Behavioral Adaptive Mechanisms Of Large Herbivore Foraging In Response To Diversity And Spatial Pattern Of Plants

Foraging of grazing livestock which occurred at plant-animal-interface is central in grazed grassland systems. The foraging behaviour defines a cardinal link between primary and secondary productivity, and affects the structure of the plant community and the utilized efficiency of grassland resources. The foraging behavior of grazing animals is an important issue in grazing ecology and grassland ecology. The foraging environment with which the grazing herbivores are confronted is highly complex in the grasslands. The grazing rangeland exhibits not only high variations in hetereogenity and diversity in space and time, but also diversity of spatial distribution pattern of plants. In this paper, we tested the effects of animal state including fasting and diet experience, plant species diversity, plant spatial distribution and spatially neighbouring relationships on sheep foraging behavior, from the perspective of animal nutrition and ecology. Based on the cognitions on the complexity of herbivore diet selection (i.e. internal state of animal was not static), we examined not only outcome of herbivore diet selection, but also process of diet selection, which will benefit to obtain deep insight into herbivore foraging strategies. The adaptive mechanism of grazing animals to complex foraging environment was discussed in conclusion. Experimental and theoretical work on herbivore foraging is of great importance for development of foraging theory, as well as maintaining biodiversity, and protecting the service functioning of grassland ecosystems, sustaining the stability of grazing grassland systems, effectively managing the grassland and developing stocking raising in grassland.Combining the artificially simulated grazing experiments, indoor cafeteria trials with analyses of experimental data, we obtained the important results and conclusions as follows. 1) We examined the effects of animal fasting (no fasting, half fasting, intermediate fasting, high fasting and over fasting treatment) and food experience on sheep foraging in indoor feeding trials. The results showed that foraging behavior of sheep strongly depended on the animal state. The fasted sheep exhibited significantly higher food intake, foraging time and intake rate than non-fasted ones. However, no effects of fasting on diet selection by sheep were detected. Food preference was significantly affected by recent dietary experience of herbivore, and animal exhibited partial preference. Sheep preferred alternatives to forages they have consumed recently. The food aversion was stronger as the exposuring time increased.2) We studied the short-term regulation of sheep foraging at four foraging phase within meal (0-0.5 h, 0.5-1 h, 1-1.5 h, 1.5-2 h) in response to altered plant diversity (2, 4, 6 and 8 species richness levels) using a indoor cafeteria trial. Kinetics of food intake was modified as plant species richness increased. At lower plant diversity, intake of sheep was highest at the beginning and then decreased continuously as satiation proceeded until satiety. At higher plant diversity, sheep maintained high and constant food intake as satiation proceeded at all times. Fasting was an important endogenous factor affecting herbivore feeding motivation, and effects of fasting decreased as satiation proceeding. Plant diversity was an important exogenous factor, and the diversity effects significantly increased with time. The complementary effects of endogenous and exogenous factors made herbivore maintain high appetite in the whole process of foraging. We therefore concluded that plant diversity can stimulate motivation to eat by sheep and obtain high food intake.3) Herbivore adopted different foraging strategies based on an evaluation of the fitness consequences of alternative behaviors to altered plant diversity. The result of process of diet selection showed that there was a trend in diet selection of sheep from being a deterministic process at low species richness to a stochastic process as plant richness increases. We suggested that the stochastic foraging pattern in complex situation with high species diversity may be a successful and optimal foraging strategy, which may be an evolutionary consequence for diversified plant resources. The changes in sheep foraging pattern as plant diversity increases implied that there was a weak positive feedback between plant diversity and herbivore foraging, and that the foraging strategy by sheep in the more diverse environments may be beneficial in conserving grassland diversity.4) We examined responses in foraging performance of sheep to change in plant species richness by using a nested species loss approach in an indoor cafeteria trial with six species richness levels (1, 2, 4, 6, 8 and 11 species). Plant diversity significantly improved nutrient intake (energy and protein) of sheep (presumably improving animal performance). Sheep preferred a diverse diet even over single species diets of the most preferred species. The diet quality selected by sheep had a slight change as the available number of plant species increased. Thus, stimulating effect of diet mixing for voluntary food intake of sheep was the most critical factor improving animal nutrient intake. The asymptotic relationship between plant species richness and voluntary food intake by sheep indicated that increasing plant species richness is a key factor to achieve maximum intake and improve performance of herbivores in the generally low-quality grasslands5) We proposed the Disturbance Selection Model for explaining plant species richness effects on herbivore performance. The central concept is that increase in number of plant species in grassland, will increase forage consumption by each large herbivore and enhance nutrient intake by modifying nutrient balance, toxin dilution and taste modulation. Higher plant species richness simultaneously intensifies herbivore diet switching frequency, and weakens the herbivore's ability to select food, thereby increasing foraging cost and disturbing the herbivore's selection of plant. The consequence of change in plant species richness for large herbivore performance depends on the trade-off between positive and negative effects. At highest plant species richness, the positive effects weaken and negative effects strengthen. The model may be useful in better understanding the relationships between plant diversity and ecosystem functioning, and informing management of large herbivores on rangelands where biodiversity conservation, as well as sustainable and profitable animal production, are required.6) The effects of plant spatial distributions on sheep foraging was examined by an artificially simulated grazing experiments by using three species with five combinations of clumped and random distribution patterns. The results showed that herbivore foraging success for preferred species depended on spatial distribution of both preferred and less preferred species as well as overall spatial characteristics of the whole food patches resulting in alteration of herbivore foraging strategy. Clumped distribution of less preferred species was always beneficial for herbivore to search and consume preferred species, while random distribution of less preferred species reduces herbivore consumption of preferred species. On the other hand, effects of spatial distribution of the preferred species on its consumption were dependent on herbivore foraging strategy. When the spatial distribution of a preferred species was clumped, the area-concentrated foraging strategy benefited its consumption. In contrast, when the spatial pattern of a preferred species was random or dispersed, the random-point-sampling benefited its consumption. We therefore concluded that herbivores can obtain higher benefits from the preferred species when there was a consistent spatial pattern between the preferred species and the entire food resources. We suggested that the dispersion of the low quality and non-preferred species in grassland may reduce herbivore's consumption of highly preferred species, thus better resisting selective grazing.7) We studied the effects of spatial neighboring relationship on sheep foraging in a manipulative experiment, where three natural plant species with different palatability was allocated to three same-size patches on the basis of different neighborhoods. We found that close spatial relationships between plant species significantly affected herbivore foraging. When there was a complex neighbor relationship of species at multiple patches in a community, the preferred species can most effectively defend against herbivory. Plants can not effectively defend themselves against herbivory if the constituted spatial pattern between plant species only compelled herbivore to make foraging selection at one scale. High complexity of spatial neighborhood resulted in herbivores passively reducing selectivity, thereby reducing the probability of damage to palatable plants in the community, or making inaccurate judgments in foraging selectivity between and within patches, thereby reducing the vulnerability of palatable plants and even the whole plant community to being eaten. We suggested that compelling herbivores to passively reduce the magnitude of foraging selectivity by establishing spatially complex neighborhoods between plant species was a compromise and optimal spatial strategy by plants to battle consumption by herbivores. This contributed not only to maintenance of plant species diversity but also to a stable coexistence between herbivores and plants in grassland ecosystems.Based on the above obtained outcomes from our experiments, we aquired the further knowledge and insights into foraging behaviour of grazing herbivore. We concluded that diet selection of grazing herbivore was high complex, and was an adaptive and evolutionary consequence for complex and diversified plant resources. The adaptive foraging behaviour of herbivore was an important determinant factor affecting ecological process of grassland, which was paramount ecological significance. We argued that the feedback effects of herbivore foraging on the structure of the plant community, plant diversity and ecosystem function of grassland, strongly depended on the complex structure of plant community itself. We therefore suggested that high complexity and diversity of grassland plant community was one of the most important conditions to ensure large generalist herbivore–plant'mutualisms'and stable coexistence. This study further emphasized the importance for protecting grassland plant diversity.