Seasonal variations in guanaco diet (Lama guanicoeMüller 1776) and food availability in Northern Patagonia, Argentina

Journal of Arid Environments (1996) 34: 215–224

Seasonal variations in guanaco diet (Lama guanicoe Muller ¨ 1776) and food availability in Northern Patagonia, Argentina

Silvia Puig, Fernando Videla, Susana Monge & Virgilio Roig Unidad de Ecologia Animal, Instituto Argentino de Investigaciones de Zonas Aridas (IADIZA-CONICET), Casilla de Correo 507, 5500 Mendoza, Argentina (Received 24 June 1995, accepted 24 September 1995) Composition and seasonal changes in guanaco diet were determined in La Payunia Protected Area through seasonal faecal sampling and food availability analysis. Diet included 32 genera of plants, mostly grasses and low shrubs. Summer use of forbs and winter use of high shrubs were more and less, respectively, than expected with regard to their availability. Seasonal alternation between grazing and browsing was detected. The herbaceous stratum provided the basic food of the guanaco; use and preference for the shrubby stratum increased when the availability of grasses and forbs decreased. Low summer selectivity may reflect the adaptability of the guanaco to the scarcity and unpredictable phenology of plants in arid environments. ©1996 Academic Press Limited Keywords: arid zones; feeding ecology; protected areas; seasonal forage preferences; guanaco

Introduction The guanaco (Lama guanicoe) belongs to the family Camelidae. Considered the largest of wild South American artiodactyls, it is the ancestor of the llama (Lama glama) which was domesticated 6000 years ago (Wheeler, 1991). South American camelids, the guanaco and llama especially, present physiological and ecological adaptations to arid conditions. They have lower forage intakes and higher efficiency in digestion of fibrous and low-quality plant species, compared with true ruminants (San Martin & Bryant, 1988). The distribution range of the guanaco is large (Wheeler, 1991; Puig, 1991) and comprises environments with marked differences in plant structure (e.g. Puna, Monte, Patagonian steppe, subantarctic forest). The ease with which the guanaco alternates between grazing and browsing habits is considered one of the factors that allow this large distribution range (Franklin, 1983). Studies on guanaco feeding habits in different environments have increased in recent years (e.g. Raedeke, 1980; Bahamonde et al., 1986; Raedeke & Simonetti, 1988; Bonino & Sbriller, 1991) but none have yet been carried out in the central region of its distribution range. This includes Northern Patagonia which presents a combination of 0140–1963/96/020215 + 10 $25.00/0

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herbaceous and shrubby communities and would therefore allow an evaluation of seasonal alternation in guanaco dietary preferences. The objective of this paper is to analyse the composition and seasonal changes of guanaco diet, in relation to environmental plant availability, in a Northern Patagonia environment.

Material and methods Study area The study was conducted in La Payunia Reserve (Mendoza, Argentina, 36°10' S and 68°50' W, 2500 km2, elevation from 1300 to 2000 m). Several ecological studies on the guanaco population have been carried out in the area (Puig & Monge, 1983; Puig, 1986; Puig, 1992; Videla, 1993). The climate is continental desert type (Consejo Federal de Inversiones, 1977). Mean seasonal temperature varies from 6°C in winter to 20°C in summer. Precipitation occurs mainly in summer, leading to a 9-month dry season (Cuello et al., 1981). Annual precipitation averages 255 mm, with high variability among years (variation coefficient 58%). The zone has signs of past strong volcanic activity; pyroclastic cones and basaltic washes alternate with gentle slopes and large plains, modelled by aeolic and hydric erosion (Gonzalez Diaz, 1972). The xerophyllous vegetation belongs to the patagonic shrubby steppe, with a moderate mean cover (58%). Sandy plains present herbaceous communities dominated by Panicum urvilleanum and Sporobolus rigens, together with Poa lanuginosa and Stipa speciosa. Slopes and basaltic scoria are covered with shrubby communities, dominated by Neosparton aphyllum and Anarthrophyllum rigidum, together with Stipa chrysophylla and S. speciosa (Martinez & Dalmasso, 1993).

Laboratory and field design During 1991–1992 five samplings were carried out, corresponding to winter (July), spring (October), summer (December and February), and autumn (May). At each sampling 10 fresh faecal pellets (considered as one sample unit) were collected from each active communal dung pile. In addition, 30 m transects for food availability analysis were made by the point-quadrat method (Daget & Poissonet, 1971). On each point of the transect, each species touched was recorded as ‘present’, independently of a previous touch of the same individual plant by another point. In all, this provided 128 samples of pellets and 125 transects. Sampling sites were selected in order to cover the main environments of La Payunia (Puig et al., pers. obs.). Each faecal sample was ground, cleared with dilute lye (aqueous sodium hypochlorite, 25% w/v), and passed through a 71 µm sieve. Slides of the material were analysed by the microhistological method of Baumgartner & Martin (1939); 50 fields per sample unit at 400 3 microscope magnification were examined. Plant reference material of La Payunia, stored at the Ruiz Leal Herbarium (IADIZA, Argentina), facilitated plant fragment identification in faecal material. Genus level, and species level when possible, were reached.

Statistical analyses Plant species were grouped in four categories, according to their life-form: grasses, forbs, low shrubs (Camephytes) and high shrubs (Fanerophytes, including the small

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arboreal genus Schinus). Every species that had been eaten by the guanaco on some occasion was considered. The relative frequency of occurrence of each species in the environment allowed us to estimate the relative importance of its forage availability. Relative frequency was determined for each transect by dividing the number of points in which a given species was touched by the sum total of frequencies for all species identified. The relative frequency of occurrence of a species in the diet was determined for each sample by dividing the number of microscopic fields in which a given species occurred by the sum total of frequencies for all species identified (Holechek & Gross, 1982). The accuracy and biases of this method have been documented (Holechek et al., 1982). Diversity in availability and diet was estimated using the standardized Shannon– Wiener function (H'; Colwell & Futuyma, 1971). Hutcheson test (Hutcheson, 1970) was applied to detect significant differences in diversity among seasons. Food-niche breadth of the guanaco was estimated using Levins’ index scaled to the environmental availability of food (B'A; Hurlbert, 1978). Mann-Whitney U-tests (Zar, 1984) were used to determine significant differences among seasons for each plant category in environmental availability and in guanaco diet. The relationship between the relative frequencies of occurrence of species in the diet and in the environment was determined by the use of rS Spearman coefficient of correlation per ranks (Siegel, 1986). Dietary preferences were detected using the electivity index of Ivlev (1961); we fixed –0·3 and + 0·3 as limits for the indifference rank (i.e. use proportional to availability). Continuity in dietary preferences was analysed throughout the year, considering three categories of preference: annual, seasonal and punctual (i.e. cases in which the preference was observed only in one sampling). The level of significance of statistical results was expressed in scientific notation when the value of p is less than 0·0001 (i.e. 6·0 e–5 = 0·00006).

Results Availability of food resources Fifty-seven plant genera were registered in the sampling sites. Availability analyses considered only genera that were included in the guanaco diet (56%). The most available genera (Fig. 1) were grasses (Panicum 31%, Stipa 23%, Poa 12%), followed by high shrubs (Neosparton 8%) and low shrubs (Hyalis 6%). A decrease in the relative frequencies of grasses and an increase in those of low and high shrubs was detected during winter and spring (Fig. 2) Plant availability was more abundant and diverse in summer (61% of mean cover, 28 available species, H' = 0·96) than in winter (49% of mean cover, 19 available species, H' = 0·76). Diversity significantly changed from July to October (t = 2·31, p = 0·021), and from December to February (t = 2·79, p = 0·005).

Guanaco diet Microhistological analysis identified 32 plant genera in the collected faecal pellets. The most frequently eaten genera corresponded to some of the most available (Fig. 1): grasses (Panicum 36%, Poa 28%, Stipa 6%) and low shrubs (Hyalis 14%). The use of low shrubs increased during winter and spring, while that of grasses decreased. The use of high shrubs increased at the beginning of summer (Fig. 2). Fanerophytes (high shrubs), with a greater proportion of ligneous tissue, were eaten proportionally less than Campehytes (low shrubs).

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0.6

Arcsin-square root of frequencies

Availability 0.4

0.2

0

–0.2

–0.4 Diet –0.6 Ho Po Sp Ar Di La As Ls Le Ac Ba At Ly Bi Ad An Br St Pa Se Qu So De Er Ve Hy Gu Ep Ne Pr Be Sc

Figure 1. Annual mean proportion of species in the environment (availability) and in the guanaco diet. Relative frequencies were transformed using the arcsin-square root to make differences between availability and diet more visible, especially for scarce species. Species abbreviations are detailed in Table 3. ( ) = grasses; ( ) = forbs; ( ) = low shrubs; ( ) = high shrubs).

B

G

C

F

Food-niche breadth of the guanaco, estimated by the Shannon–Wiener function, reached its highest value in December (H' = 0·89), but remained below 0·75 during the rest of the year. Significant differences were found between December and October (t = 3·94, p = 9·8 e–5) and between December and February (t = 4·01, p = 6·0 e–5). Levins’ index, which takes account of availability, enhanced seasonal diet differences, showing very low values in winter (B'A = 0·02) and high ones in summer (B'A = 0·45). Dietary preferences A significant and positive association was obtained between the distribution of species frequencies in the diet and in the environment, considering the annual averages (rs = 0·6482, p = 3·0 e–4) as well as averages for each sampling date (Table 1). Significant differences between seasonal availability and use of two plant categories were found: forbs were eaten more during summer and autumn, and high shrubs were eaten less during autumn and winter, than expected with regard to their availability (Table 2). An important percentage of forbs (75%), grasses (67%) and low shrubs (50%), but only 22% of high shrubs, were identified as preferred by the guanaco (Fig. 3). Continued use throughout the year was detected in 56% of grasses (Table 3), among them those of high quality were preferred annually (Hordeum) or seasonally (Poa), unless availability was very high (Panicum). Coarse grasses (Sporobulus, Stipa) were avoided throughout the year. Forbs generally showed low availability; most of them were included and preferred only in the summer diet. Astragalus, with known toxic effects on livestock, was the only forb always avoided. Among low shrubs with

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continued use (67%), Hyalis was almost always preferred, Verbena and Atriplex sometimes preferred, and most of the others showed avoidance. Preference for some high shrubs (Prosopidastrum, Adesmia, Ephedra) in winter was detected. Use of Ephedra occurred throughout the year, but it was eaten with avoidance during summer, while the rest of the high shrubs were avoided all year round.

1.0 Relative frequency (%)

(a) 0.8 0.6 0.4 0.2 0

J

A

S

O

N

D J F Months

M

A

M

J

J

S

O

N

D J F Months

M

A

M

J

J

Relative frequency (%)

(b)

0.2

0

J

A

Figure 2. Relative frequencies of plant categories in environment (–––) and guanaco diet (– – – –) throughout the year. (a) grasses (j) and low shrubs (.), (b) forbs (×) and high shrubs (m).

Table 1. Spearman rank correlation between species relative frequencies in the environment and in guanaco diet during each sampling month. N=32 in all cases

Month July October December February May

rs

Significance level

0·7512 0·7064 0·7688 0·4616 0·4872

p<0·0001 p=0·0001 p<0·0001 p=0·0102 p=0·0067

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Table 2. Environmental food availability and guanaco diet comparison per plant category in each sampling month, using Mann-Whitney U-test. Numbers are Ψ statistic values and the significance levels; no and nd are, respectively, numbers of availability and diet data compared

Plant categories Sample month

no

Grasses

Forbs

Low shrubs

High shrubs

0·49 p=0·63 0·11 p=0·91 0·47 p=0·64 0·41 p=0·68 1·58 p=0·11

1·00 p=0·32 1·18 p=0·24 4·69 p=3e–6 3·23 p=1e–3 2·87 p=4e–3

1·83 p=0·07 2·05 p=0·04 0·79 p=0·43 0·93 p=0·35 1·15 p=0·25

2·39 p=0·02 1·16 p=0·24 1·44 p=0·15 1·65 p=0·10 5·18 p=2e–7

nd

July 14 40 October 21 4 December 25 40 February 21 4 May 25 40

Discussion Considering preference, and intensity and continuity of use of different food items, grasses (Panicum, Poa and Hordeum) as well as low shrubs (Hyalis and Ephedra) are important components of the diet of the guanaco in La Payunia. The herbaceous stratum is the most important, for it provides 82% of the annual diet, although use and preference for the shrubby stratum increases in winter, when availability of grasses and forbs have decreased in quality and quantity. Similar results have been obtained in other environments (Raedeke, 1979; Bahamonde et al., 1986; Bonino & Sbriller, 1991). 1

Electivity index

P

0.3 I 0 –0.3

A

–1 Ho Po Sp Ar Di La As Ls Le Ac Ba At Ly Bi Ad An Br St Pa Se Qu So De Er Ve Hy Gu Ep Ne Pr Be Sc

Figure 3. Dietary preferences of guanaco according to electivity index of Ivlev (1961). Categories considered are preference (P), indifference (I) and avoidance (A). Species abbreviations are detailed in Table 3. ( ) = grasses;( ) = forbs; ( ) = low shrubs; ( ) = high shrubs.

B

G

C

F

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Most camelids typically feed on the herbaceous stratum (grasses, grasslikes (Juncaceae and Ciperaceae), forbs). Ability to alternate seasonally between grazing and browsing, according to forage availability, allows us to consider the guanaco as an ‘adaptable mixed feeder’, following the classification of Hofmann (1973). A similar

Table 3. Guanaco preference (P), indifference (I) and avoidance (A) for plant species in La Payunia throughout the year. Punctual preference (i.e. detected only in one sampling) is indicated as (p)

Dietary preferences Plant species Grasses Ho Hordeum spp. Br Bromus spp. Po Poa spp. St Stipa spp. Sp Sporobolus rigens Pa Panicum urvilleanum Ar Aristida spp. Se Setaria mendocina Di Digitaria californica Forbs Qu Quenopodium pappulosum La Lappula redowsky So Solanum atriplicifolium As Astragalus pehuenches De Descurrainia sp. Ls Lesquerella mendocina Er Erodium cicutarium Le Lecanophora heterophylla Low shrubs Ve Verbena spp. Ac Acantholippia seriphioides Hy Hyalis argentea Ba Baccharis darwini Gu Gutierretzia spathulata At Atriplex lampa High shrubs Ep Ephedra ochreata Ly Lycium chilense Ne Neosparton aphyllum Bi Brachiclados lycioides Pr Prosopidastrum glovosum Ad Adesmia spp. Be Berveris grevilleana An Anarthrophyllum rigidum Sc Schinus spp.

Annual

Spring

Summer

Autumn

Winter

P

P I

I

P

P A A I P I

I P(p) P

P(p) P(p) P(p) A P(p) P P(p) P

P P

P A P

I

P

P

A A P(p) I

A

A

P(p)

A A P A P A A A

P(p) P

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ability was described for the alpaca (Lama pacos), because it also changes its dietary preferences between grasses and forbs according to availability (San Mart´ın, 1987). Dietary flexibility enables the guanaco to survive in environments where the herbaceous stratum is not dominant. In these environments, the greatest use of this stratum occurs in winter, when there is a decrease in availability of the predominant plant category: lichens in the Atacama Desert (Raedeke & Simonetti, 1988), xerophyllous shrubs in Monte (Balmaceda & Digiuni, 1979), and deciduous trees in the Valdivian forest (Guerra, 1982) and in the magellanic forest-steppe ecotone (Raedeke, 1980). In the latter there is a mosaic of habitats where guanaco decreases the competitive pressure of sheep by moving into the forest (Raedeke, 1980). The high preference for forbs in summer and the fact that the toxic Astragalus was the only forb eaten with avoidance indicates that the guanaco develops an avoidance behaviour against Astragalus in La Payunia. The same behaviour was detected in llama and alpaca in Peru, ´ where these species were significantly less often intoxicated by Astragalus spp. than sheep, in spite of the longer food retention period in the digestive tract of camelids (Espinosa et al., 1989). Fowler (1989) proposes that this avoidance behaviour derives from a process of co-evolution with the native flora, since it has not been observed in camelids translated to new environments. The guanaco diet in La Payunia confirms the generalistic foraging behaviour of this species. It comprises 32 plant species (56% of those present in the environment), including grasses, forbs, and low and high shrubs. Raedeke (1980) suggests that the wide food-niche used by the guanaco is the result of evolution in absence of competitors. The high food diversity (H' between 0·60 and 0·89 in La Payunia) may also reflect the adaptation of this camelid to arid environments, where vegetation generally shows low availability and low quality for most of the year (San Mart´ın, 1991). Proportions of plant categories in the guanaco diet showed significant seasonal changes, and a positive association with changes in food availability. Enlargement of food-niche breadth corresponded to seasonal increases in diversity of available plants. These results do not follow what should be expected in regard to optimal foraging theory: that a herbivore responds to an increase in food diversity by increasing its diet selectivity. No summer decrease in food-niche breadth of the guanaco has been detected in other studied environments: Atacama desert (Raedeke & Simonetti, 1988), Monte (Balmaceda & Digiuni, 1979), Valdivian forest (Guerra, 1982) and magellanic forest-steppe ecotone (Raedeke, 1980; Bonino & Sbriller, 1991). Enlargement of the food niche when environmental availability increases in diversity can be interpreted as another adaptation to arid environments, where high climatic fluctuations reduce the predictability of phenological and nutritional changes in the vegetation (Miller & Gaud, 1989). This situation could force the herbivores to develop a ‘cafeteria style’ (Hansen et al., 1985), continually sampling the available forages to evaluate the species quality (Goss-Custard, 1981). Food diversity in winter in La Payunia is low; moreover an important proportion of these species are inappropriate for the guanaco owing to their phenological condition (aging of leaves and absence of buds). On the other hand, some of the most eaten species are still available in winter. Quintana et al. (1994) describe a similar situation as ‘forced selectivity’ by low food diversity in winter. The nine plant species added to the guanaco diet in summer show high preference indices, suggesting that a nutritional advantage is acquired by their addition. Nevertheless, their low availability would not allow the guanaco to supply its food requirements through a selective type of behaviour. We thank Prof. Monica ´ Cona for her invaluable support during the collection and processing of the data, Ing. Eduardo Martinez Carretero for his assistance in botanical and phytosociological aspects, and Biol. Rafael Gonz´alez del Solar for his editorial suggestions. This study was

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supported by the Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas de Argentina through a research grant (PID 3-094400/88).

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