1.Introduction

Predicting the impacts of livestock grazing on natural communities has become a major concern in rangeland ecology research, especially where grazing is widespread and its impacts may be in conflict with the sustainable use of natural resources and biodiversity conservation. Major advances have been made in revealing the patterns and processes of vegetation changes associated with grazing, generalizing these patterns within a given landscape or region, and reassessing the appropriate paradigm to describe vegetation dynamics. There is uncertainty, however, as to whether the current knowledge is the key to successful environmental management of rangeland ecosystems. The central aims of this study are to link ecological theories and concepts with land management to promote species diversity, functional diversity, and the sustainable use of natural resources and to develop rigorous ecological benchmarks across Mongolian rangeland ecosystems that would forewarn land managers that actions must be taken to stabilize resilience and minimize the probability of crossing an ecological threshold.

2.Methods

Grazing gradients provide an ideal tool for examining vegetation responses to a realistic range of long-term grazing impacts. Livestock density and grazing intensity are usually highest close to livestock camps or sources of water. Many rangeland studies have sampled vegetation and soils along grazing gradients based on distance from a source of water, or from human settlements or livestock camps to determine the grazing impacts on rangeland structure and function. Throughout this study, I therefore used a grazing gradient approach to test various ecological theories and concepts, and to apply the results to land management. I also attempted to generalize our insights into ecological theories and management application by establishing grazing gradients at multiple ecological sites that are defined as a distinctive kind of land with specific physical characteristics that differs from other kinds of land in its ability to produce a distinctive kind and amount of vegetation. Three study areas were situated near Kherlen Bayan Ulaan (47°12'N, 108°44'E), Mandalgobi (45°46'N, 106°16'E), and Bulgan (44°05'N, 103°32'E) in Mongolia's Khenti, Dundgobi, and South Gobi provinces, respectively. Across the three study area, ten ecological sites were located at different landscape positions in grassland, shrubland, or halophytic shrubland vegetation physiognomies.

3.Threshold changes in vegetation along a grazing gradient in Mongolian rangelands

The concept of threshold has become important in ecology, but the nature of potential threshold responses of vegetation to grazing in rangeland ecosystems remains poorly understood. I aimed to identify ecological thresholds in vegetation changes along a grazing gradient and to examine whether threshold changes were expressed similarly at a range of ecological sites. To accomplish this, I surveyed the vegetation along grazing gradients at 10 ecological sites, each located at different landscape positions in Mongolia's central and southern rangelands. Evidence for a threshold in changes in floristic composition along the grazing gradient was examined by comparing linear models of the data with nonlinear models fitted using an exponential curve, an inverse curve, a piecewise regression, and a sigmoid logistic curve. Three nonlinear models (piecewise, exponential, and sigmoid) provided a much better fit to the data than the linear models, highlighting the presence of a discontinuity in vegetation changes along the grazing gradient. The shapes of the best-fit models and their fit to the data were generally similar across sites, indicating that the changes in floristic composition were relatively constant below a threshold level of grazing, after which the curve changed sharply. Except for two sites, the best-fit models had relatively narrow bootstrap confidence intervals (95% CI), especially around threshold points or zones where the rate of change accelerated, emphasizing that the results were robust and conclusive. The present results provided strong evidence for the existence of ecological thresholds in vegetation change along a grazing gradient across all ecological sites. This suggests that vegetation responses to grazing in the study areas are essentially nonlinear. The recognition that real threshold changes exist in real grazing gradients will help land managers to prevent the occurrence of undesirable states and promote the occurrence of desirable states, and will therefore permit a major step forward in the sustainable management of rangeland ecosystems.

4.Management applicability of the intermediate disturbance hypothesis across Mongolian rangeland ecosystems

The current growing body of evidence for diversity-disturbance relationships suggests that the peaked pattern predicted by the intermediate disturbance hypothesis (IDH) may not be the rule. Even if ecologists could quantify the diversity-disturbance relationship consistent with the IDH, the applicability of the IDH to land management has rarely been addressed. I examined two hypotheses related to the generality and management applicability of the IDH to Mongolian rangeland ecosystems: that the diversity-disturbance relationship varies as a function of landscape condition and that some intermediate scales of grazing can play an important role in terms of sustainable rangeland management through a grazing gradient approach. I quantified the landscape condition of each ecological site using an ordination technique and determined two types of landscape conditions, relatively benign and harsh environmental conditions. At the ecological sites characterized by relatively benign environmental conditions, diversity-disturbance relationships were generally consistent with the IDH and maximum diversity was observed at some intermediate distance from the source of the grazing gradient. In contrast, the IDH was not supported at most but not all sites characterized by relatively harsh environmental conditions. The intermediate levels of grazing were generally located below the ecological threshold representing the points or zones at which disturbance should be limited to prevent drastic changes in ecological conditions, suggesting that there is little "conundrum" with regard to intermediate disturbance in the studied systems in terms of land management. I suggest that the landscape condition is one of the primary factors that cause inconsistencies in diversity-disturbance relationships. The ecological threshold can extend its utility in rangeland management because it also has the compatibility with the maintenance of species diversity. The present results thus suggest that some intermediate scales of grazing and ecological thresholds are mutually supportive tools for sustainable management of Mongolian rangelands.

5.Two-phase functional redundancy in plant communities along a grazing gradient in Mongolian rangelands

The concept of functional redundancy is at the core of theory relating changes in ecosystem functioning to species loss. However, few empirical studies have investigated the strength and form of the relationship between species and functional diversity (i.e., the presence of functional redundancy in ecological communities) in this context. In particular, ecologists know little about how local extinctions in real communities might impact functional diversity. Here, I examined the relationship between species and functional diversity in plant communities along a grazing gradient across Mongolian rangeland ecosystems. I applied a recently described measure of functional diversity that incorporates species' dissimilarities defined from plant functional traits and tested several hypothesized forms of the relationship between species and functional diversity using linear and nonlinear modeling techniques. I found a significant sigmoid logistic relationship between species richness and functional diversity in relatively benign environmental conditions. This indicates high functional redundancy at low levels of species richness followed by a rapid increase at intermediate levels, until functional diversity reaches an asymptote at high levels (i.e., two-phase functional redundancy). In contrast, I generally observed a positive linear relationship between these parameters in relatively harsh environmental conditions, indicating low functional redundancy. Observed functional redundancy probably resulted from two factors, intrinsic redundancy in species' functional traits and extrinsic redundancy caused by nonrandom compositional change that is nonrandom with respect to functional traits. Lack of either intrinsic or extrinsic redundancy may result in low functional redundancy. Two-phase functional redundancy suggests that functional traits are abruptly lost from a community below a certain level of species richness, and a community then shifts into a contrasting state that has a few limited functional groups characterized by disturbance-resistant traits, as a consequence of disturbances such as livestock grazing. The present results represent a major step forward in predicting the consequences of livestock grazing on the functioning of Mongolian rangeland ecosystems.

6.Ecological benchmarks underpinned by ecological theories: toward sustainable management of Mongolian rangeland ecosystems

The development and application of operational procedures for sustainable management of rangeland ecosystems need to be linked closely with a broader body of ecological theories. However, the current state-of-the-art in rangeland management is still disconnected from nonequilibrium concepts in ecological theory. In particular, the procedures do not fully account for the potential nonlinearity of vegetation responses to grazing, or that some aspects of vegetation dynamics might be driven by abiotic factors. The goal of this chapter is to identify an ecological benchmark that is able to link ecological theories and concepts of rangeland ecosystems with practical management applications. I focused responses to grazing by individual species and functional groups in relation to ecological thresholds with repeated measures from the same ecological sites to account for rainfall variability across Mongolian rangeland ecosystems. In all ecological sites, even under fluctuations in rainfall, there were robust combinations of indicator species that could be used to forewarn managers that actions must be taken to minimize the probability of crossing an ecological threshold. Depending on the landscape condition of each site, functional groups that share the traits of perennial life history, grass or forb growth form, linear leaf shape, and alternate leaf attachment or the functional group of woody shrubs drastically decreased prior to the crossing of an ecological threshold. Thus, across Mongolian rangeland ecosystems the responses of certain functional groups to grazing appear to predict the crossing of an ecological threshold. However, these responses of functional groups sometimes masked important signs of species' responses prior to the threshold being reached. Therefore, at an ecological site scale it would be better to assess rangeland states in relation to ecological thresholds based on individual plant species' responses to grazing. Nonetheless, the ecological benchmarks derived in this chapter can improve a land manager's ability to prevent adverse changes in states prior to the threshold being reached.

7.Conclusions

First, this study provides strong evidence of the existence of an ecological threshold in vegetation changes along a grazing gradient across all ecological sites. Second, my findings show that the relative importance of grazing in creating species diversity varies as a function of landscape condition. The study also demonstrates that some intermediate levels of grazing can play an important role in sustainable rangeland management by contrasting with an ecological threshold which has an explicit potential to be used in rangeland management. The utility of the ecological threshold can be extended to rangeland management because it is compatible with the maintenance of species diversity. Thus, some intermediate scales of grazing and ecological thresholds are mutually supportive tools for the sustainable management of Mongolian rangeland ecosystems. Third, this study reveals a relationship between species diversity and functional diversity in plant communities along a grazing gradient across Mongolian rangeland ecosystems and provides strong evidence for the existence of two-phase functional redundancy in plant communities along grazing gradients. This complements the former finding that some intermediate scales of grazing can play an important role in the maintenance of species diversity. Disturbance by domestic livestock grazing may allow for the retention of functional complementarities and possibly ecosystem functioning on Mongolian rangelands. Finally, I described rigorous ecological benchmarks that link important ecological theories and concepts in rangeland ecosystems with practical management applications. These ecological benchmarks can improve a land manager's ability to prevent adverse state changes prior to the ecological threshold being reached and provide practical management strategies for promoting both species diversity and possibly ecosystem functioning and the sustainable use of Mongolian rangeland ecosystems.

放牧地生態学の役割は、放牧地管理および生物多様性保全の観点から家畜放牧が自然環境とその変容に与える影響を予測することにある。しかし、放牧地生態系を捉える考え方は二極化しており、理論に基づく考え方に厳密にとらわれ過ぎることで、現実の系に即した管理への応用が十分に達成されていないのが現状である。また、放牧などの撹乱による種の共存に支えられる種多様性の高い群集が土地管理の上でも重要とは限らないため、放牧地の持続的利用と生物多様性保全の両立は困難となる場合が考えられる。

申請者が研究の対象としたモンゴルの放牧地生態系では、社会主義体制の崩壊により、放牧圧を適切に分散させていたと考えられる土地利用コントロールシステムが消失した。それに伴って、放牧圧の局所集中が顕在化するようになり、実際に土地荒廃が引き起こされている。放牧地生態系において、放牧は主要な撹乱要因であるため、このような土地利用の変化は系全体における撹乱体制の変化につながる。

このような撹乱体制の変化に伴う土地荒廃を予防する手段として、適切な管理基準の確立が強く求められている。さらに、生態系の頑健性を維持しつつ持続的な利用を行っていくためには、導かれる管理基準は生物多様性の保全を包含した基準である必要がある。

そこで申請者は、放牧地生態系における種多様性および機能的多様性の維持かつ持続的土地利用を可能にする頑健な生態学的基準を提供するため、生態学の理論体系を放牧地管理に結び付け、その上で予測的価値を持った生態学的管理基準を抽出すること、を大きな目的として研究を行った。本研究は、第三章から第六章の各論から成り立っており、以下に結果の要約とその総合的な評価を記す。

まず、第三章では、放牧傾度に沿った種組成の変化に生態学的閾値が存在することを明示した。気候条件、景観位置、植生タイプ、放牧家畜の種類がサイト間で異なるにもかかわらず、一貫して植生の変化に不連続性があることが見出され、研究地域における植生の放牧に対する応答が本質的に非線形であることが明らかとなった。

第四章では、モンゴルの放牧地生態系における中規模撹乱仮説(IDH)の一般性および管理への適用性を検証した。相対的に緩やかな環境条件下では、種多様性と撹乱の関係はIDHの予測に一致し、放牧傾度上の中間距離で種多様性が最大となった。一方、厳しい環境条件下では、IDHは支持されなかった。これらから、種多様性と撹乱の関係は各景観位置の環境条件に依存することが明らかとなった。中規模の撹乱レベルは、概して生態学的閾値を越えないレベルに位置づけられ、放牧による中規模撹乱が種多様性維持の観点からだけでなく土地管理においても重要な役割を持つことが示された。本章の結果により、中規模撹乱と生態学的閾値の概念がこの系での持続的な土地管理における相互補完的なツールであることが示唆された。

第五章では、放牧傾度に沿った植物群集における種多様性と機能的多様性の関係を明らかにした。緩やかな環境条件下では、種数と機能的多様性の関係はS字型のロジスティック曲線で描かれた。これは、低い種数レベルにおける機能的冗長性から、徐々に種が増えるにつれて急激に機能的多様性が増加し、高い種数レベルでまた機能的多様性が飽和するという二段階の機能的冗長性を表す。逆に厳しい環境条件下では、種数と機能的多様性の関係は正の線形関係となり、機能的冗長性が低いことが示された。本章から得られた二段階の機能的冗長性は、放牧などによる撹乱の結果、群集がある一定の種数を下回ると急激に機能的特性が失われ、限られた機能群しかもたない対照的な状態へシフトすることを示唆するものといえる。

最後に、第六章では、第三章から五章にかけて得られた知見を実践的な土地管理に応用するために、第三章で得られた生態学的閾値をベースラインとして予測的価値の高い生態学的基準を抽出することを試みた。各生態的特性群および各植物種の閾値近辺の挙動の解析を組み合わせることで、持続的土地管理に直接結びつく生態学的基準を明らかにした。得られた基準を軸として管理論を構築し、第三章から第五章で得た知見を持続的土地管理の観点から統合した。

以上のように、本研究は、独自の視点から生態学理論を絶妙に組み込んだ仮説を基に、モンゴルの放牧地生態系を例として生物多様性の維持と持続的土地利用が両立することを実証することに成功した極めて独創的な研究である。また、放牧地利用における生態学的管理基準の抽出は生態学的観点から見た放牧地管理への提言に直接繋がるため、とくに今後土地荒廃の危険性が指摘されているモンゴルにおいては、その社会的貢献は極めて大きい。よって、審査委員一同は、博士(農学)の学位を与えるに十分値する論文であると判断した。