IntroductionTop

Large herbivores’ diet is often limited by the quantity and quality of available forage, and also by their intrinsic anatomical adaptations that facilitate feeding (McNaughton, 1986). These facts make herbivores to select grazing areas of abundant and nutritionally good forage quality (Pelliza Sbriller et al., 1997). According to its feeding habits, guanaco (Lama guanicoe Müller), a native ungulate widely distributed in South America, may be considered a generalist of intermediate selectivity, since its diet include most of the plants present in its natural habitat (Baldi et al., 2004). Guanaco is the largest native ungulate that inhabits Patagonia, the region that covers the southern tip of South America in Argentina and Chile. In Argentina, this region comprises different physiographic environments, from arid and semiarid plateaus in the east, steppe-forest ecotone in the piedmont of the Andes going west, to dense Nothofagus spp. forests at higher altitudes of the Andean cordillera. In continental Patagonia, guanaco’s habitat includes arid and semiarid grassland steppes and shrublands, and open ranges in the forest-steppe ecotone (Franklin, 1982; Baldi et al., 1997), but excludes denser forest areas dominated by Nothofagus spp. or by other tree species. In the southern island of Tierra del Fuego (TF), instead, guanacos’ habitat includes Nothofagus spp. forests and adjacent meadow areas (Bonino & Fernández, 1994; Montes et al., 2000). One of the possible reasons for the expansion of guanacos’ habitat into Nothofagus forests is that the puma (Felis concolor), the main predator of guanaco in continental Patagonia, has not been yet recorded in this island. In TF, Nothofagus spp. forests grow at low altitude, and their physiography present a matrix of forest embedded of open wetlands (meadows), located in depressions of the landscape. These meadows, regionally named “vegas”, although highly overgrazed in the past, are sites of high potential productivity, mainly provided by tender species of the Cyperaceae, Juncaceae, and Poaceae families, which in general supply excellent forage quality (Cassola, 1988). In continental Patagonia, guanacos’ diet mainly consists of grasses, grass-like, herbs, and shrub species (Puig et al., 2011). In the southern area of guanaco’s distribution in TF, and in spite of the availability of tender herbs and grasses in the vegas, the consumption of lenga beech leaves [Nothofagus pumilio (Poepp. & Endl.) Krasser], and to a lesser extent of ñire [(Nothofagus antarctica (G. Forst.) Oerst.] and several other low abundant shrubs, seems to constitute an important part of its diet all year round (Raedeke, 1980; Bonino & Pelliza Sbriller, 1991; Soler Esteban et al., 2011; Muñoz & Simonetti, 2013).

The forest environments dominated by lenga trees in TF are considered of great ecological, economic, and landscape value (Bava & Rechene, 2004; González et al., 2006b). While the understory of these forests usually presents lower vegetation cover as compared to vegas (Boelcke, 1957), and guanaco seems to prefer these vegas for feeding purposes (Bank et al., 2003; Clausen et al., 2006; Puig et al., 2011), evidences show that it also feeds on lenga beech saplings and on some shrub species (Soler Esteban et al., 2011). Vegas are critical habitats for guanacos during the breeding season, allowing increased reproductive success as compared to other habitats (Bank et al., 2003; Wurstten et al., 2013). On the other hand, while lenga beech saplings may be an important component of the guanacos’ diet, the negative effects of its continuous browsing and trampling may compromise future forest development (Rebertus & Veblen, 1993; Bava & Rechene, 2004). The aim of this study was then to determine: 1) the composition of the diet of free ranging guanacos during spring and summer in three representative areas of TF which combine vegas and Nothofagus forests; and 2) which of the forage plants grown in the forest understory guanacos do prefer, or avoid, in these seasons of great food abundance. The results of this study will provide scientific information to know and compare guanacos’ dietary preferences when grazing in different environments of insular and continental areas of Patagonia. Determining the incidence of Nothofagus spp. in guanacos’ diet could also help understand its effects on lenga regeneration in TF, and provide essential information for designing management plans aimed at preserving not only guanacos’ habitat, but also the sustainable growth and development of Nothofagus spp. forests.

Materials and methodsTop

Study area

The study area, located in the center of Tierra del Fuego province (TF), Patagonia Argentina (54º 40’ S, 67º 35’ W, Figure 1), consists of three typical paddocks situated in landscapes that alternate undulated areas with valleys. One paddock was located in “Estancia María Cristina” (MC), and two in “Corazón de la Isla Reserve” (Reserve 1 and Reserve 2, R1 and R2, respectively, Figure 1 and Table 1). Their landscapes are composed of lenga beech forests with some very scattered ñire trees, surrounded by adjacent wet vegas in which herbs and grasses of the Cyperaceae, Juncaceae, and Poaceae families prevail. The regional climate is characterized by short, cool summers, and long, snowy, and frozen winters. Annual average wind speed outside the forests is 8 km h-1, reaching up to 100 km h-1 during storms (Martínez Pastur et al., 2009). Average annual temperature is 4.5 °C; annual precipitation averages 467 mm and, in the form of either rain or snow, is evenly distributed along the year (Pisano, 1977). In the three paddocks, the forest had been used for more than 50 years for timber production based on selective cut of the best lenga beech trees (high-grading). At present and after livestock exclusion (at least 10 years ago), natural populations of guanaco are the only ungulates grazing in these sites.

Table 1. Main physiographic and stand characteristics of the study sites in Tierra del Fuego, Argentina. For altitude, slope and basal area, mean values (±S.E.) are presented.

Feces collection for determining feeding habits

In each paddock and within their respective forest area, we selected a representative sub-area (~ 15 ha) to sample guanacos’ feces. Although guanacos may feed in the vegas or inside the forest, they showed a particular feces deposition behavior, concentrated in specific places within the Nothofagus spp. forests [named dung-piling, (Franklin, 1982)], and rarely in vegas (Soler Esteban et al., 2013). In each site, fresh fecal samples (recognized because they presented a typical greenish color as compared to older feces), were collected during spring (November 2009) and summer (February 2010). Feces collection was done from five active dung-pilings distant at least 50 m one to each other, mixed together and put in plastic bags totalizing 100 to 150 g each. Spring and summer feces were then considered as independent samples.

Botanical and fecal analyses

The botanical identification of plant tissues of reference plants collected in the three study sites was done by determining epidermal and non-epidermal plant fragments based on the micro-histological analysis method (Williams, 1969) adapted for Patagonian plants by Latour & Pelliza Sbriller (1981). At the same time of feces collection, we also sampled plants of all taxa present in either the vegas or in the forest understory. The analysis involved drying the plant material and feces at 60 ºC for 48 h, milled in an analytical mill (IKA A11) until reaching particle sizes of about 1 mm, and mounted on microscope slides for further identification. This microscopic identification allowed performing species-specific patterns of plant (items) for comparison with those found in feces. Microscopic observations of fecal samples showed different frequencies of species epidermis, so those results were expressed as percentage values of the items, at genera or species levels. Fecal samples were mounted on five microscope slides and 20 fields per slide were examined using 100 × magnifications. These analyses allowed grouping the results of these observations into six vegetation groups: trees, grasses, herbs, grass-like, shrubs, and lichens. Quantification of items composing each diet was done according to Holechek & Vavra (1981), and Holechek & Gross (1982). All analyses were conducted at the Botany laboratory, Universidad Nacional de la Patagonia “San Juan Bosco”, in Comodoro Rivadavia (Chubut, Argentina).

Biomass availability in the forest understory

To determine biomass availability in the forest understory, we placed 3 transects per paddock. The transects, 320 m long each, were located in parallel and distant about 300 m one to the other, starting from the edge of each vega toward the interior of the forest. Along each transect, 1 m2 quadrats were placed at 10, 20, 40, 80, 160 and 320 m. The understory biomass inside each quadrat was cut at 2 cm aboveground, collected, separated into grasses, herbs, shrubs and trees, put in plastic bags, taken to the lab, oven-dried at 75 °C for 48 h, and weighed. Biomass was expressed as kg DM ha-1, and was harvested in spring and summer simultaneously with feces collection.

Diet selectivity

Diet selection was analyzed considering aboveground biomass present in the forest understory during spring and summer, and used as an estimation of food availability. This was performed according to Ivlev’s selectivity index modified by Jacobs, 1974 (Lechowicz, 1982), as:

Di = (ri - pi) / (ri + pi-2* ri* pi)

where: ri is the relative abundance of plant group i in the guanacos’ feces, and pi is relative abundance of that group in the field. Di varies between -1 (strong avoidance) and 1 (strong selection), while a value near 0 indicates that the forage resource is consumed proportionally to their availability in the environment. According to Puig et al. (1996), the limit to determine if an item is consumed according to its availability in the environment ranges from –0.3 to 0.3. The selectivity of guanaco, by considering understory groups of grasses, herbs, shrubs and trees (lenga leaves and twigs of seedlings and saplings) per site and season, was evaluated by relating the percentage of each group in the diet with its relative availability in the understory.

To analyze the selectivity of the diet, we purposely excluded from the analysis grass-like species belonging to the Juncaceae and Cyperaceae families (grass-like). Apart from being their biomass insignificant in the understory as compared to that found in the adjacent vegas, where they dominate, our objective was to determine selectivity only for those species typical of that understory.

Data analyses

The assumption of normality (Shapiro-Wilks) was investigated for the variables species and groups of species percentage in the diet. Mean differences of dietary composition between seasons was performed by using the Student T test (p<0.05) for paired samples, with n=3.

Linear mixed-effect models were used to analyze the effects of treatments: Site: MC, R1 and R2, and Season: spring and summer, on the biomass available. Treatments (Site, Season and Site *Season interaction) were considered as a fixed effect, and the variable transect was considered as a block (random effect). The amount of the model variance that was explained by differences among sites was calculated by dividing the random effect variance by the total variance.

The mean differences of D index between seasons was performed by using the Student T test (p<0.05) for paired samples, with n=3.

This test was applied by using the Satterwai correction in the case the variances were not homogeneous (p<0.05). All statistical analyses were performed by using the INFOSTAT Statistical Software (Di Rienzo et al., 2015).

ResultsTop

Guanacos’ diet composition

The percentages of the different groups of plants composing guanacos’ diet during spring and summer, in the three study sites, is shown in Table 2. In the two seasons studied and considering all sites together, guanacos’ diet presented high percentage of tree lenga beech leaves and twigs (38.7%), followed by grass-like (27.6%) and grass species (27.3%), and showing a low presence of herbs (3%), Lichens (2.1 %) and shrubs (1.2%). The most important items found in the diet, with frequencies higher than 10%, were Nothofagus sp. (mainly lenga beech leaves and shoots), Uncinia lechleriana, Poa sp, Carex sp. and Festuca spp. In spring, 55% of the diet was composed of species only found in the forest understory (Nothofagus sp., Osmorhiza chilensis, Berberis sp., Gaultheria sp. and lichens), while 30% corresponded to grass-like species only found in vegas. The rest of the diet (15%) corresponded to other grass species found in either the forest understory or in the nearby vega. During the summer, the percentage of Nothofagus spp. in the diet significantly decreased in all three sites (29%, p: 0.03) and increased the grass component (40%, p: 0.003). The grass-like species showed a mean percentage of 25% in the diet in the two seasons analyzed. Of these grass-like species, U. lechleriana marginally decreased (p: 0.05) its frequency during the summer (Table 2).

Table 2. Percentage values (± S.E.) of plant species found in guanacos’ diet during spring and summer, in the three sites (María Cristina (MC), Reserve 1 (R1), and Reserve 2 (R2), respectively. Student T test (p<0.05) for paired samples, * denotes significantly differences between seasons. The + denotes species characteristics of the vegas which presented low biomass in the forest.

Biomass availability in the understory

Total biomass showed significant differences among sites, being higher in R2 in both seasons analyzed and lower during spring in R1 and in summer in MC (Table 3). The higher values found in R2 could be related to the higher lenga seedling and sapling biomass availability as compared to the other sites. The lowest lenga seedling and sapling biomass availability in R1 and MC were registered during the summer season (Table 3). Biomass of all other analyzed groups of species did not show significant differences. However, biomass of grasses presented a maximum value (~160 kg DM ha-1) in R2 and a minimum (~50 kg DM ha-1) in MC, both during the spring. All other herb species showed a peak of biomass (~115 Kg DM ha-1) in R2 during the summer and a minimum (~18 kg DM ha-1) in R1 also during the summer. Shrub biomass was in general scarce (< 1 kg DM ha-1) in all three sites. In neither case, the seasons considered nor the interaction between season and site did show significant differences (Table 3).

Table 3. Average values of available understory biomass in kg DM ha-1 (± S.E.) in spring and summer, and in each studied site (Estancia María Cristina (MC), Reserva Corazón de la Isla 1 (R1), Reserva Corazón de la Isla 2 (R2). Different letters indicate significant differences (p< 0.05, LSD Fisher) between sites and season.

Considering the relative availability of each group related to the total understory biomass, it was observed that the group of grasses predominated, followed by trees and herbs. Shrubs, showed a very limited availability (Figure 2).

Guanaco selectivity of forest understory species

The Ivlev’s index showed that lenga regeneration (seedlings and saplings) were selected or consumed in relation to its availability in all three sites and during both seasons (Figure 3). In spite of their low availability, shrubs were also highly selected. Herbs were rejected in all three sites during both seasons. Grasses, instead, were rejected during spring and consumed according to their availability, or slightly selected, during summer, being significant the consumption difference between seasons (spring and summer) (T: 3.29, p: 0.03, Figure 3).

DiscussionTop

Guanacos’ diet composition

In the particular environments that compose the landscapes of Tierra del Fuego, in which Nothofagus forests are intermixed with vegas, guanacos’ populations may show either migratory or sedentary behavior (Raedeke, 1982; Schiavini et al., 2009; Moraga et al., 2015). Some studies showed that guanacos migrate to lowlands and coastal areas of Tierra del Fuego during cold winter periods, returning to forest areas of higher altitudes in spring and summer (Raedeke, 1980; Bonino & Fernández, 1994; Montes et al., 2000), or mainly during the summer (Moraga et al., 2015). Schiavini et al. (2009), however, found that there are sedentary guanacos populations that remain in the forests and surrounding vegas of Tierra del Fuego all year around. While guanacos in the three studied sites may have shown different migratory behavior, we restricted our study to determine their diet during the relatively warm period comprising from mid spring to mid-summer, when the populations wandered around Nothofagus forests and vegas. During these periods, high amounts of leaves and twigs of seedlings and saplings of Nothofagus spp., and also of grass species (mainly of Poa sp. and Festuca sp.), were consumed by guanacos; grass-like species such as Carex sp. and U. lechleriana completed its diet. The consumption pattern was similar in the three analyzed sites. These results are in accordance with some studies carried out in other areas of Tierra del Fuego (either in Chile or Argentina) (McNaughton, 1983; Bonino & Pelliza Sbriller, 1991; Muñoz González, 2008; Soler Esteban et al., 2013), and emphasize the importance that Nothofagus spp. leaves and twigs have in guanacos’ diet during the spring in this island (Soler Esteban et al., 2011). Our results, however, highly contrast with guanacos’ diet determined in other environments outside Tierra del Fuego. In continental Patagonia, for instance, it has been demonstrated that guanaco diet overlap with that of sheep during the summer season, when both species feed on grasses and grass-like species found in meadows (Baldi et al., 2004). During the winter months, instead, guanaco shifts its preference to shrubs (Senecio spp., Mulinum spp., Berberis spp.), which comprise about 55 to 80% of its diet during this cold period (Baldi et al., 2006). In the north-central region of Argentina (in the provinces of Mendoza and Córdoba), several studies revealed that guanacos mainly consume grass species, shifting their preferences to some portions of woody species only when grasses are very scarce (Puig et al., 1997; Barri et al., 2014). It is interesting to note that this north-central region includes a variety of environments in which the trees Polylepis australis and Maytenus boaria are abundant. While leaves of these species are highly preferred by domestic grazers (cows, goats, and sheep) (Renison et al., 2015), guanacos prefer to graze on short grasses, avoiding foraging on these and other tree species (Flores et al., 2013).

Despite the abundant offer of herbaceous species during spring and summer in the areas surveyed in Tierra del Fuego, our results showed some preference of guanacos to browse on Nothofagus spp. leaves and twigs to satisfy part of their nutritional requirements. This particular feeding behavior may be related to the efficiency of gastric digestion that guanacos have (Marín et al., 2006). In fact, guanacos and other camelids regurgitate and re-chew the forage they eat, and are much more efficient than ruminants in extracting protein and energy from poor quality forages (San Martin & Bryant, 1989; Pinto Jiménez et al., 2010). On the other hand, the corporal mass plays an important role in the foraging strategy (Gordon, 2003). In this regard, it has been proposed that an inverse relationship exists between body size and selectivity, being the guanaco a herbivore with an intermediate selectivity (species from 50 to 200 kg of body weight (Baldi et al., 2004).

Guanaco selectivity of forest understory species

The analysis of diet selectivity of guanacos, taking into consideration exclusively species grown in the forest understory (excluding grass-like species), showed that they prefer to consume woody vegetation (leaves and twigs of lenga seedlings and saplings) and grasses only during the summer, avoiding selecting grasses during the spring, and herbs during either spring or summer. Other studies carried out in Tierra del Fuego emphasized the high frequency of lenga leaves and twigs found in the diet of guanaco, although neglected the importance of shrub species in that diet (Martínez Pastur et al., 2010; Soler Esteban et al., 2011; Arias et al., 2015). In our study, however, although we recorded little abundance of shrubs in the forest understory and that these species were infrequent in the diet, the Ivlev’s index indicated that shrubs are highly preferred should they are available.

On the other hand, although the availability of herbs was relatively high related to the total available biomass inside the forest (and much higher if we consider the adjacent vegas), they were rejected by the guanacos, coinciding with the results presented by other studies (Soler Esteban et al., 2011; Muñoz & Simonetti, 2013). Grasses, instead, although rejected during spring, they were slightly selected during the summer and consumed according to their availability. Contrasting with our results, Puig et al. (2011) indicated that in steppe zones of Central Argentina, grasses and graminoids species were preferred by free ranging guanacos, and that woody species were rejected. Furthermore, these authors reported that in these steppe zones, wetland meadows constitute a focal sector for guanacos foraging. Based on the classification proposed by Hofmann (1989) the guanaco could be then defined as an intermediate herbivore, or opportunistic (mixed) feeder. This is so because it forages on a highly diverse range of food sources (González et al., 2006a), having developed physiological and morphological adaptations for including in its diet a high proportion of woody species (Searle & Shipley, 2008). In continental Patagonia, the negative impacts of understory grazing by domestic and wild ungulates has been considered as one of the most important disturbances that affect Nothofagus forests regeneration (Veblen et al., 1992; Relva & Veblen, 1998; Raffaele et al., 2011; Barrios-Garcia et al., 2012). In this region, guanacos avoid grazing on Nothofagus forests. In Tierra del Fuego, instead, guanacos select feeding areas within Nothofagus forest in spring and summer, despite these are seasons of great abundance of other forages outside the forest. The significant consumption of lenga could adversely affect the development of regeneration of the species, and this aspect should be carefully considered in planning and implementing management plans for sustainable regeneration and growth of Nothofagus forests. In order to cast light on this issue, it is necessary that future research should be focused in determining vegetation dynamics comparing grazed and ungrazed lenga forests in Tierra del Fuego.