The relative importance of food quantity and quality in large herbivore diet selection (papers II-III)

a) How do large herbivores with high metabolic rate survive in poor environment such as pastures?

The question of the use of food resources by large herbivores in relation to their availability is a recurrent question in ecology (Wam et al. 2010). It opposes two hypotheses: the forage-abundance hypothesis where ungulates feed in response to the forage-abundance of forages and the selective-quality hypothesis where the selectivity of feeding depends on the nutrient quality of forages (Weckerly 1994).

 Taxonomic niche

According to our results from paper II, diets of chamois and mouflon appeared to be conservative, at least for the main plant species making up the diet in spring and summer, within the daily home range. By the word “conservative”, I mean that whatever the availability (except 0) the proportion of plant intake (but not the selection!) was constant (fig.30). Moreover, no significant relationship were found between the proportion of plants in the diet and the local plant availability. Our results also underlined the strong avoidance of chamois and mouflon for the most abundant plant species occurring in the pastures, i.e. Carex sempervirens and Sesleria caerulea. On the contrary, they focused on rare evergreen plant species, such as the evergreen shrub Helianthemum nummularium or on leguminous rich in nitrogen and easily digestible such as Onobrychis montana or Lotus corniculatus.

Figure 30. Effect of plant availability (relative abundance in terms of biomass in the field) on proportion in the diet for two of the most abundant plant species in the diets of chamois and mouflon (Onobrychis montana on the left and Helianthemum nummularium on the right). In green: mouflon population, in red: chamois in sympatry, in black: chamois in allopatry.

This selectivity observed both for chamois and mouflon was not completely in agreement with previous studies. Indeed, La Morgia & Bassano (2009), Bertolino et al. (2009), Garcia-Gonzalez et al. (1996) recorded that diets of chamois (Rupicapra rupicapra for the two first studies, Rupicapra pyreneica for Garcia-Gonzalez et al.) were composed of a large proportion of grasses (respectively about 30.2% in August to 46.9% in September, 48% in autumn to 67%

in spring, 13.7% in autumn) in addition to forbs, in line with the “intermediate feeder” chamois diet type. Like in our study, both La Morgia & Bassano (2009) and Bertolino et al. (2009)

63 underlined the presence of Cistaceae (the family of Helianthemum nummularium) in the diets, but in lower proportions.

Even though mouflon have been classified as grazers in many studies (see references in Marchand et al. 2013), our study confirmed the non-compulsory grassy diet of mouflon as demonstrated in Marchand et al. (2013) (fig.31). In the Bauges Massif, their diet in pasture was mainly composed of forbs, with a low proportion of grasses (Dactylis glomerata especially).

Hypotheses about the unusual diet compositions of chamois and mouflons are given in the Synthesis, perspectives and directions.

Figure 31. Ternary plot of the diet compositions of mouflon populations reviewed in Marchand et al. 2013.

 Functional niche

Functional niche of diets was described with the analogous of the CWM (Community Weighted Mean) metric, using three plant functional traits in relation with diet quality (nitrogen, phosphorus and leaf dry matter content). It also reflected the weak importance of functional availability (CWM available). Indeed, whatever the available nitrogen, phosphorus or water, animals were able to build their functional niche in a constant way, in spring and summer (fig.3 in paper II). In autumn, availability influenced the functional niche, but we hypothesized that during this period, selection could not compensate for the general decrease in the quality of plants available (fig.3 in paper II).

As stated in the introduction, 14-18% of crude protein has been estimated as the optimum percentage for maximum body growth of deer (French et al. 1956, Magruder et al. 1957). In our study, the diets of chamois and mouflon contained from 14% in autumn to 24% in spring of crude protein, which was greatly above the threshold of the deer. It suggested that chamois and mouflon, with their high metabolic rate, due to their low body mass (Demment & Van Soest 1985, White & Seymour 2003) selected for and found enough of a high quality diet. Hence,

Faeces Rumen

chamois and mouflon were able to reach their energy requirements whatever the availability.

Moreover, we showed that they were able to survive in poor environments such as pastures mainly composed of grass species by selecting rare plants of higher quality than the average available vegetation (diet rich in nitrogen, rich in water, easily digestible; fig.2 in paper III).

 Functional diversity

Although the preferences did not depend on the environment (CWM of the diet independent of the available CWM), we wondered whether the functional diversity (variation of functional trait value) of the diet depended on the functional diversity of the environment (48h home range around faeces). For that, we calculated the functional dispersion index (FDis, Laliberté &

Legendre 2010), which estimates the mean distance between species trait and the CWM. For each season, we tested the relationship between the FD of the diet and the available FD, as in paper II for the CWM.

Except for LDMC in autumn where there was a positive relationship (slope=0.91, p<0.05), the functional diversity of the diet estimated with functional dispersion did not longer depend on the functional diversity of the available plant in the environment (fig.32, p>0.05).

Main result:

Chamois and mouflon fed on plants of high quality rather than on plants of high biomass and of low-quality. We assumed that it allowed them to reach their energetic requirements for growth and reproduction.

Figure 32. Effect of habitat FD (calculated as functional dispersion) on diet FD for nitrogen content (on the left) and LDMC (on the right).

65 b) Methodological considerations

In this work, the carbon content in plants had a relatively low coefficient of variation compared to nitrogen, phosphorus and water content. Consequently, even if Elser et al. (2000) advise the use of nutrient ratios, we decided to not include the C:N and C:P ratio in our analyses as their variations tracked the N and P variability. Moreover, we thought that the interpretation of single components was easier, as we are not aware of optimum ratio values required for chamois, mouflon and roe deer growth and reproduction.

In addition, the carbon content measured with the CHN analyzer included the indigestible and digestible carbohydrates. As the carbohydrates on which herbivores rely on are the digestible ones, which constitutes the proxy of energy, the total carbon content value does not give precise information on what is really used by the animal. Indeed, the lignin, considered as deterrent for herbivores as it is indigestible (Robbins 1983), is highly composed of carbon and could explain a large proportion of the total carbon content. The carbon content measured was not adapted to test the hypothesis that in winter, ungulates (white-tailed deer) would focus on plant rich in energy (Berteaux et al. 1998). For their part, they estimated the energy through the percentage of dry matter digestibility in food.

Finally, Elser et al. (2000) advised to use N:P ratio for animal studies. For the same reason as before, we preferred to focus on the two components independently, as we did not know the optimum values that large herbivores need to reach for their development.

c) At which scale do herbivores make their diet selection?

Additional analyses on the feeding selection scale by chamois and mouflon have been realized by P.A. Dupeyras (2014, M2 internship). The hypotheses are detailed in 2.2.c.

The results showed that a large part of the key-resources (resources on the right of the red axis x = 1 on fig.33) of both chamois and mouflon had a selection pattern similar as “SP1” (selection pattern 1 on the fig.34). Key-resources were selected at the 48h home range (4^th-order selection) and not at the seasonal home range (3^rd-order selection) (fig.34). Therefore, animals did not choose sites where the preferred resources were abundant (1^st hypothesis rejected). The most abundant plant species (Sesleria caerulea, Carex sp.) were eaten in low proportion or not eaten and led to pattern “SP2” or “SP3”. Hence, those patterns suggested that the home range selection did not depend on forage resources but on other factors such as sociality, fear or body physiology.

In addition, “SP1” highlighted a strong inter-individual variability in the proportion of key-resources eaten, which was in line with the significant among-individual variation observed in paper I.

Main result:

Herbivores selected their diet at the fine-scale (4^th order of selection), i.e. within the 48h home range.

While availability did not seem to be the strongest determinant shaping the diet, the selection of plants should, at least partly, depends on functional traits. We used this approach to determine the plant characteristics involved in the diet selection hereafter (6.4).

Figure 33. Mean selection ratio for the 25 key-resources in the diets of chamois and mouflons.

Vertical red line (x = 1) corresponds to the axis where selection is neutral (from Dupeyras 2014). Selection ratios need to be interpreted carefully as they can be high in two cases: when the resource is rare and highly used, or when the resource is abundant and overused.

Figure 34. Selection pattern (SP) at the different orders of selection. 1: Helianthemum nummularium from chamois in pasture, 2: Sesleria caerulea from mouflon in pastures, 3: Carex sp. From chamois in pasture, 4: Betulaceae from chamois in forest. A3: Relative abundance of available resources within the seasonal home range (4.3.c). A4: Relative abundance of available resources within the 48h home range (4.3.b). U4: Relative abundance of used resources measured in the faeces (4.1). Each individual is represented by two lines linking the relative abundance of the used resource at two spatial scale (A3 and A4) (from Dupeyras 2014).

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6.3 How do introduced species influence plant-herbivore interactions? Are