Functional Habitat Heterogeneity and Large Herbivore Seasonal Habitat Selection in Northern Botswana

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Functional Habitat Heterogeneity and Large HerbivoreSeasonal Habitat Selection in Northern BotswanaAuthor(s): Richard W.S. Fynn , Michael Chase & Achim RöderSource: South African Journal of Wildlife Research, 44(1):1-15. 2014.Published By: Southern African Wildlife Management AssociationDOI: http://dx.doi.org/10.3957/056.044.0103URL: http://www.bioone.org/doi/full/10.3957/056.044.0103

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Functional habitat heterogeneity andlarge herbivore seasonal habitatselection in northern Botswana

Richard W.S. Fynn1*, Michael Chase2 & Achim Röder3

1Okavango Research Institute, University of Botswana, Private Bag 285, Maun, Botswana2Elephants Without Borders, P.O. Box 682, Kasane, Botswana, and

Institute for Conservation Research, San Diego Zoo Global, 15600 San Pasqual Valley Rd,Escondido, CA 92027, U.S.A.

3Department of Environmental Remote Sensing and Geoinformatics, University of Trier, D-54286,Trier, Germany

Received 28 April 2013. Accepted 1 October 2013

This study aimed to determine the functional seasonal attributes for herbivores of the majorhabitats and landscapes of the Savuti-Mababe-Linyanti ecosystem (SMLE) of northern Botswanaand how various herbivore species responded to this heterogeneity. Floodplain grasslandsand dambo grasslands provided the only significant green forage and biomass during thelate dry season, whereas short grasslands of the Mababe Depression provided the highestforage quality of all habitats during the wet season. The ability to provide reliable forage anddrinking water in floodplain,swamp and dambo grasslands attracted large concentrations ofzebra and buffalo during the dry season, which was mediated by fire. Large concentrationsof zebra were observed in mineral-rich grasslands of the Mababe Depression during the wetseason, whereas buffalo were not observed in these open grassland landscapes in thisseason. Other herbivore species appeared to use the same landscapes year-round wherelechwe were observed mainly in floodplain and swamp landscapes on the western edge ofthe Linyanti Swamps, while wildebeest and impala were observed in floodplains and adjacentwoodlands near permanent water suggesting that these species had access to sufficientresources at a landscape scale. Thus various herbivore species responded differently tofunctional heterogeneity across seasons and scales.

Key words: Botswana, seasonal resources, forage quality, drinking water, foraging strategy,resource limitation.

INTRODUCTIONThe size and stability of large herbivore populationsis dependent upon the ability to adapt to stronginter-annual and inter-seasonal variation in foragequantity and quality, while minimizing the risk ofpredation (Rettie & Messier 2000; Owen-Smith2004; Hopcraft et al. 2010). During the wet-seasonextra demands on females by rapidly maturingfoetuses and for lactation after calving result inelevated demands not only for protein and energybut also for minerals such as Ca, Mg, Na and P(Murray 1995; Parker et al. 2009). During the latedry-season nutrient and energy levels in foragegenerally decline to below maintenance levels inmost habitats (Sinclair 1975; Ellis & Swift 1988;Owen-Smith 2008) but this is associated withreduced demand for these resources outside thewet-season period of calving and lactation (Parker

et al. 2009). Thus, highest forage quality is neededduring the wet-season and only adequate foragequality during the dry season, provided that foragequantity is sufficient (Owen-Smith 2004; Parkeret al. 2009). During extended droughts, however,forage quantity may become severely limitingleading to depletion of body stores and populationcollapses in ecosystems where adaptive foragingoptions are limited (Walker et al. 1987; Ellis & Swift1988; Fynn & Bonyongo 2011).

A habitat may meet the resource requirements ofa herbivore in one season but often not in otherseasons, and thus is functional for a specific herbi-vore species only in the season that its mainte-nance requirements are met. Thus, functionalwet-season habitats are those that are able toprovide intake rates of energy, protein and mineralsat levels that meet the maintenance requirementsof pregnant and lactating females and rapidlygrowing calves (Parker et al. 2009; Hopcraft et al.

South African Journal of Wildlife Research 44(1): 1–15 (April 2014)

*To whom correspondence should be addressed.E-mail: [email protected]

2010). These habitats also need to provide lowpredation risk, especially during the calving period(Smuts 1978; Riginos & Grace 2008; Hopcraftet al. 2010). Functional dry-season habitats arethose that are able to provide dependable ade-quate-quality forage as a reserve resource for thelate dry season, as well as a key resource ofrarely eaten, low-quality forage that can bufferherbivore populations against the effects of drought(Illius & O’Connor 2000; Owen-Smith 2002: 255).Functional dry-season habitats are inextricablylinked to permanent water supplies with dependabledrinking water generally dictating where herbivoresmay forage over the dry season but the impor-tance of water will differ across species (Hayward& Hayward 2012). In its simplest form, functionalheterogeneity can be defined as an ecosystemcontaining a mix of functional wet- and dry-seasonhabitats within viable migration distance of eachother.

Habitats that are able to maintain forage produc-tion during the moisture-limited dry season (func-tional dry-season habitats) are generally limited towetlands with a shallow water table (e.g. Vesey-FitzGerald 1960, 1963; Acres et al. 1985; Roberts1988). Grasses adapted to conditions of greaterproductivity and litter accumulation in wetlandhabitats select for structural compounds (carbon,cellulose and lignin) to support high-biomass accu-mulation and the ability to penetrate their own litter(Fynn et al., 2011) leading to dilution of nutrients(Jarrell & Beverley 1981) and decreased digestibility(Hobbs & Swift 1988;Wilmshurst et al.2000).Thusnutritious and digestible forage (a key functionalaspect of wet-season habitats) is usually limited toless productive moisture-limited habitats on shallowupland soils or on saline soils (McNaughton &Banyikwa 1995; Murray 1995; Grant & Scholes2006;Hopcraft et al.2010).For example, the highlyproductive, high-rainfall areas of the Serengetiwoodlands have lower forage-quality than thelow-rainfall Serengeti plains but provide more reli-able forage and greenery over the dry season(McNaughton & Banyikwa 1995; Murray 1995).

Our key question was what functional attributesdo the various habitat types of the SMLE offer toherbivores during the wet- and dry-season and whatare the landscape and environmental featuresdetermining these attributes? Recognizing thatsoil moisture is the primary factor limiting theproductivity of African savannas (Desmukh 1984),our first hypothesis (H1) was that, owing to theirmore favourable soil moisture regimes for plant

growth, wetland habitats would be moreproductive and provide more reliable forage forherbivores over the dry season (in terms of green-ness and biomass) than the various drylandhabitats of the region. However, our secondhypothesis (H2) was that greater productivity andbiomass accumulation in wetland habitats, wouldresult in lower nutrient concentrations in foragethan dryland habitats (Jarrell & Beverley 1981;Wilmshurst et al. 2000). Finally, our third hypothesis(H3) was that differences in body size and mouthadaptations among herbivore species would resultin different distributions of these species in relationto the abundance of resources across habitattypes (Wilmshurst et al. 2000; Arsenault & Owen-Smith 2008). Thus the objectives of our study wereto: (1) determine the key functional seasonalattributes of the various habitats in the SMLE;(2) link these functional seasonal attributes tounderlying landscape and environmental drivers;(3) determine seasonal habitat selection by variouslarge herbivores in relation to seasonal attributesof various habitats and in relation to type of herbi-vore; and (4) compare our findings with those ofthe 1991–1993 study (Joos-Vandewalle 2000)when the hydrology of the ecosystem was verydifferent.

METHODS

Study areaThe Savuti-Mababe-Linyanti ecosystem (SMLE)

makes up part of the >80 000 km2 northern conser-vation area of Botswana (Fig. 1).The climate of theregion is semi-arid and can be classified into threemain seasons: (1) the wet-season period, whichvaries inter-annually in length according to theunpredictable timing of the start and end of goodrainfall events (Fig. 2); (2) the cool dry season,which commences after the last rains to when dailytemperatures start to rise at the end of August and(3) the stressful hot dry season between Septemberand the first rains of the next season, characterizedby an absence of rain for four to five months prior toits commencement and maximum daily tempera-tures mostly between 35 and 40°C (Fig. 2).

The SMLE region was shaped by prehistoric in-flows of large river systems depositing alluvialsediments, over which lie deep aeolian deposits ofKalahari sands (Mendelsohn et al. 2010). Thedominant soil system over much of the SMLE is amosaic of sand-filled paleoriver channels amongan alluvial matrix (Photo 1d in online supplement),

2 South African Journal of Wildlife Research Vol. 44, No. 1, April 2014

which gives rise to a landscape mosaic of Mopanewoodland (Photo 1a,b online) on alluvial soils andSandveld woodland (Photo 1c online) on deepsands (Table 1). In the eastern part of the ChobeEnclave the sand/alluvial mosaic gives way to alandscape mosaic of low-lying poorly drainedgrasslands, known as dambos, vleis or wet mead-ows, among more elevated and better drainedwoodlands (Vesey-FitzGerald 1963; Acres et al.1985; Roberts 1988) (Fig. 1; Table 1; Photo 2c,donline). Dambos have been defined as ‘Sea-sonally waterlogged, predominantly grass-

covered, shallow, linear depressions, frequentlywithout a marked stream channel’ (Acres et al.1985) (see Photo 2e online). In the south easternpart of the SMLE a large paleolake sump known asthe Mababe Depression (Teter 2007) replaces thedominant sand/alluvial mosaic with heavy claysoils characterized by open grassland and sparseAcacia woodland (Table 1; Fig. 1; Photo 3 online).The Okavango and Kwando river systems arisingin the distant high-rainfall Angolan highlandsgive rise to riverine woodland, swampland andfloodplain grassland habitats of the Okavango

Fynn et al.: Large herbivore seasonal habitat selection in northern Botswana 3

Fig. 1. Map of the the Savuti-Mababe-Linyanti ecosystem (SMLE) of northern Botswana, which consists of a mix ofseasonal habitats and landscapes such as the Mababe Depression, Savuti Channel, Selinda Spillway, LinyantiSwamps and Chobe Enclave dambo grasslands. M-S = Mopane-Sandveld mosaic.

Delta, Selinda Spillway, Linyanti Swamps andSavuti Channel (Mendelsohn et al. 2010) (Table 1;Fig. 1; Photos 2a,b and 4c,d online). The SavutiChannel river system is an outflow from theLinyanti Swamps and supports importantfloodplain grassland for wildlife along its coursedown to its distal end in the Savuti Marsh at thenorthern end of the Mababe Depression – (Fig. 1;Photo 3b online). Riverine woodland occurs adja-cent to river channel, swamp and floodplain grass-land of the region (Table 1; Photo 4a,b online).

Herbivore seasonal habitat selectionTo document the broad-scale wet- and dry-

season distribution patterns of the various herbi-vore species in the SMLE, we made use of severalaerial survey data sets: (1) a survey conducted byR. Fynn in the late dry season between 31 Octoberand 1 November 2010 and in the mid wet seasonon 19 February 2011; (2) three late dry-seasonsurveys conducted by M. Chase between 25 and

28 October 2010, 2 and 4 September 2011 and on3 September 2012. Surveys by R. Fynn were con-ducted at 152 m (500 ft) above ground level usinga four-seater Cessna light aircraft with two observ-ers and the pilot while surveys by M. Chase wereconducted at 91 m (300 ft) above ground levelusing a four-seater Cessna light aircraft withthree observers and the pilot. For the purposes ofthis paper our aim was not to obtain a populationcensus of each herbivore species but rather todocument their seasonal concentrations. Aerialsurvey data do not provide sufficient accuracyof animal positions to quantify fine-scale habitatselection but we were able to quantify selection ofbroader-scale habitat/landscape types for buffaloand zebra in the key dry-season range of theChobe Enclave (observations of other specieswere too low in this region to analyse). The ChobeEnclave has three functional landscape types:(1) dambo landscapes (well-drained upland wood-land to poorly drained lowland dambo grassland),


Fig. 2. Daily rainfall (bars) and maximum daily temperatures (points) for the three seasons of the study. Data are fromthe Okavango Research Institute (ORI) automatic weather station, which is the closest station to the research area.

(2) the Mopane-Sandveld mosaic woodland land-scape and (3) the Linyanti Swamps landscape (amix of tall reed and sedge swamp habitat andshort, medium and tall grass floodplain habitats onislands). Jacobs index (Jacobs 1974) was calcu-lated to determine the degree of positive or nega-tive selection for a specific landscape type bybuffalo or zebra according to the equation D =(r – p)/(r + p – 2rp), where r is the proportion ofobservations in a specific landscape type and p isthe proportional area of that landscape type.Values between 0 and –1 indicate negative selec-tion and between 0 and +1 indicates positiveselection.

Habitat samplingTo characterize functional differences in resources

between habitats we determined grass height,biomass, greenness and forage quality in each ofthe major habitats of the SMLE. Habitats (seeTable 1) were sampled in the late dry season (lateOctober/early November 2010) and in the mid wetseason (mid–late January 2011) at 15 differentsites per habitat type, except for Sandveld wood-land habitats, which were only sampled at eightsites in the late dry season (15 in the wet season)and Mababe short grassland habitats, which wereonly sampled at 10 sites in the wet season (15 inthe dry season). Positions of sites sampled in thedry season were not specifically relocated duringwet-season sampling. At each site, a 50 m tapemeasure was laid down subjectively in what wasconsidered representative vegetation of that habi-tat type or parts of the habitat that were observedto be specifically favoured by herbivores, such asgreener areas during the dry season (e.g. Photo 2online). Thus during the dry-season survey we se-lected for the parts of habitats able to producegreen leaf at this time, such as the lower-lyingparts of floodplain grassland and dambo grass-lands.At each site a 0.25 m2 quadrat was placed atthe 0, 10, 20, 30 and 40 m marks on a tape mea-sure with each of the five quadrats on a transectrepresenting a sub-sample with the mean of thefive quadrats being considered a sample. In eachquadrat, we estimated grass greenness (percentgreen vs brown leaf and stem) and measuredgrass height by laying a 30 × 15 cm brown paperbag on top of the sward at the centre of the quadratand measuring its height above the soil surface.During the wet-season survey, we determinedgrass composition by enumerating all grassesrooted within the quadrat and visually estimated

the aerial cover value (% of quadrat area) of eachspecies. In both the wet- and dry-season surveywe determined grass standing biomass by clippingall grasses rooted within the quadrat at groundlevel, placing them in brown paper bags andoven-drying in the lab at 60°C to constant massand then weighing. After weighing we cut up thegrass into smaller segments using clippers, thor-oughly mixed these segments with those of thegrasses of the other four quadrats on that transectand selected a subset for milling and nutrient analy-sis (only samples from the wet-season surveywere analysed). Nutrient analyses (protein andminerals) were conducted at the feed analysis lab-oratory of the University of KwaZulu-Natal inPietermaritzburg, South Africa. Protein analyseswere conducted using Dumas combustion on aLECO CNS combustion analyser. Mineral analyseswere conducted by acid digestion and elementalanalysis on an Inductively Coupled Plasma massspectrometer (ICP).

Statistical analysesFor each parameter (grass height, grass bio-

mass, grass greenness, grass nutrient content),we calculated the mean value for each habitat typeas the mean of the number of samples/sites withineach habitat type. Before testing for statistical dif-ferences among means, tests of normality of thedata were run using the Shapiro-Wilk normalitytest in R and homogeneity of variance using the‘leveneTest’ function in the ‘car’ package of R(R Core Team 2013). If parameters did not satisfyassumptions of either normality or homogeneity ofvar iance they were analysed using thenon-parametric Kruskal-Wallis test followed bymultiple comparison tests using the ‘kruskalmc’function in the ‘pgirmess’ package in R (R CoreTeam 2013). Data for protein and sodium, how-ever, satisfied assumptions of Anova and wereanalysed by Anova followed by the Tukey HonestSignificant Difference test using the ‘TukeyHSD’function in R (R Core Team 2013).

RESULTS

Functional characteristics of seasonalhabitats

Floodplain grassland and dambo grassland sup-ported the only significant biomass and greennessduring the late dry-season (Fig. 3a,b; Photo 2 inonline supplement), as well as reliable drinkingwater. Despite being dominated by tall productive


6 South African Journal of Wildlife Research Vol. 44, No. 1, April 2014Ta

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When considering nutrient concentrations, how-ever, Mababe region short grasslands, had higherconcentrations Mg, Na, K, P and protein thanfloodplain grassland but only Ca and protein werehigher than dambo grasslands (Fig. 4). Savutiregion short grasslands had lower forage qualitythan Mababe region short grasslands but did havehigher Na and K concentrations than floodplain

grassland (Fig. 4c,d) and higher protein thandambo grassland (Fig. 4f). The Mababe regionlandscape consists of short grasslands dominatedby short digestible annual grasses (Appendix 1 on-line; Photo 3d online), and occur as a narrow band(generally <1 km wide) around the edges of thesouthern half of the Mababe Depression (Mababeregion) between the tree line on the edge of thedepression and deeper soils further into thedepression, which are dominated by taller grassessuch as Cenchrus ciliaris and a Bothriochloa sp.(not surveyed). The Savuti region landscape isgenerally open grassland savanna with varyingdensities of Acacia hebeclada and mainly shortgrassland, dominated by saline-tolerant lawngrasses (Appendix 1 online; Photo 3a online) butalso with patches of taller grassland (not sur-veyed). Detail on grass composition in each habitattype may be found in Appendix 1 online (nomen-clature for grasses according to Germishuizenet al. 2006).

Herbivore seasonal habitat selectionDuring the 2010 late dry season many zebra

(Equus burchelli) herds were observed in floodplain


Fig. 3. Average grass standing biomass (a, c) and greenness (b, d) during the late dry season and mid wet season ofthe major habitats in the Savuti-Mababe-Linyanti ecosystem of northern Botswana. Bars represent 95% confidenceintervals. For within season contrasts, habitats with the same letters are not significantly different (P < 0.05).

grassland and adjacent woodland landscapes ofthe upper half of the Savuti Channel, in the dambograssland landscape of the Chobe Enclave(Figs 5a and 6a; Photo 2 online) and some herds inwoodland landscapes adjacent the LinyantiSwamps (Fig. 6a; Photo 1b online). In the 2010late dry season in the Chobe Enclave, 79% ofzebra observations were in dambo landscapeswith a positive Jacobs index (J.I.) of 0.57 and selec-tion against woodland landscapes (J.I., –0.46)(Fig. 6a; Table 2). This trend was reversed, how-ever, in the 2011 and 2012 late dry seasons withzebra observations in woodland landscapes being73% (J.I., 0.78) and 82% (J.I., 0.86), respectively,while in dambo landscapes observations were27% (J.I., –0.47) and 14% (J.I., –0.73), respec-tively (Fig. 6b,c; Table 2). Buffalo (Synceruscaffer) were observed in similar landscapes to ze-bra during the late dry season, such as floodplaingrassland and adjacent woodland landscapes of

the Savuti Channel and woodland landscapes ofthe Chobe Enclave adjacent the Linyanti Swamps(Figs 5a and 6a–c; Photo 2 online). In contrast tozebra, however, buffalo strongly selected againstdambo landscapes in all years (J.I. –0.92 to –1.00)(Table 2). In all years, zebra strongly selectedagainst foraging in the Linyanti Swamp land-scapes (J.I. = –0.48 to –1.00), while buffalo hadstrong positive selection for swamp landscapes inthe 2010 (J.I. = 0.49) and 2011 (J.I. = 0.56) late dryseasons (Table 2).

In contrast to the late dry season, zebra werelargely observed concentrated in the open grass-land landscapes of the Mababe Depression(Savuti and Mababe regions) during the wet-season survey (Fig. 5b; Photo 3c online), whereasbuffalo were not observed in these open grass-lands but rather in woodland and dambo land-scapes (Fig. 5b). In both seasons, wildebeest(Connochaetes taurinus) were mainly concentrated


Fig. 4. Average mid wet-season grass nutrient content of key seasonal habitats of the Savuti-Mababe-Linyantiecosystem of northern Botswana. Bars represent 95% confidence intervals. Habitats with the same letters are notsignificantly different (P < 0.05).

around the Savuti and Mababe regions (Figs 5 and6; Photo 3a online). Similarly, impala (Aepycerosmelampus) were most commonly observed in bothseasons around the Savuti and Mababe regionsas well as in the floodplain and woodland land-scapes adjacent the western Linyanti Swamps,while lechwe (Kobus leche) were mainly observedin floodplain/swamp landscapes of the westernLinyanti Swamps (Figs 5 and 6a; Photo 4c online).

DISCUSSIONSeveral habitats of the SMLE appear to provideimportant functional heterogeneity of resourcesover the different seasons of the year that are likelynecessary for the maintenance of relatively stableand productive herbivore populations in the region(e.g. Owen-Smith 2004). Forage quality in theshort grasslands of the Mababe Depression,especially the annual-dominated grasslands in theregion of Mababe, was generally better than wet-land habitats (Fig. 4), in support of H2.The MababeDepression represents the sump of PaleolakeMababe (Teter 2007), which as it underwentprogressive desiccation and evaporative concen-tration of solutes, combined with erosion of calcium-and phosphorus-rich calcretes in local water-sheds, experienced deposition of large amountsof P and other minerals in the upper sediment layersof the lake sump (Teter 2007), translating into highermineral content in grass, especially P (Fig. 4). Thedigestible leafy nature, sufficient leaf biomass andheight and high nutrient concentrations of theshort annual-dominated grasslands in the Mababeregion constitute the characteristics of a high-quality resource (see Owen-Smith 2002: 255),which is critical for maintaining high populationproductivity (Cook et al. 1996; Person et al. 2003;Pettorelli et al. 2003; Owen-Smith 2004; Parkeret al. 2009). Similarly, previous volcanic eruptionshave provided extremely fertile saline soils onthe Serengeti short-grass plains, which form themajor wet-season range for Serengeti ungulates(Maddock 1979), where concentrations of Naand P in grasses are much higher than in theirhigher-rainfall and more productive dry seasonrange (Murray 1995). Apart from the higher foragequality of the Serengeti short-grass plains andthe Mababe Depression grasslands, another keyfeature of these two important wet-season rangesis that they are open grassland dominated by shortgrasses (Photo 3c,d online), which ensures highvisibility, thereby reducing predation risk.Selectionfor low-predation-risk habitats may be a key

Fynn et al.: Large herbivore seasonal habitat selection in northern Botswana 9Ta

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priority in the hierarchy of foraging decisions madeby herbivores (Sinclair & Arcese 1995; Rettie &Messier 2000; Riginos & Grace 2008), especiallyduring calving, with new-born calves being most vul-nerable to predation. Thus zebra, wildebeest andimpala were observed in these short grasslands ofthe Mababe Depression during the wet season(Fig. 5b), supporting the observations of a previousstudy (Joos-Vandewalle 2000).

In support of H3, buffalo with their large bodysize-mediated demands for high forage quantity(Wilmshurst et al. 2000) and poorly adaptedmouthparts for efficiently cropping short grass(Codron et al. 2008) were not observed in shortgrass areas selected by zebra, such as the shortgrasslands of the Mababe Depression during thewet season, or the short high-quality re-growth inthe dambo grasslands during the dry season


Fig. 5. (a) Dry-season (31 October – 1 November 2010) and (b) wet-season (19 February 2011) concentrations ofseveral herbivore species in the Savuti-Mababe-Linyanti ecosystem (SMLE) of northern Botswana (surveys conduc-ted by R. Fynn). Imagery is MODIS 16-day EVI satellite image product (MOD13Q1).


Fig. 6. Dry-season concentrations of several herbivore species in the Savuti-Mababe-Linyanti ecosystem (SMLE) ofnorthern Botswana (surveys conducted by M. Chase). (a) 25 – 28 October 2010; (b) 2–4 September 2011; (c) 3 Sep-tember 2012. Imagery is MODIS 16-day EVI satellite image product (MOD13Q1).

(Figs 5 and 6; Table 2). In support of this, severalstudies have shown buffalo to avoid short grass-lands that were actively selected by other grazers(Jarman & Sinclair 1979; Traill & Bigalke 2006;Bhola et al. 2012). Another possibility is that buf-falo herds cannot reach the dambo grasslandsduring the dry season, which are 10–15 km outfrom dependable dry-season water provided bythe Linyanti Swamps, whereas zebra herds havebeen shown to regularly forage between 10 and20 km out from the Boteti river in Makgadikgadiduring the dry season (Bradley 2012). This mayexplain why buffalo and not zebra used swamphabitat (Table 2) with buffalo being forced into for-aging in the Linyanti Swamps when forage innearby woodlands had been depleted by the latedry season. While our coverage of the woodlandsfurther inland from the Linyanti Swamps was lim-ited during the wet-season survey (Fig. 5b), thedata suggest that during the wet season buffalomerely used landscapes further out from theirdry-season concentration areas along major waterbodies (Fig. 5b), as observed for the adjacentChobe Riverfront region (Skarpe et al. 2004). Ourobservations of both zebra and buffalo wet- anddry-season concentrations are supported by re-cent satellite-collar tracking studies of both zebraand buffalo in the SMLE (K. Sianga, unpubl. data).

While floodplain and dambo grasslands displayedpoorer quality grazing than the grasslands of theMababe Depression (Fig. 4), they provided theonly significant biomass and green forage duringthe late dry season (Fig. 3a,b), which supportedH1. These functional attributes of floodplain grass-land and dambo grasslands were probably respon-sible for the observed concentrations of zebra andbuffalo in landscapes consisting of either swamps,floodplains and dambos and their adjacent wood-lands during the late dry season (Figs 5a and 6a;Table 2). Woodlands adjacent swamps, floodplainsand dambos provide additional heterogeneity offoraging options during the dry season because oftheir easy access to drinking water (adjacentswamps and floodplains) and alternate foragingoptions to those of these various wetland habitats.

Wetland habitats such as dambo grassland(Roberts 1988; Scoones 1995; Parrini & Owen-Smith 2010; Macandza et al. 2012) and floodplaingrassland (Tomlinson 1980; Prins & Beekman1989; Jarman 1972; Bartlam-Brooks et al. 2011)have been observed to be key dry-season habitatsin many African wildlife and pastoral systems. Thisis because deep-rooted perennial grasses are

able to access their characteristic shallow watertables (Acres et al. 1985; Roberts 1988), enablingthem to provide reliable forage during the dryseason. Thus during the late dry season, flood-plain grassland and dambo grassland may providea reserve resource for herbivores, which owing toits lower quality mediated by greater productivity(Jarrell & Beverley 1988; Hobbs & Swift 1988;Wilmshurst et al. 2000), is forage that is ignoredfor most of the annual cycle until more preferredresources have been depleted (see Owen-Smith2002: 255). In the event of a severe drought yearthe most productive and lowest quality compo-nents of wetlands such as tall, productive grasses,sedges and reeds, that are normally never eaten inaverage years, may provide a buffer resource oflow-quality forage (see Owen-Smith 2002: 255),otherwise known as a key resource (Illius &O’Connor 2000).

When, however, the normally coarse and indi-gestible vegetation in the productive parts of thesefloodplain and dambo grasslands is removed byburning or trampling, the resultant emergence offresh re-growth provides a bridge of green forageover the dry season known as a bridging resource(see Owen-Smith 2002: 241). Dry-season fires inmoist sites that enabled regrowth throughout thedry season were shown to greatly elevate proteinintake for sable antelope (Hippotragus niger) rela-tive to years when these moist sites did not burn,indicating the importance of fire in wetland habitatsfor dry-season nutrition (Parrini & Owen-Smith2010). The Rukwa valley floodplains in Tanzania(Vesey-FitzGerald 1960) and the Yaere floodplainsin Cameroon (Pamo 1998) provide classic exam-ples of the production of bridging resources afterfire and trampling, which supported large concen-trations of herbivores during the late dry season.These examples support our observations of largeconcentrations of zebra in dambo landscapesafter fire during the late dry season of 2010 andwith far fewer observed in these dambo landscapesin subsequent dry seasons when fire was absent(Figs 5a and 6a–c; Table 2; Photo 2c,d online).

In contrast to our observations of concentrationsof wildebeest in both the wet and dry seasons atSavuti and Mababe (Figs 5 and 6), in a previousstudy in the early 1990s wildebeest were neverobserved to remain on the Mababe Depressionover the dry season but rather to migrate to theChobe Enclave (Joos-Vandewalle 2000). It is likelythat a key difference between the 1991–1993 studyperiod and the current study period is that there is


now water year-round in both the Savuti andMababe regions of the Mababe Depression,whereas during the previous study period therewould have been no dry-season drinking wateravailable (Joos-Vandewalle 2000). Similarly,placement of artificial waterholes in KrugerNational Park, South Africa may have disruptedmigratory behaviour of certain zebra and wilde-beest populations (Smuts 1978). At the landscapescale, functional wet-season resources (nutritiousgrazing) are available in the Savuti and Mababeregions (Fig. 4), while localized floodplain grass-land around the Savuti and Mababe Marshes nowprovide dry-season grazing as well as drinkingwater. Thus both wildebeest and impala inthe SMLE appear to find these landscape-scalegradients in functional resource heterogeneitymore favourable to the regional-scale gradientsutilized by zebra, which also supported H3. Themuch larger zebra population is probably forced tomaintain its migration to obtain sufficient forage. Inthe Serengeti ecosystem most of the wildebeestmigrate on regional-scale functional resourcegradients (Maddock 1979), but some wildebeestchoose to be sedentary as they are able to findsufficient functional heterogeneity of resources atthe landscape scale (Bell 1970; Maddock 1979).Lechwe are wetland specialists and appeared tobe mainly restricted to suitable areas of shortfloodplain grassland in the western section of theLinyanti Swamps (Fig. 6a; Photo 4c online).

In conclusion, this study revealed that a mix ofinherent environmental factors, such as floodplains,dambos, woodlands and paleolakes, and inducedenvironmental factors, such as fire, played a keyrole in providing adaptive foraging options over theannual cycle but in different combinations andscales for different herbivore species.Conservationmanagers need to understand what mixes of habitattypes provide critical functional heterogeneity ofseasonal resources for different herbivore speciesand how these seasonal resources may be optimallymanipulated by applying fire in appropriate seasonsand landscape types (e.g. fire in wetlands for dry-season grazing and in drylands for wet-seasongrazing).

ACKNOWLEDGEMENTSWe thank the Botswana Ministry for the Environ-

ment, Wildlife and Tourism as well as the Depart-ment of Wildlife and National Parks for permissionto conduct this research. Cybertracker software(see http://cybertracker.org/) was used in the aerial

surveys by R. Fynn. Funding for aerial surveys andfieldwork by R. Fynn was provided by the Officeof Research Development at the University ofBotswana (UBR/RES 3/2). Funding for aerialsurveys by M. Chase was provided by BotswanaDepartment of Wildlife and National Parks,San Diego Zoo Global, The Paul G. Allen FamilyFoundation, Forest Conservation Botswana andMadeleine and Jerry Delman-Cohen. A. Röder’scontribution to the study was under the frame of‘The Future Okavango Project’ funded by the Ger-man Federal Ministry for Education and Research(BMBF). Field and laboratory assistance byKeoikantse Sianga is much appreciated. PilotsMike Holding and Louis Brink did an outstandingjob during the aerial surveys. We thank Wilder-ness Safaris for logistical support. This paper hasbeen greatly improved by comments from MattHayward, Isak Smit, Norman Owen-Smith, GrantHopcraft, Casper Bonyongo and an anonymousreviewer.

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Corresponding Editor: M.W. Hayward


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Functional Habitat Heterogeneity and Large Herbivore Seasonal Habitat Selection in Northern Botswana