大型草食动物采食对植物多样性与空间格局的响应及行为适应机制
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摘要
动物、植物界面(plant-animal-interface)上的家畜采食(herbivore foraging)是草地放牧系统的核心特征。作为动物与植物生产的一个关键因素,采食影响草地植被结构,决定草地资源转化效率。动物采食行为无论对放牧生态学和草地生态学都是极其重要的研究内容。天然草地放牧家畜所面临的采食环境是极其复杂的,这种复杂性不仅表现植物在时空尺度上的高度异质性(heterogeneity)和多样性(diversity)变化,也体现植物空间分布格局(spatial pattern)变化。由此,本论文以大型草食家畜——绵羊作为研究对象,基于动物营养学和生态学两个角度,从影响动物采食行为的动物体本身因素、植物个体因素,到植物群落的空间因素,系统地研究了放牧绵羊本身的动物体状态(饥饿与营养经历),植物物种多样性、以及植物种群的空间分布和植物群落空间关系对家畜采食的影响;基于对动物采食行为本身的复杂性认识,本研究不仅考虑动物采食选择的结果,同时进行动物采食选择过程的分析,全面深入地揭示动物的食性选择策略,探讨放牧家畜对复杂多变采食环境的适应机制。这些研究不仅能发展动物采食理论,同时对于恢复或维持生物多样性、保护草地生态系统的服务功能、维持草地放牧系统的稳定性,有效地进行草地放牧管理,推动草地畜牧业发展都具有极其重要的理论价值与实际指导意义。
本研究采用一系列严格的试验设计及分析技术,包括舍饲自助餐试验、人工模拟草地放牧试验,以及大量的实验室分析测试、试验数据统计与比较分析,获得了以下重要研究成果:
(1)不同饥饿程度(零饥饿、半饥饿、中度饥饿、高度饥饿和极度饥饿)及不同采食经历对绵羊采食影响的试验研究表明,动物体状态显著影响绵羊的采食行为。饥饿状态下,绵羊的采食量、采食时间和采食速度都明显高于非饥饿状态下,饥饿状态对绵羊的食性选择无显著影响;动物对某种食物的偏食性强度明显受其对该食物的采食经历影响,即动物具有明显的部分偏食性(partial preference),当采食某种食物一段时间后,动物会产生明显的食性转移;这种短期食物回避(transient food aversion)强烈地受动物对该食物的持续接触时间(duration of exposure)影响,动物对某一食物接触持续的时间越长,对其产生的饱腻感越强,因此,对该物种偏食性强度降低的幅度就越大,即,转移回避的强度越大。
(2)采用舍饲自助餐实验,开展了不同植物物种多样性(2种、4种、6种、8种)水平下的绵羊短期采食调控(即,四个采食阶段0-0.5 h、0.5-1 h、1-1.5 h、1.5-2 h的采食行为)试验。研究结果显示,不同植物多样性显著影响绵羊的采食调控模式。在较低物种多样性条件下,绵羊的采食模式表现为:随着采食的进行,其采食量和采食速率逐渐降低;在较高植物物种多样性条件下,随着采食的进行,绵羊表现为维持持续较高的食欲状态;进一步分析表明,动物饥饿是刺激动物采食的重要内源因素,随着采食的进行,即饱腹的增加,饥饿的刺激作用逐渐减弱,相反,植物多样性是刺激动物采食的重要外源因素,随着采食的进行,植物多样性的刺激作用逐渐增强。内源因素和外源因素之间的相互补偿使动物在整个采食过程中维持较高的食欲。因此,维持较高的植物多样性是刺激动物食欲(appetite)、提高动物采食动力(motivation to eat)的重要因素。
(3)通过对不同植物多样性水平下,绵羊偏食性的餐变异(连续采食16餐)分析发现,随着可选择植物物种数的增加,绵羊具有不同的食性选择模式。在较低植物多样性条件下,绵羊偏食性的时间模式是稳定的、趋向确定性的,随着植物多样性的增加,食性选择模式为多变的、趋向随机的,即,动物的采食策略表现为由确定性采食(deterministic foraging)向随机性采食(stochastic foraging)的变化过程。随着植物多样性的增加,动物采食模式的变化是对复杂采食环境的适应性采食策略,高多样性条件下的随机性采食是动物对多样化食物资源行为适应进化的结果。草食动物对变化植物多样性的行为响应具有重要的生态意义,高多样性条件下的动物随机性采食策略可能更有利于维持草地植物多样性;相反,在较低植物多样性条件下,动物的采食不利于其多样性的维持,因此,植物多样性与动物采食间具有一种弱的正反馈关系。本研究结果进一步表明,提高草地植物多样性能够降低动物的采食选择,从而有利于放牧动物对草地植物的均匀利用,进而保护草地植物多样性。
(4)采用“嵌套式物种丢失法”(nested species loss approach)进行了草地植物多样性变化对动物采食性能的影响研究。研究发现,植物多样性的增加显著提高了绵羊的营养物质(能量和蛋白质)摄入量,进而提高了动物生产性能。随着物种多样性的逐渐增加,绵羊的采食量表现出一个逐渐显著增加的模式;绵羊总是偏于采食一个多样化的日粮(diet),这种偏食强度甚至要优于动物最喜食的物种;随着可利用物种数的增加,动物采食选择获得的日粮质量(即日粮营养物质浓度)仅有微小的变化。因此,多样化日粮对动物随意食物采食量(voluntary food intake)的刺激作用是提高动物营养获取、改善其生产性能的关键,这说明,在普遍低质的天然草地,提高和保护草地植物物种多样性能够作为促进动物采食性能的重要措施。该结果对于放牧草地管理以及动物生产都具有极其重要的理论价值,也从一个新的视角证明了保护草地植物多样性的重要意义。
(5)在生物多样性的生态系统功能认识的理论假说基础上,针对草地植物多样性对草食动物的生产性能的作用结果,提出了“干扰选择模型”(Disturbance selection model)。该模型的要点是:草地较高的植物多样性,通过调节动物营养平衡、毒素稀释和口味调节等作用,显著地提高动物的采食量和营养物质摄入量,从而提高动物的生产性能;高植物多样性同时也降低动物进行食性选择的能力,增加采食选择的成本,对动物的采食选择带来了干扰;植物多样性对动物采食性能的作用依赖于这两个正、负作用间的权衡(trade-off)。同时,在最高植物多样性条件下,植物多样性对动物产生的正作用减弱,负作用增强,因此,植物多样性有利于提高草食动物的生产性能,但过高的多样性对动物采食的作用结果并不是最佳的。该模型的提出不仅有利于提高我们对植物多样性的生态系统功能的深入认识和理解,而且,对放牧家畜生产与草地管理方面也具有重要的理论价值。
(6)采用人工模拟放牧试验法,检验了不同植物(高、中、低偏食物种)的空间分布特征(聚集或随机分布)对绵羊采食选择影响。试验结果表明,植物空间分布特征显著地影响放牧动物的采食选择。草食动物对偏食物种的选择性采食结果依赖于偏食物种本身的空间分布、非偏食物种的空间分布,以及整个采食区域内食物资源的总体空间分布特征。偏食物种本身的空间分布对其采食的影响依赖于动物的采食策略,当偏食物种为聚集分布时,集中区域采食策略(area-concentrated foraging strategy)有利于动物的采食成功,而随机点取样策略(random-point-sampling strategy)不利于采食;相反,当偏食物种为随机分布时,随机点取样策略有利于其采食成功,而集中区域采食策略不利于采食;当整个食物资源的空间分布特征趋于聚集分布时,动物通常采取集中区域采食策略,而趋于随机分布时,动物则通常采取随机点取样策略。由此推论,当某一植物种群的空间分布模式与整个采食区域食物资源分布的空间特征相一致时,动物将更容易采食到该物种。此外,非偏食物种的聚集分布总是有利于动物寻找采食偏食物种,而草地低质非偏食物种的随机分散分布能够显著降低动物对偏食植物种的采食,从而更好地抑制草食动物的放牧选择性,有利于草地植物多样性保护,提高草地群落稳定性。
(7)通过人工模拟放牧试验,研究了植物空间关系对绵羊采食选择的影响,分析了植物对动物采食的联合防御效应(associational plant defense)。结果表明,不同模式的植物空间邻居关系显著影响动物对偏食物种以及所有植物物种的采食。当不同植物物种之间在多个斑块存在复杂的邻居关系时,偏食植物物种能够最有效地防御动物采食;当植物物种之间所构成的空间关系模式迫使草食动物仅在一个尺度下做采食决策时,植物通常不能够有效地防御动物对偏食物种的采食;误导或干扰动物进行食性选择的能力从而达到防御动物采食的目的是植物采取的一种极端空间防御策略,因为这种策略过度地限制了动物对所有植物的总采食量;而直接迫使动物被动地降低选择性是植物采取的一种折衷的、最佳的空间防御策略,这种防御策略不仅有助于维持草地植物物种多样性及其功能,同时有利于草地生态系统中动、植物的稳定共存。
本研究通过对大型草食动物在不同草地植物多样性背景下的采食行为响应、不同植物种群的空间分布模式和植物物种间的空间邻居关系对动物采食选择影响,以及动物体本身的采食调控规律等深入系列的研究,获得了对草地家畜采食行为的深入理解和进一步认识:放牧家畜具有极其复杂的选择性采食行为,这种复杂的采食行为是对草地复杂采食环境的适应进化结果,草食动物的适应性采食行为(adaptive foraging behaviour)是决定草地生态过程的关键因素,具有重要的生态意义;动物采食对草地的反馈作用结果强烈地依赖于草地植物本身的复杂结构,草地植物的复杂性与多样性是确保草地动、植物之间互惠与稳定共存的重要基础。本研究从动物采食行为的视角进一步证实了草地植物多样性在生态系统中的重要作用以及保护草地植物多样性的重要性。
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 grasslands
5) 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.
本研究采用一系列严格的试验设计及分析技术,包括舍饲自助餐试验、人工模拟草地放牧试验,以及大量的实验室分析测试、试验数据统计与比较分析,获得了以下重要研究成果:
(1)不同饥饿程度(零饥饿、半饥饿、中度饥饿、高度饥饿和极度饥饿)及不同采食经历对绵羊采食影响的试验研究表明,动物体状态显著影响绵羊的采食行为。饥饿状态下,绵羊的采食量、采食时间和采食速度都明显高于非饥饿状态下,饥饿状态对绵羊的食性选择无显著影响;动物对某种食物的偏食性强度明显受其对该食物的采食经历影响,即动物具有明显的部分偏食性(partial preference),当采食某种食物一段时间后,动物会产生明显的食性转移;这种短期食物回避(transient food aversion)强烈地受动物对该食物的持续接触时间(duration of exposure)影响,动物对某一食物接触持续的时间越长,对其产生的饱腻感越强,因此,对该物种偏食性强度降低的幅度就越大,即,转移回避的强度越大。
(2)采用舍饲自助餐实验,开展了不同植物物种多样性(2种、4种、6种、8种)水平下的绵羊短期采食调控(即,四个采食阶段0-0.5 h、0.5-1 h、1-1.5 h、1.5-2 h的采食行为)试验。研究结果显示,不同植物多样性显著影响绵羊的采食调控模式。在较低物种多样性条件下,绵羊的采食模式表现为:随着采食的进行,其采食量和采食速率逐渐降低;在较高植物物种多样性条件下,随着采食的进行,绵羊表现为维持持续较高的食欲状态;进一步分析表明,动物饥饿是刺激动物采食的重要内源因素,随着采食的进行,即饱腹的增加,饥饿的刺激作用逐渐减弱,相反,植物多样性是刺激动物采食的重要外源因素,随着采食的进行,植物多样性的刺激作用逐渐增强。内源因素和外源因素之间的相互补偿使动物在整个采食过程中维持较高的食欲。因此,维持较高的植物多样性是刺激动物食欲(appetite)、提高动物采食动力(motivation to eat)的重要因素。
(3)通过对不同植物多样性水平下,绵羊偏食性的餐变异(连续采食16餐)分析发现,随着可选择植物物种数的增加,绵羊具有不同的食性选择模式。在较低植物多样性条件下,绵羊偏食性的时间模式是稳定的、趋向确定性的,随着植物多样性的增加,食性选择模式为多变的、趋向随机的,即,动物的采食策略表现为由确定性采食(deterministic foraging)向随机性采食(stochastic foraging)的变化过程。随着植物多样性的增加,动物采食模式的变化是对复杂采食环境的适应性采食策略,高多样性条件下的随机性采食是动物对多样化食物资源行为适应进化的结果。草食动物对变化植物多样性的行为响应具有重要的生态意义,高多样性条件下的动物随机性采食策略可能更有利于维持草地植物多样性;相反,在较低植物多样性条件下,动物的采食不利于其多样性的维持,因此,植物多样性与动物采食间具有一种弱的正反馈关系。本研究结果进一步表明,提高草地植物多样性能够降低动物的采食选择,从而有利于放牧动物对草地植物的均匀利用,进而保护草地植物多样性。
(4)采用“嵌套式物种丢失法”(nested species loss approach)进行了草地植物多样性变化对动物采食性能的影响研究。研究发现,植物多样性的增加显著提高了绵羊的营养物质(能量和蛋白质)摄入量,进而提高了动物生产性能。随着物种多样性的逐渐增加,绵羊的采食量表现出一个逐渐显著增加的模式;绵羊总是偏于采食一个多样化的日粮(diet),这种偏食强度甚至要优于动物最喜食的物种;随着可利用物种数的增加,动物采食选择获得的日粮质量(即日粮营养物质浓度)仅有微小的变化。因此,多样化日粮对动物随意食物采食量(voluntary food intake)的刺激作用是提高动物营养获取、改善其生产性能的关键,这说明,在普遍低质的天然草地,提高和保护草地植物物种多样性能够作为促进动物采食性能的重要措施。该结果对于放牧草地管理以及动物生产都具有极其重要的理论价值,也从一个新的视角证明了保护草地植物多样性的重要意义。
(5)在生物多样性的生态系统功能认识的理论假说基础上,针对草地植物多样性对草食动物的生产性能的作用结果,提出了“干扰选择模型”(Disturbance selection model)。该模型的要点是:草地较高的植物多样性,通过调节动物营养平衡、毒素稀释和口味调节等作用,显著地提高动物的采食量和营养物质摄入量,从而提高动物的生产性能;高植物多样性同时也降低动物进行食性选择的能力,增加采食选择的成本,对动物的采食选择带来了干扰;植物多样性对动物采食性能的作用依赖于这两个正、负作用间的权衡(trade-off)。同时,在最高植物多样性条件下,植物多样性对动物产生的正作用减弱,负作用增强,因此,植物多样性有利于提高草食动物的生产性能,但过高的多样性对动物采食的作用结果并不是最佳的。该模型的提出不仅有利于提高我们对植物多样性的生态系统功能的深入认识和理解,而且,对放牧家畜生产与草地管理方面也具有重要的理论价值。
(6)采用人工模拟放牧试验法,检验了不同植物(高、中、低偏食物种)的空间分布特征(聚集或随机分布)对绵羊采食选择影响。试验结果表明,植物空间分布特征显著地影响放牧动物的采食选择。草食动物对偏食物种的选择性采食结果依赖于偏食物种本身的空间分布、非偏食物种的空间分布,以及整个采食区域内食物资源的总体空间分布特征。偏食物种本身的空间分布对其采食的影响依赖于动物的采食策略,当偏食物种为聚集分布时,集中区域采食策略(area-concentrated foraging strategy)有利于动物的采食成功,而随机点取样策略(random-point-sampling strategy)不利于采食;相反,当偏食物种为随机分布时,随机点取样策略有利于其采食成功,而集中区域采食策略不利于采食;当整个食物资源的空间分布特征趋于聚集分布时,动物通常采取集中区域采食策略,而趋于随机分布时,动物则通常采取随机点取样策略。由此推论,当某一植物种群的空间分布模式与整个采食区域食物资源分布的空间特征相一致时,动物将更容易采食到该物种。此外,非偏食物种的聚集分布总是有利于动物寻找采食偏食物种,而草地低质非偏食物种的随机分散分布能够显著降低动物对偏食植物种的采食,从而更好地抑制草食动物的放牧选择性,有利于草地植物多样性保护,提高草地群落稳定性。
(7)通过人工模拟放牧试验,研究了植物空间关系对绵羊采食选择的影响,分析了植物对动物采食的联合防御效应(associational plant defense)。结果表明,不同模式的植物空间邻居关系显著影响动物对偏食物种以及所有植物物种的采食。当不同植物物种之间在多个斑块存在复杂的邻居关系时,偏食植物物种能够最有效地防御动物采食;当植物物种之间所构成的空间关系模式迫使草食动物仅在一个尺度下做采食决策时,植物通常不能够有效地防御动物对偏食物种的采食;误导或干扰动物进行食性选择的能力从而达到防御动物采食的目的是植物采取的一种极端空间防御策略,因为这种策略过度地限制了动物对所有植物的总采食量;而直接迫使动物被动地降低选择性是植物采取的一种折衷的、最佳的空间防御策略,这种防御策略不仅有助于维持草地植物物种多样性及其功能,同时有利于草地生态系统中动、植物的稳定共存。
本研究通过对大型草食动物在不同草地植物多样性背景下的采食行为响应、不同植物种群的空间分布模式和植物物种间的空间邻居关系对动物采食选择影响,以及动物体本身的采食调控规律等深入系列的研究,获得了对草地家畜采食行为的深入理解和进一步认识:放牧家畜具有极其复杂的选择性采食行为,这种复杂的采食行为是对草地复杂采食环境的适应进化结果,草食动物的适应性采食行为(adaptive foraging behaviour)是决定草地生态过程的关键因素,具有重要的生态意义;动物采食对草地的反馈作用结果强烈地依赖于草地植物本身的复杂结构,草地植物的复杂性与多样性是确保草地动、植物之间互惠与稳定共存的重要基础。本研究从动物采食行为的视角进一步证实了草地植物多样性在生态系统中的重要作用以及保护草地植物多样性的重要性。
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 grasslands
5) 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.
引文
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