baldi et al. 04 competition guanacos sheep patagonia

HIGH POTENTIAL FOR COMPETITION BETWEEN GUANACOS AND SHEEP IN PATAGONIA

RICARDO BALDI,1,2 Centre for Ecology and Hydrology, Hill of Brathens, Banchory AB31 4 BW, United KingdomALICIA PELLIZA-SBRILLER, Instituto Nacional de Tecnología Agropecuaria (INTA), EEA Bariloche, CC277 (8400) Bariloche,

ArgentinaDAVID ELSTON, Biomathematics & Statistics Scotland, Macaulay Land Use Research Institute, Craigiebuckler, Aberdeen AB15

8QH, United KingdomSTEVE ALBON, Centre for Ecology and Hydrology, Hill of Brathens, Banchory AB31 4BW, United Kingdom

Abstract: Guanacos (Lama guanicoe) are the largest native Artiodactyl in South America and the most widely dis-tributed. In arid Patagonia, densities are low and negatively related to domestic sheep numbers in space and timeconsistent with interspecific competition theory. Although guanacos and domestic sheep have been described asintermediate feeders sharing food resources, no studies have been conducted to compare their diets in sympatricconditions and explore whether the potential exists for direct interspecific competition. We assessed the diet ofboth species across 9 different sites and 2 seasons by microhistological analysis of fecal samples. We found that (1)guanacos and sheep are generalist herbivores feeding on a wide range of plant species; (2) both are intermediatefeeders able to include both monocotyledoneous and dicotyledoneous plants in their diet; (3) both are able tochange their diets seasonally; and (4) food niche overlap is high, particularly in summer when food resources aremore scarce than in spring. We conclude that the potential for competition between guanacos and sheep is highand could have played a major role in the demise of guanacos. Consequently, current management practicesfocused on maximizing sheep numbers are not compatible with the recovery of guanaco populations.

JOURNAL OF WILDLIFE MANAGEMENT 68(4):924–938

Key words: arid lands, conservation, diet selection, dietary overlap, Guanaco, Lama guanicoe, interspecific competi-tion, Patagonia, ranching, sheep.

924

Guanacos are the largest and still the mostwidely distributed of the native Artiodactyls inSouth America (Franklin 1982). Since they wereintroduced at the end of the 19th century, domes-tic sheep have inhabited guanaco range over thePatagonian steppe. Recent evidence has shownthat in arid Patagonia, guanaco densities are neg-atively related to sheep abundance in space andtime, consistent with the hypothesis of interspe-cific competition (Baldi 1999, Baldi et al. 2001).Competition for food with introduced herbivoreswas a major reason for guanaco populationdemise (Raedeke 1979). However, no comparativestudies have examined the foraging ecology ofsympatric guanacos and sheep. Our objective wasto explore the potential for interspecific compe-tition by studying diet selection of both species.

Different herbivore species with relatively simi-lar body mass and living in sympatry should bepredisposed to interspecific competition for food(Schoener 1974, Belovsky 1986). The logic under-lying this statement is that body mass has a majorrole in shaping foraging strategies (Bell 1970, Jar-

man 1974). In a seminal study on the ecology ofAfrican antelopes (family Bovidae), Jarman (1974)found a general, inverse relationship betweenbody size and selectivity, with the species <50 kgbody mass tending to be highly selective (in termsof plant species and parts) browsers. In contrast,the species >200 kg fed unselectively, primarily ongrasses. Between these browser–grazer extremes,Jarman (1974) found a range of intermediateselectivity, with species of intermediate body mass(between 50 and 200 kg) feeding on browse andgrass in important proportions. Therefore, simi-lar-sized herbivores should overlap in the use offood resources. In particular, under food-limitingconditions, resource overlap is expected to resultin interspecific competition (Wiens 1977, Belovsky1984, de Boer and Prins 1990).

Although adult guanacos (80–120 kg) are heav-ier than sheep (Merino breed, 40–60 kg), bothspecies can be classified within the “intermediateselectivity” range proposed by Jarman (1974). Interms of their feeding habits, guanacos have beendescribed as intermediate feeders able to includegrasses and dicotyledoneous plants in their diet(Raedeke 1980, Bahamonde et al. 1986, Boninoand Pelliza-Sbriller 1991, Puig et al. 1996). Sheephave been described both as grazers and interme-diate feeders across a range of ecosystems from

1 Present address: Unidad de Investigación EcologíaTerrestre, Centro Nacional Patagónico, 9120 PuertoMadryn, Argentina.

2 E-mail: [email protected]

J. Wildl. Manage. 68(4):2004 925GUANACOS AND SHEEP: POTENTIAL FOR COMPETITION • Baldi et al.

temperate grasslands to arid savanna–shrublands(Coppock et al. 1986, Janis and Ehrhardt 1988,Edwards et al. 1995, Pelliza-Sbriller et al. 1997).However, no studies have explored dietary simi-larities between sympatric guanacos and sheep inarid Patagonia because guanacos have been stud-ied in protected areas where sheep are excluded(Franklin 1983, Ortega and Franklin 1988, Puiget al. 1996).

Current theory about the diversity of feedingstrategies in ungulates enables us to make pre-dictions about the feeding ecology of guanacosand sheep under the hypothesis proposed by Jar-man (1974) and others. First, if feeding strategiesare primarily the result of body size, then bothguanacos and sheep will be classified as interme-diate feeders. In this case, both graminoids anddicotyledoneous plants will be important compo-nents of guanaco and sheep diets, but guana-cos—being larger—will be expected to include ahigher proportion of grass in their diet. Second,if guanacos and sheep are intermediate feeders,then we expect both to increase the proportionof woody dicotyledoneous plants in their summerdiets, when annual grasses and forbs are less avail-able in the environment. Third, if guanacos andsheep are intermediate feeders, both will be gen-eralist herbivores and will include most of theavailable plant species in their diets. Finally, ifguanacos and sheep are intermediate feeders,then food niche overlap will be high, particularlyin summer when forage plants are scarcer than inspring. Our objectives were to assess the diets ofguanacos and sheep, analyze variation across dif-ferent sites and seasons, and compare diet com-positions between species and in relation to theplants available.

STUDY AREAWe conducted our study during spring

(Sep–Nov 1997) and summer (Dec 1997–Mar1998) across 9 sites within Chubut Province,Argentine Patagonia (Fig. 1). All sites were situ-ated on the arid and semi-arid Patagoniansteppe. For a description of climatic variables, seeBaldi et al (2001).

The vegetation of arid and semi-arid Patagoniais structured in herbaceous and shrub steppes(Beeskow et al. 1995). Shrubby layers often areoverlapped forming shrub clumps, while grassesare found mainly around the shrub clumps andalso isolated in open sites with no shrubs(Bertiller et al. 1991). In terms of vegetation com-position, the area has been classified in 2 phyto-

geographical provinces (León et al. 1998). Prin-cipal species representing the Patagonian Province(sites 1–7) are the shrubs quilembay (Chuquiragaavellanedae), yaoyín (Lycium chilense), neneo(Mulinum spinosum), and Nassauvia spp.; and thegrasses flechilla (Stipa tenuis) and coirón poa (Poaligularis). Foliage cover varies from 35 to 65%, butmay increase substantially in rainy periods whenannual plants contribute a high proportion ofthe total cover (Beeskow et al. 1995). The MontePhytogeographic Province (sites 8, 9) is charac-terized by open, tall shrubland that covers40–60% of the soil surface (Ares et al. 1990). Theupper layer (1.5–2 m) is dominated by the shrubsjarilla (Larrea divaricata), piquillín (Condaliamycrophylla), and molle (Schinus johnstonii). Anintermediate layer (0.5–1 m) is dominated byquilembay, and a lower layer is dominated by thesame grass species as in the Patagonian Province.

With 1 exception, sites were privately ownedranches (3,500–28,000 ha), and sheep ranchingfor wool production was the exclusive productiveactivity. The exception was site 5, a provincialwildlife reserve (Reserva Cabo Dos Bahías) of1,200 ha, where sheep ranching was terminatedin 1973 and guanaco density ranged from 37.8 to43.9 animals/km2 (Baldi et al. 2001). The reservehas a permanent warden, and tourism is the onlyhuman-related activity. Sheep also were absent atsite 8 due to management practices, while noguanacos were at site 4, where the highest sheepdensity (50.0–80.0 animals/km2) has been esti-mated (Baldi et al. 2001). Thus, guanacos andsheep were sympatric in 6 sites (sites 1, 2, 3, 6, 7,

Fig. 1. Location of our study sites (1–9) within Chubut Province,in the Argentine Patagonia, Sep 1997–Mar 1998.

J. Wildl. Manage. 68(4):2004926 GUANACOS AND SHEEP: POTENTIAL FOR COMPETITION • Baldi et al.

9), and densities ranged from 1.3 to 9.0 guana-cos/km2 and from 8.4 to 33.0 sheep/km2 (Baldiet al. 2001).

METHODS

Vegetation Surveys and Feces CollectionWe sampled 6–12 vegetation plots at each site

(80 plots across all sites). We surveyed for vegeta-tion composition at 4 randomly oriented, 50-mtransects originating at the center of each plot.Records were made at 1-m intervals using a grad-uated stick (5-mm diameter) placed vertically inthe vegetation. We recorded plant species andstatus (green or dry) for every hit, giving 200points/plot. Surveys were replicated duringspring and summer in all plots. We collected gua-naco and sheep feces within 200 m of vegetationplots in spring and summer. Only fresh pelletswere collected. We pooled pellets of the sameherbivore species found around each single plot,resulting in 1 sample/herbivore by plot by season(n = 189 samples).

Data AnalysisVegetation.—Species were grouped into 7 differ-

ent functional types according to life forms(Raunkaier 1937, Kent and Coker 1992): (1) ever-green, and (2) deciduous phanerophytes (woodydicotyledoneous plants >30 cm height); (3)chamaephytes (woody dicots <30 cm height, allevergreen); perennial herbs either (4) mono-cotyledoneous (grasses) or (5) dicotyledoneous(forbs); and the annual herbs either (6) monocotsor (7) dicots. We estimated availability in terms ofrelative cover of each functional type by dividingthe number of hits on that plant functional typeby the total number of hits on all vegetation.

We estimated species richness (S) using the Jack-knife procedure, which is recommended whenthe estimate is based in the number of speciesoccurring in plots or quadrats (Krebs 1989:336):

S = s + [(n – 1)/n]k,

where s = observed number of species present inn plots, n = number of plots sampled, and k =number of species that occur in only 1 plot. Weinvestigated seasonal differences in species rich-ness using t-tests for paired samples.

Feces.—Guanaco and sheep feces were oven-dried at 60 °C for 48 hr and ground to <1 mm ina Wiley mill (DALMAU, Buenos Aires, Argenti-na). We then performed microhistological analy-

sis on feces. Plant epidermal fragments in thefecal samples were identified at the level of genusor species when possible, and their frequency ofoccurrence was recorded. Procedures for micro-histological analysis followed the methodologyproposed by Sparks and Malechek (1968). We ob-tained 5 subsamples from each sample, and 20fields of each subsample were examined by micro-scope using 100X magnification. Overall, 100fields for each sample and 189 samples resultedin 18,900 fields being examined.

The potential bias associated with the fecalmicroanalysis technique as an estimate of herbi-vore diet composition has been discussed (Dear-den et al. 1975, Vavra and Holechek 1980,Holechek et al. 1982). Differential digestibilitymay cause overestimation of shrubs and grassesin the diet and underestimation of the readilydigested forbs (Kessler et al. 1981, Norbury1988). However, when compared with othermethods to study diet composition, includingmicroanalysis of rumen samples, microhistologi-cal analysis of feces provided similar results(Mohammad et al. 1995). In addition, fecal analy-sis detected more plant species than the rumenmacroanalysis technique (Kessler et al. 1981).Although the most reliable technique to assessdiet was reported to be esophageal fistulation(McInnis 1976), fecal analysis is the most practi-cal technique for evaluating dietary compositionunder field conditions. Also, fecal analysisappears suitable for comparisons between speciesof herbivores. After foraging trials conducted inthe United Kingdom, guanacos and sheep didnot differ in their efficiency in overall dietdigestibility and voluntary intake (Fraser andGordon 1997) or in digesting perennial grasses(Fraser and Baker 1998). Therefore, we assumedthat microhistological comparisons of feces fromthe 2 species did not introduce systematic biasinto the assessment of diet.

Diet.—We analyzed guanaco and sheep diet at 3different levels of resolution: (1) proportion ofmajor vegetation groups, such as monocotyledo-neous, woody dicotyledoneous, and herbaceousdicotyledoneous (forbs) plants; (2) proportion ofdifferent functional types; and (c) key plant spe-cies, defined as species representing ≥10.0% ofthe diets of guanaco or sheep in any site–seasoncombination. We analyzed differences in the pro-portion of individual plant categories betweenherbivore species and seasons using t-tests(paired samples, with level of significance set to P< 0.05). Data are presented as percentage of each


plant functional type (or species) in the feces andassumed to represent the diet. We compared gua-naco and sheep diets by site and season, usingKulcyznski’s similarity index (Oosting 1956) toestimate the overlap in terms of percentage ofdifferent plant functional types and plant species.Also, Kulcyznski’s index was used to estimatewithin-species similarities between spring andsummer diets and to estimate similarity betweenthe percentage of functional types in the dietsand their availability in the environment as a gen-eral estimate of selectivity.

We analyzed statistical significances of dietaryoverlap, diet similarity between seasons, and dietselection in relation to environmental availabilityusing the following randomization tests. We cal-culated a matrix containing all possible similaritycombinations for every case (Fig. 2). To testwhether the values of interest were significantlydifferent from those expected at random, wedesigned a program in GENSTAT (Lawes Agricul-tural Trust 1996). After calculating mean overlapor similarity across sites, the program randomly re-ordered the rows in the matrix and calculated themean for every new diagonal. The simulation wasrun 1,000 times in each case. Observed meanoverlap or similarity were accepted as biologicallysignificant (P < 0.05) if ≤5% of the means obtainedthrough randomization were higher than ob-served mean overlap or similarity (Manly 1997).

RESULTS

Dietary RangeGuanacos ate 76 different plant species (74.5%

of 102 species identified) grouped in 60 differentgenera, while sheep ate 79 plant species (77.5% ofthose available) belonging to 63 genera (Table 1).We found only 3 plant species eaten exclusivelyby guanacos and 6 eaten just by sheep. Thus, 73plant species were found in the diets of both gua-nacos and sheep (Table 1).

Seasonal VariationMonocotyledoneous and Dicotyledoneous Plants.—

Mean percentage of monocotyledoneous plantsin the feces of guanacos was higher in springthan in summer (64.9 vs. 45.3%, t7 = 3.57, P <0.01), and, conversely, the percentage of dicotyle-doneous plants (woody and herbaceous com-bined) was significantly higher in summer thanin spring (t7 = –3.6, P < 0.001). Considering dicotsalone, the percentage of woody plants in the dietof guanacos was higher in summer than in spring

(t7 = –4.4, P < 0.01, range in summer =21.0–56.0%, range in spring = 8.0–39.0%). Herba-ceous dicotyledoneous (forbs) did not differbetween spring and summer diets (16.6 vs. 17.2%,t7 = –0.22).

The percentage of monocotyledoneous anddicotyledoneous plants in the diet of sheep didnot change significantly between spring and sum-mer (t6 = –1.27 and t6 = 1.72, respectively). With-in the dicots selected by sheep, we found no sig-nificant differences in the proportions of woodyplants (t6 = –0.76) or forbs (t6 = 1.25) betweenseasons.

Plant Functional Types within Monocotyledoneous andDicotyledoneous Plants.—The reduction in monocotsin the summer diet of guanacos largely was due toa decrease in presence of annual grasses (17.3 vs.4.4%, t7 = 7.36, P < 0.01). The increase in woodydicots in summer was associated with a doublingof the percentage of evergreen and deciduous

Fig. 2. Schematic representation of the matrix we used tostudy the significance of (A) dietary overlap between guana-cos and sheep.The diagonal represents overlap values acrosssites within season; G(1– 9) = guanaco diet across sites,S(1–9) = sheep diet across sites. Mean overlap was comparedagainst simulated means obtained through randomization. (B)Matrix to study the similarity between the diet of guanacosand the availability of plant types in the environment, withinseason; A(1–9) = availability of plant functional types acrosssites. Mean similarity between diet and environment was com-pared against simulated means obtained through randomiza-tion, Patagonia, Argentina, Sep 1997–Mar 1998.


Table 1. Plants available (+) and in the feces (x) of guanaco and domestic sheep in arid Patagonia, Argentina, Sep 1997–Mar 1998.

Study sitesa

1,2,3,4 5,6,7 8,9 Guanaco Sheep

Evergreen phanerophytes Anacardeaceae Schinus johnstonii + + + x x Asteraceae Brachyclados megalanthus + + x x

Chuquiraga avellanedae + + + x xChuquiraga hystrix + + x xNardophyllum obtusifolium + + + + +

Chenopodiaceae Atriplex lampa + + x xVerbenaceae Junelia alatocarpa + + + x xZygophyllaceae Larrea divaricata +

Larrea nitida +Berberis heterophyla + x xColliguaja integerrima + x x

Deciduous phanerophytesAsteraceae Brachyclados licioides +

Cyclolepis genistoides + + + x xPsila spartioides + x x

Ephedraceae Ephedra ochreata + + + x xFabaceae Prosopidastrum globosum + + + x x

Prosopis alpataco + + + x xProsopis denudans + + + x x

Nictaginaceae Bouganvillea spinosa + xOleaceae Menodora robusta + + x xRhamnaceae Colletia hystrix +

Condalia microphylla + + x xSolanaceae Lycium spp. + + + x x

ChamaephytesAsteraceae Baccharis crispa +

Baccharis darwini + + + x xBrachyclados caespitosus +Chuquiraga aurea + + x xChuquiraga erinacea + + x xNassauvia glomerulosa +Nassauvia ulicina + + x xSenecio mustersii + x x

Frankeniaceae Frankenia patagonica + + x xRosacea Acaena platyacantha

Tetraglochin caespitosum + x xUmbelliferae Mulinum spinosum + + + x xVerbenaceae Acantholippia seriphioides + + x x

Junelia seriphioides +

Perennial grasses and graminoidsCyperaceae Carex sp. + + x x

Eleocharis sp. + x xPoaceae Distichlis spp. + + x x

Festuca pallescens + xHordeum sp. + + + x xPoa ligularis + + + x xSporobolus rigens + + + x xStipa chrisophylla + + x xStipa longilumilis + + + x xStipa neaei + + + x xStipa pampeana + + + x xStipa speciosa + + + x xStipa tenuis + + + x x

Perennial forbsAsteraceae Baccharis melanopotamica +

Gamochaeta stachidifolia + + x xGrindelia chiloensis + xPerezia recurvata + + + x x

(continued on next page)


panerophytes (11.4 vs. 20.9%, t7 = –3.32, P = 0.01and 5.3 vs. 14.4%, t7 = –3.68, P < 0.01). Althoughherbaceous dicots seemed to be equally presentin both seasons (t7 = –0.22), this concealed oppo-site trends in perennial forbs, which increasedsignificantly in summer (3.5–12.7%, t7 = –5.59, P <

0.001) while annual forbs decreased significantly(13.1–4.6 %, t7 = 3.79, P < 0.01).

Although monocots made up a similar propor-tion of the diet in both seasons for sheep, thisconcealed an increase in perennial grasses insummer (29.1–40.2%, t6 = –3.7, P < 0.01) while

Table 1. continued.

Study sitesa

1,2,3,4 5,6,7 8,9 Guanaco Sheep

Psila tenela + +Senecio gillesii + + x xTaraxacum officinale +

Calyceraceae Boopis anthemoides + + + xCariophyllaceae Cerastium arvense +

Herniaria cinerea + + + x xParonychia sp. + + x xSpergula ramosa + +Spergularia sp. + +

Fabaceae Adesmia villosae xAstragalus moyanoii + + x xHoffmansegia erecta + x xHoffmansegia trifoliata + + + x x

Iridaceae Sisirinchium junceum + + + x xPolemonaceae Gilia laciniata + + + x xPortulacaceae Calandrina colchaguensis + + x xSantalaceae Arjona tuberosa + + + x xSchropulariaceae Calceolaria sp. + + x x

Cardionema ramosissimmum + + x x

Annual grassesPoaceae Bromus brevis + + x x

Bromus sp. + + x xBromus trinii + + + x xSchismus barbatus + + + x xVulpia spp. + + + x x

Annual forbsAsteraceae Duseniella patagonica + x x

Facelis retusa + + + x xSenecio chrysocomoides + x

Brassicaceae Lepidium mirianthum + + x xCariophyllaceae Arenaria serpillifolia +

Cerastium junceum + + x xSpergula villosa + + x

Convolvulaceae Dichondra microcalyx + x xCruciferae Capsella bursapastoris + x x

Draba australisFabaceae Adesmia smithii

Vicia pampicola +Geraniaceae Erodium cicutarium + + + x xHydrophyllaceae Phacelia sp. +Labiatae Marrubium vulgare +Loasaceae Loasa bergii +Onagraceae Camissonia dentata + + + xPlantaginaceae Plantago patagonica + + + x xPoligonaceae Poligonum stypticum +Rosaceae Aphanes parodii + + x xRubiaceae Galium sp.

Rebulnium richardianum + x xUmbelliferae Bowlesia incana + + + x xUrticaceae Parietaria debilis +

a Sites were grouped according to regional proximity.


annual grasses declined (11.6–3.7%, t6 = 2.81, P <0.05). Sheep decreased the percentage of annualforbs in their diet in summer (26.7–6.3%, t6 = 2.7,P < 0.05), but this was not balanced by a signifi-cant increase in perennial forbs (t6 = –1.92). Thepresence of each of the 3 woody dicot types in thediet did not differ between seasons (evergreen t6= –1.35, deciduous phanerophytes t6 = 0.69,chamaephytes t6 = –2.03).

Key Plant Species.—Although most of the plantsin the environment were found in guanaco and

sheep diets (Table 1), only 17 plant species rep-resented ≥10.0% of either species’ diet in anysite–season combination (Table 2). Overall, these17 species accounted for 80.0% of the plant frag-ments found in the feces of guanacos and sheep.

From spring to summer, guanacos significantlydecreased the percentage of the perennial grasscoirón poa, the annual grass Schismus barbatus andthe annual forb alfilerillo (Erodium cicutarium) intheir diet. In contrast, guanacos increased the pres-ence of perennial dicots in their diet in summer aspercentages of the evergreen shrubs quilembayand molle, the deciduous shrub Prosopidastrum glo-bosum, and the perennial Cariophyllaceae forbswere significantly higher than in spring (Table 2).

We found an increase in the percentage of theperennial grass Stipa spp. and the perennial Cario-phyllaceae forbs in the summer diet of sheep. Con-versely, we recorded a decrease in the deciduousshrub Cyclolepis genistioides, the annual grass Schismusbarbatus, and the annual forb alfilerillo (Table 2).

Diet Similarity between SeasonsMean similarity indexes between guanaco spring

and summer diets were not significant for plantfunctional types (66.4%, range = 52.0–79.0%) orfor key plant species (56.8%, range = 48.7–68.9%;Table 3). In contrast to guanacos, mean similarity

Table 2. Mean (SE) seasonal percentage of key plant species in the diet of guanacos (df = 7) and sheep (df = 6), and seasonaldifferences within species, Patagonia, Argentina, Sep 1997–Mar 1998.

Guanacos Sheep Spring Summer Spring Summer

Perennial grasses Distichlis spp. 0.30 (0.20) 2.36 (1.23) t = –1.89, P = 0.10 0.67 (0.40) 3.67 (2.01) t = –1.75, P = 0.13Sporobolus rigens 0.31 (0.22) 1.22 (0.72) t = –1.47, P = 0.18 0.39 (0.34) 2.88 (1.79) t = –1.34, P = 0.23Stipa spp. 26.93 (3.29) 29.55 (2.20) t = –0.56, P = 0.59 18.72 (3.48) 26.71 (3.93) t = –3.15, P < 0.05Poa spp. 16.09 (3.13) 9.10 (2.56) t = 2.46, P < 0.05 6.93 (1.82) 6.86 (2.08) t = 0.05, P = 0.96

Annual grasses Schismus barbatus 12.63 (2.33) 2.58 (0.66) t = 5.10, P < 0.01 9.11 (1.94) 2.74 (1.14) t = 2.38, P = 0.06

Evergreen phanerophytes Atriplex spp. 0.53 (0.38) 0.56 (0.23) t = –0.08, P = 0.94 1.42 (1.32) 1.80 (1.39) t = –1.84, P = 0.12 Condalia microphylla 0.00 1.45 (1.27) t = –1.51, P = 0.18 0.02 (0.02) 0.86 (0.86) t = –1.00, P = 0.36Junellia spp. 1.84 (1.26) 0.57 (0.26) t = 0.96, P = 0.37 1.46 (1.34) 1.83 (1.23) t = –0.64, P = 0.55Chuquiraga avellanedae 5.46 (1.16) 9.07 (1.36) t = –2.95, P < 0.05 5.06 (2.14) 8.48 (3.97) t = –0.85, P = 0.43 Schinus spp. 2.46 (0.67) 9.20 (2.20) t = –3.90, P < 0.01 2.77 (2.51) 6.14 (2.07) t = –2.10, P = 0.08

Deciduous phanerophytes Ciclolepys genistoides 0.47 (0.20) 1.38 (1.27) t = –0.68, P = 0.52 12.16 (6.99) 0.06 (0.05) t = –3.51, P < 0.05Prosopidastrum globosum 0.30 (0.18) 5.66 (2.04) t = –2.87, P < 0.05 0.38 (0.27) 4.37 (2.24) t = –1.74, P = 0.13 Prosopis spp. 0.03 (0.03) 3.22 (1.79) t = –1.75, P = 0.12 0.48 (0.43) 2.21 (1.17) t = –2.27, P = 0.06

Perennial forbs Perennial Cariophyllaceae 1.99 (0.91) 8.09 (2.35) t = –2.54, P < 0.05 1.16 (0.58) 11.61 (4.32) t = –2.52, P < 0.05

Annual forbs Duseniella patagonica 0.28 (0.20) 0.00 t = 1.40, P = 0.21 2.98 (1.67) 0.83 (0.75) t = 1.08, P = 0.32 Erodium cicutarium 9.97 (1.74) 2.14 (1.04) t = 3.88, P < 0.01 17.92 (4.00) 3.80 (2.40) t = 2.63, P < 0.05Plantago patagonica 1.06 (0.64) 0.58 (0.22) t = 0.75, P = 0.48 2.40 (2.10) 0.42 (0.22) t = 1.04, P = 0.34

Table 3. Between-season diet similarity (%) for guanacos andsheep by study site, at 2 levels of resolution: plant functionaltypes and plant species in the diet, Patagonia, Argentina, Sep1997–Mar 1998.

Plant functional types Key plant species

Site Guanacos Sheep Guanacos Sheep

1 68 58 61 36.5 2 58 49 49.7 30 3 52 50 48.7 36 4 68 61 5 63 57 6 79 54 68.9 46.2 7 70 75 63.6 55.7 8 68 56 9 73 72 49 67.5

Mean 66.4 60.9 P = 0.028 56.76 45.3 P = 0.034


between spring and summer diets for sheep wassignificant for both proportions of plant func-tional types and key plant species found in thediet (Table 3).

Feeding SelectivityIn spring, diet compositions of guanacos and

sheep were similar to the relative availability ofplant functional types in the environment (Table4). In summer, guanacos were more selective astheir diet was different to the availability of plantfunctional types in the environment. Whereas,sheep diet composition in summer was similar tothe proportion of plant functional types available(Table 4).

Dietary Niche Overlap between Guanacosand Sheep

Plant Functional Types.—For the 6 sites where gua-nacos and sheep were sympatric, interspecificdietary overlap ranged from 49.0 to 84.0% in springand from 76.0 to 93.0% in summer (Table 5). Al-though dietary overlap in spring appeared to behigh, it was not significant (P = 0.088, Table 5).

However, we found strong evidence of dietaryoverlap between guanaco and sheep diets in sum-mer (Table 5).

Key Plant Species.—Mean overlap between gua-nacos and sheep was significantly high both inspring and in summer (Table 5). Furthermore,mean dietary overlap between guanacos andsheep was significantly higher in summer than inspring (t5 = –3.32, P = 0.02). We found a strong,positive correlation in the proportions of the 17key plant species between guanaco and sheepdiets, particularly in summer (correlation coeffi-cients: spring = 0.80, summer = 0.92; Fig. 3).

Seasonal Changes in Food AvailabilityThe number of plant species (species richness)

estimated for each site ranged from 33 in site 4 to57 in site 6 during spring but from 25 to 31 spe-cies in the same sites during summer (Fig. 4).Mean species richness in spring (45.8) was signif-icantly higher than in summer (32.2 species; t8 =7.28, P < 0.001), mainly due to a decrease in the

Table 4. Similarity (%) between proportion of plant functionaltypes found in the diets of guanacos and domestic sheep andthe proportion available in the environment by study site andseason, Patagonia, Argentina, Sep 1997–Mar 1998.

Guanaco Sheep Site Spring Summer Spring Summer

1 71 67 49 57 2 75 64 74 62 3 64 65 80 64 4 80 85 5 84 73 6 68 65 69 68 7 58 66 63 62 8 56 55 –9 76 57 75 59

Mean 69 64 70 65.29P = 0.006 P = 0.344 P = 0.014 P = 0.018

Table 5. Dietary overlap (%) by season between guanacos andsheep—in terms of plant functional types and key plantspecies found in feces—for study sites where the species weresympatric, Patagonia, Argentina, Sep 1997–Mar 1998.

Plant functional types Key plant species Site Spring Summer Spring Summer

1 49.1 77.3 39.1 72.3 2 66 78.1 58.6 66.6 3 52.1 76.1 44.5 68.8 6 67.5 93.5 67.3 74.4 7 69 89.9 65.1 79 9 84.4 75.7 66.4 71.3

Mean 64.5 81.7 56.8 72.1P = 0.088 P = 0.000 P = 0.018 P = 0.000

Fig. 3. Correlation between proportions of key plant species inguanaco and domestic sheep diets by season (values areangular transformed) in the Argentine Pategonia, Sep1997–Mar 1998. 1 = Stippa spp., 2 = Poa spp., 3 = Schismusbarbatus, 4 = Chuquiraga spp., 5 = Erodium cicutarium, 6 =Schinus spp., 7 = perennial Cariophyllaceae, 8 = Prosopidas-trum globosum, 9 = Prosopis spp., 10 = Distichllis sp., 11 =Junellia spp., 12 = Cyclolepis genistioides, 13 = Plantagopatagonica, 14 = Condalia microphylla, 15 = Sporobolusrigens, 16 = Atriplex spp., 17 = Duseniella patagonica.


number of species of annual forbs from 10–15species in spring to 1–8 species in summerdepending on the site.

The percentage of live plant tissue, in terms ofproportion that was green, was significantly lowerin summer than in spring for all the functionaltypes forming the herbaceous layer (Fig. 5, df = 8in all cases). While 72.0% of perennial grass tissuewas green in spring, only 27.0% remained greenin summer (t = 5.11, P < 0.001). The proportionof perennial forbs that were green declined from94.0 to 14.0% (t = 13.65, P < 0.001), while 92.0%of annual grass material was green during springand only 3.0% in summer (t = 29.16, P < 0.001).Proportion of annual forb green material

decreased from 87.0% in spring to 14.0% in sum-mer (t = 20.72, P < 0.001). Among the woodydicots, percentage of green tissue of evergreenphanerophytes was higher in summer than inspring (66.0 vs. 61.0%, t = –2.57, P < 0.05), whilefor chamaephytes green tissue was lower in sum-mer (t = 2.40, P < 0.05).

DISCUSSIONBoth guanacos and sheep are intermediate

feeders, able to change their diets seasonally, interms of plant functional types and species.Although guanacos and sheep are generalist her-bivores using a wide range of plant species avail-able in the environment, most of their diets arecomprised of only 20% of the species they eat.Also, guanacos and sheep overlap in their diets,especially in summer, when food resources aremore scarce in terms of plant species and avail-able live plant material. Thus, our results suggesta high potential for interspecific competition forfood between guanacos and sheep.

Foraging BehaviorGuanacos and sheep changed their diets sea-

sonally, although changes in the diet of guanacoswere more pronounced than changes in thesheep diet. The higher proportion of monocotsin the diet of guanacos compared to sheep dur-

Fig. 4. Species richness (Jackknife estimate) expressed asnumber of plant species by site and season, Patagonia,Argentina, Sep 1997–Mar 1998.

Fig. 5. Mean percentage of green tissue by plant functional type by season across all study sites, Patagonia, Argentina, Sep1997–Mar 1998.


ing spring may be a consequence of the guana-co’s larger body size, as larger animals are expect-ed to tolerate lower-quality food in higher pro-portions than smaller animals (Demment andVan Soest 1985, Illius and Gordon 1991).

As predicted from the Jarman (1974) hypothe-sis, both herbivore species included significantproportions of monocotyledoneous and dicotyle-doneous plants in their diets. However, in accor-dance with theory, the percentage of dicotyledo-neous plants in the diet of sheep tended to behigher than in the diet of guanacos during thespring season when guanaco included moremonocots in their diet than did sheep.

As predicted for an intermediate feeder, guana-cos increased the percentage of woody dicots inthe summer diet. Guanacos seasonally changedthe proportions of plant functional types andeven the proportion of plant species within func-tional types. In contrast, sheep tended to eat thesame proportions of monocots and dicotsbetween seasons. Although sheep increased thepercentage of perennial grasses and decreasedthe percentage of annual forbs and grasses intheir diet from spring to summer, the diets weresimilar in terms of plant species. As predicted forintermediate feeders, guanacos and sheep weregeneralist herbivores and included ≥75% of theavailable plants in their diets.

Contrary to Jarman (1974), Hofmann (1973,1989) has argued that the most important differ-ence separating species of ruminants is their mor-phologic adaptation to consume different diets.According to Hofmann (1973), browsers arethought to have a digestive system adapted forthe rapid excretion of highly lignified parts of thecell wall of dicotyledoneous plants. Whereas,grazers have adaptations in the rumen to slowdown the passage of plant material and toincrease the extent of digestion of the less ligni-fied cell wall component of monocotyledoneousplants (Hofmann 1973, 1989). However, by mod-eling data on digestion for 21 species of Africanruminants, Gordon and Illius (1994) found nodifferences in mean retention times in the gutbetween browsers and grazers after accountingfor variation due to body mass and food type(grass or browse). Instead, rumen fermentationrate was found to be higher in smaller animals(Gordon and Illius 1994), and potentialdigestibility of the diet was negatively correlatedwith body mass (Gordon and Illius 1996) inaccordance with the implications of Bell (1970)and Jarman’s (1974) hypotheses. In addition,

Robbins et al. (1995) found that the extent offiber digestion did not differ between the feedingcategories proposed by Hofmann and Stewart(1972) but was positively related to body size.

Guanacos show some morphologic adaptationsexpected to favor selective feeding strategies.Camelids are not true ruminants but are func-tionally ruminants because plant fermentationtakes places in the foregut and they remasticatethe cud (Heller et al. 1984). In accordance withHofmann’s (1973) description for concentrateselectors, all camelids have high gut motility andhigh rates of food passage relative to sheep andcattle (Maloiy 1972, Hintz et al. 1973). In addi-tion, camelids have a relative narrow muzzle,bifid upper lip, and a mobile tongue (Mukasa-Mugerwa 1981, Janis and Ehrhardt 1988). All ofthese characteristics would place guanacos withinthe category of browsers according to Hofmann(1973). Nevertheless, our results show that gua-nacos are intermediate feeders as predicted fromthe Jarman (1974) hypothesis.

Current work on the guanaco diet agrees withprevious research in other parts of their range.For example, in La Payunia, a protected area inwestern Argentina, guanacos changed their dietfrom mainly grasses and forbs in summer toshrubs in winter, when the former decreased inquality and availability (Puig et al. 1996). Twostudies in Tierra del Fuego described guanacos asgeneralist herbivores in a woodland–steppe eco-tone. In the first, guanacos included up to 61% ofgrass and 30% of forbs and shrubs plus epiphytesand lichens in their diet (Raedeke 1980). In a sec-ond study, guanacos varied the proportion ofgrasses in the diet from 38 to 90% depending onthe availability of dicotyledoneous plants (Boni-no and Pelliza-Sbriller 1991).

Although Hofmann (1989) classified sheep as“bulk/roughage feeders” or grazers, in contrastto Jarman’s mass criteria, sheep are able to in-clude more forbs and dwarf-shrubs in their dietcompared to cattle (Grant et al. 1985, Hodgson etal. 1991). In an African nomadic pastoral ecosys-tem, sheep were intermediate, generalist herbi-vores including up to 80% of shrubs and forbs inthe diet depending on the season (Coppock et al.1986). Similarly, in the Australian dry rangelands,sheep consumed less grasses and forbs and moreshrubs during the dry season (Edwards et al.1995). In arid Patagonia, sheep diet compositionincluded between 20 and 40% of woody dicotyle-doneous plants, especially during the dry sum-mer (Pelliza-Sbriller et al. 1997).


Diet SelectionAs predicted for intermediate feeders, guana-

cos and sheep are generalist herbivores capableof including a high proportion of the plantspecies available in their diets. Both speciesshowed selectivity in the plant functional typesand species they ate. Just 17 of 102 plant speciesrepresented ≤80% of both species’ diets.

Guanaco diet composition was similar to theavailability of functional types during spring(Table 4). Guanacos were particularly nonselec-tive in terms of functional types in site 5, whereno sheep were present (similarity diet–environ-ment = 84%; Table 4). We found a similar trendin sheep, which showed a greater selectivity insympatric sites than in site 4 where guanacos wereabsent (Table 4).

In a comparative study on diet selection of gua-nacos and sheep conducted in the uplands of theUnited Kingdom, Fraser (1998) found that bothspecies selectively ate the green leaves of broad-leaved grasses. Illius (1997) argued that the mor-phology of mouthparts imposes constraints onselectivity, and these constraints are more severein large animals, limiting the quality of the dietthey can select. Gordon and Illius (1988) foundthat grazers have a relatively wider incisor arcadethan browsers, and this difference increasinglydiverges well above 100 kg in body weight. Meanincisor arcade breadth (IAB) reported for sheepwas 31.9 mm (Gordon and Illius 1988). Similarly,we estimated mean IAB = 31.4 mm (SE = 1.7, n =8) for adult guanacos (R. Baldi, Centro NacionalPatagónico, unpublished data). The design ofour study does not allow us to draw conclusionson diet selection beyond the plant species level.However, we suggest that the differences betweenguanacos and sheep in terms of body size are notlarge enough to result in differences in their IABand hence in their ability to select plant func-tional types and species.

Diet OverlapThe changes leading to more similar diets in

summer largely were due to an increase in thepercentage of the grass Stipa spp. in sheep dietsand an increase in the proportion of the decidu-ous shrubs Prosopidastrum globosum in guanacodiets. Also, the percentage of the deciduousshrub Cyclolepis genistoides declined in sheep dietsfrom spring to summer.

Presumably, the seasonal changes in the pro-portion of different plant species in the dietreflect changes in forage availability and quality

(Owen-Smith 1992). The nutrient content of grassdeclines as it matures, while woody dicots contin-ue to produce new leaves long after the rainscease because they are deeper rooted (Noy-Meir1973, Fischer and Turner 1978, Sala et al. 1989).In Patagonia, the percentage of crude protein inthe grasses Stipa spp. and Poa spp. falls from6–10% in spring to 2–4% in summer (Somlo et al.1985). Likewise, the concentration of nitrogen ingreen leaves of these grass species falls from3.0–3.5 to 0.5–0.8% (Carrera et al. 2000). Somedeciduous shrubs follow the same seasonal trendas grasses, showing pulses of assimilation closelyrelated to short-term rainfall (Bertiller et al.1991). This is the case of Cyclolepis genistoides,important in the diet of sheep during spring andalmost absent in the summer diet when its crudeprotein content falls from 5 to 1%. Other decidu-ous shrubs, such as Prosopidastrum globosum, whichare more important in guanaco summer diets thanin spring, develop new leaves during the summerand may increase their protein content from 5 to9% (Somlo et al. 1985). Conversely, we found nosignificant differences in nitrogen concentrationin green leaves of evergreen shrubs between latewinter and early summer (Carrera et al. 2000).

In addition to changes in quality, mean plantspecies richness was significantly lower in sum-mer than in spring—due mainly to the reductionin the number of annual forbs species togetherwith a reduction in the relative availability ofperennial forbs. Furthermore, from spring tosummer, a highly significant reduction occurredin the percentage of green tissue in the herba-ceous layer (perennial and annual grasses andforbs). Thus, changes in plant cover and avail-ability of green tissue, together with changes infood quality, may influence seasonal diet compo-sition and food niche overlap of guanacos andsheep. Our findings suggest that forage resourcesfor the herbivores are more limited in summerthan during spring.

Our results on relative dietary overlap betweenguanacos and sheep agree with studies on othermammalian herbivores. In the arid lands of Aus-tralia, dietary overlap between the bridled nailtailwallaby (Onychogalea fraenata), the black-stripedwallaby (Macropus dorsalis), and domestic cattle washigher during the driest conditions, when all 3species included more dicotyledoneous plants intheir diet (Dawson et al. 1992, Ellis et al. 1992). Inthe same ecosystem, sheep and red kangaroos(Macropus rufus) increased their dietary overlapduring dry periods (Dawson and Ellis 1994). While


in North America, dietary overlap between prong-horn (Antilocapra americana) and domestic sheepwas higher when both food abundance and quali-ty decreased in winter (Schwartz and Ellis 1981).

de Boer and Prins (1990) argued that interspe-cific competition for food between 2 speciesexists when 3 conditions are met: habitat overlapmust occur, species must share a common diet,and food resources must be limited. Becausehigh habitat overlap in sympatric conditionsmight be possible in the absence of competition(Vandermeer 1972, Strong 1983), this overlapshould occur when food availability is not limit-ing (Belovsky 1984). Accordingly, competing spe-cies must change their resource-use patterns anddistribution as a result of the interspecific inter-action (Wiens 1989, de Boer and Prins 1990, Put-man 1996). Therefore, we expect populationdensities of potentially competing species to behigher in habitats where species live allopatrical-ly than in habitats where they live sympatrically(Belovsky 1984, 1986; van der Wal et al. 1998).

Our results support recent findings on the pat-terns of guanaco abundance under the hypothesisof interspecific competition. Across the same siteswe studied, Baldi (1999) and Baldi et al. (2001)found that (1) guanaco abundance is inverselyrelated to sheep numbers in space and time; (2)guanaco densities were negatively related to bothtotal plant cover and availability of preferredplant species (the grasses Stipa spp. and Poa spp.,which were positively associated with sheep den-sity); and (3) guanaco densities are ≤23 timeslower than sheep densities in sympatric condi-tions. Therefore, low guanaco densities across therange may well be the outcome of interspecificcompetition for food with sheep because bothherbivores share a common diet, particularlywhen forage is scarce during the hot, dry summer.

When guanacos spread across the South Ameri-can arid lands 10,000 years ago, they had no com-petitors because the megafauna was already impov-erished and became extinct at the end of thePleistocene (Pascual 1996). Not until the 20th cen-tury, after Europeans introduced the domesticsheep in Patagonia, was the potential high for com-petitive interactions. While the most productivelands were used for sheep ranching, guanacos werepushed to marginal lands where their preferredplant species were less abundant (Baldi et al. 2001).

MANAGEMENT IMPLICATIONS Farmers likely will keep high sheep densities as

long as the local productivity of forage species is

sufficiently high. Overgrazing affects the struc-tural patterns of vegetation, reducing total plantcover and replacing highly palatable plant spe-cies with species of low palatability. At a highsheep grazing pressure, the reduction in peren-nial grass cover mainly was due to the decrease offlechilla, Stipa speciosa, and coirón poa, all keyplant species in the diets of guanacos and sheep(Bisigato and Bertiller 1997). Also, large vegeta-tion patches dominated by palatable shrubs andgrasses were replaced by smaller patches with sig-nificantly lower species richness, dominated bythe unpalatable shrubs jarilla and dwarf shrubs(Bisigato and Bertiller 1997).

Today, land modification due to overgrazing isa major threat across arid Patagonia. Over 95% ofthe land is privately owned. Most of these privateranches were dedicated to sheep husbandrythroughout the 20th century. Overgrazing led todesertification of approximately 30% of thesteppe (del Valle 1998). Even sheep husbandryhas declined in recent decades because largeexpanses of degraded land can no longer sup-port high numbers of sheep. Alternatives to cur-rent farming activities in Patagonia must be con-sidered, and, in the longer term, a change ofattitude needs to be promoted in the local com-munities regarding indigenous wildlife. Marinemammals and birds are perceived as naturalattractions and a benefit to the regional economy,but continental species like guanacos are stillseen as an obstacle to human development. Alter-native strategies should be applied depending onlocal conditions, including controlled harvestingof guanacos, shearing, tourist attractions on pri-vate ranches, and the creation of protected areasin the Patagonian steppe. We believe that therecovery of ecologically functional populations ofnative wildlife in Patagonia is still possible but willdepend strongly on informed management deci-sions and conservation action.

ACKNOWLEDGMENTSFundación Patagonia Natural provided logistic

support. Students from the Universidad Nacionalde la Patagonia helped collect data, the Direc-ción de Fauna Silvestre and Organismo Provin-cial de Turismo of Chubut provided work per-mits, and the wardens at Cabo Dos Bahias reserveand landowners provided cooperation. M. Stan-ganelli and M. Valverde helped with plant identi-fication. We thank I. J. Gordon, T. N. Coulson,and R. van der Wal for early discussion of ourwork, and 2 anonymous referees for their sugges-


tions. This study was funded by the Wildlife Con-servation Society and the British Council.

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