinson 1981) favour wooded savannas.

The type of landscape preferred by roan appears to be based primarily oncontrasts in physiognomy. Roan generally avoided sandstone substrate, exceptgrassland on sandstone in parts of the year. This suggests that soil type is ofsecondary importance in the selection of landscape type. The hypothesis thatroan select for regions of particular soil types with elevated nutrient levels withinlandscapes is therefore not entirely supported.

Seasonal grass sward selection

In the early wet season, roan foraging sites had a high proportion of green leafrelative to old leaf and a low stem density. During this season, when foraging



Table4.Seasonaldi

fferencesingrassswardstructurebetweenroan

FSsandNFSsinseveralimportantlandscapeunits,NylsvleyN.R.,SouthAfrica.Landscapeunits

arerankedforseasonaluse/availabilityratios(leftcolumn).SignsindicatehowthevaluesforFSsdi

fferfrom

NFSsforeachvariableperseason:"=significantlyless

(P<0·05);("

)marginally

less(0·10<P<0·05);+=significantlymore(P<0·05);(+)marginally

more(0·10<P<0·05);0=notmarginally

different(P>0·10)

LSUSeason,rank

Quantity

Quality

Deterrent

Height

Clusters

n(FS)

n(NFS)

Greenleaf+

sheethdensity

(m"1)‘low’1

Oldleaf

density(m

"1)

‘high’2

Green:oldleaf

ratio

Leaf:stem

ratio

Stem

density

(m"1)‘high’

Leaf

table

Height

110mm

Height

200mm

Height

320mm

Alluvialgrassland

Earlywet,5

(")

—+

0—

——

——

2512

Latewet,1

00

00

00

103

Earlydry,2

0—

+0

00

2715

Latedry,1

Felsitegrassland

Earlywet,1

—0

—+

—("

)5

6Latewet,2

—0

—0

00

99

Earlydry,1

00

00

0("

)0

00

66

Latedry,6

Sandstonegrassland

Earlywet,2

0—

++

——

69

Latewet,5

Earlydry,5

Latedry,2

(+)

00

+—

—0

(")

—6

6

Felsitesavanna

Earlywet,6

Latewet,3

0—

0+

—0

356

Earlydry,3

0—

(+)

+—

00

——

93

Latedry,5

1‘low’referstothelowergrassswardlayer(<150mm).

2‘high’referstotheuppergrassswardlayer(150–300mm).



sites included swards regrowing after dry season burns, sward quality rather thanquantity attracted roan. In the late wet and early dry seasons roan appearedattracted to green leaf quantities in the upper sward layer in both felsite andalluvial grasslands, tolerating high stem densities there. As the dry seasonprogressed, a favourable leaf:stem ratio was the main discriminating factorrather than green leaf density, which had reached low values by then. Althoughold leaf density tended to be high in the most preferred landscape units, roanoften foraged in sites with lower densities. This suggests that old leaf acted as adeterrent rather than as an attractant. An additional factor in the selection ofgrass swards seems to be the clustering of edible biomass. This may havefacilitated the intake of largely stem-free material, particularly in less preferredareas.

Bell (1984) offered a working hypothesis explaining regional differencesin plant nutrient levels linked to soil moisture and nutrient status. Alluvialsoils with low water infiltration in drainage depressions are nutrient enriched.The high soil water availability promotes carbohydrate photosynthesisand grasses growing in such regions tend to have nutrient concentrationsreduced by a high cell wall fraction. In higher lying areas, where soil nutrientlevels are lower, soil water availability is also reduced. Grasses growing inthese regions tend to contain nutrients that are less diluted by structuralcarbohydrates.

This working hypothesis may help explain seasonal sward selection by roanantelope. Grass swards on alluvium and on moderately nutrient rich felsiteenabled the intake of nutrient-rich plant material by roan, possibly together withmuch plant fibre in the late wet and early dry seasons. Our data suggest thatswards on alluvium did not provide larger quantities of green leaf in the dryseason than did other grasslands, despite their more clayey substrate. Theapparent lack of stem-avoidance by roan when foraging in preferred alluvial andfelsite grasslands when leaf quantities were high suggests that roan are stem-tolerant in open habitats with intermediate to high nutrient levels. It appearsthat stem density is an effective deterrent only in nutrient-poor and woodedhabitats, where roan tended to discriminate more strongly between foraging andnonforaging sites.

Sward quality appeared to be particularly important to roan in the early wetand late dry seasons, and food quantity in the late wet and early dry seasons.We suggest that roan may be stem-tolerant in grassland on substrate withintermediate to high nutrient levels. The second hypothesis, that roan select forgrass swards offering particularly favourable features of food abundance orquality, is largely supported.

We conclude that the factors governing grass sward selection by roan appearbasically similar to those influencing other grazing ungulates (Duncan 1975;Murray & Brown 1993). Nevertheless, roan appear to be particularly tolerant todeterrents in open areas with higher soil nutrient status. This feature, as well astheir extensive use of grasslands growing in areas with less fertile soils suggeststhat roan either (i) tolerate low quality food better than other large herbivores or(ii) discriminate better at plant part level than do other ruminant species. Thisnutritional tolerance or their selection power at plant part level may underlietheir distribution in nutrient-poor savannas.



AcknowledgmentsThe Directorate of Nature Conservation of the former Transvaal ProvincialAdministration permitted the roan study to be carried out in the Nylsvley N.R.Stuart Walls and Veronika Straass assisted with fieldwork. Arend Brunstingprovided helpful comments on a previous draft. The study formed a componentof the South African Savanna Biome Study funded by the Council for Scientificand Industrial Research.

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(Manuscript accepted 19 May 1997)



Seasonal selection of soil types and grass swards by roan antelope in a South African savanna

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Transcript of Seasonal selection of soil types and grass swards by roan antelope in a South African savanna