Chapter 1 Introduction

Grasslands have long been utilized for cultivation and grazing. Recently, their value in maintaining biodiversity (wide array of taxa) has been recognized as well. Spatial heterogeneity is a major driver of species diversity. Most papers have reported a positive relationship between habitat heterogeneity and species diversity; for example, because more heterogeneity is equivalent to more niches, it allows more species to coexist. Thus, from a management viewpoint, optimization of heterogeneity has been acknowledged as an effective approach to maintaining high species richness, and consequent healthy ecological functioning.

Disturbance is the main factor generating spatial heterogeneity. The disturbance regime includes characteristics such as spatial distribution of the disturbance; disturbance frequency, return interval, and rotation period; and disturbance size, intensity, and severity. In the context of communities at the landscape scale, because the disturbance regime controls the proportion of the landscape at each succession stage, it is essential to consider the disturbance regime when examining the dynamics of spatial heterogeneity. According to the previous findings, biodiversity is maximized where multiple kinds, frequencies, severities, periodicities, sizes, shapes, and/or durations of disturbance occur concomitantly in a spatially and temporally distributed fashion. Seeking for a appropriate disturbance regime to maintain spatial heterogeneity is an effective conservation manner.

In Mongolian grasslands, Marmots modify the soil's physical properties and chemical properties through activities such as burrowing, grazing, and urinating, thereby affecting the distribution of plant species. Because marmots disturb grasslands heterogeneously in space, they can be a potentially important tool for conservation management. However because ground-dwelling rodents damage a range of agricultural crops and cause soil erosion around the world, managing rodents in arid or semi-arid grasslands should consider their negative influence as well. Because the effects of keystone species or ecosystem engineers on ecosystems at broad scale are context-dependent, a priori evaluation of the impact of marmots on communities at landscape scale is needed.

My final goal is therefore to evaluate the impact of marmot on Mongolian steppe at the landscape and local scales based on the theory of landscape ecology, thus providing an appropriate tool for the sustainable management of healthy Mongolian steppe. Specifically, by clarifying the effects of disturbance by marmots on species diversity (vegetation) and soil among landscape divisions (Chapter 2) and between land use divisions (Chapter 3), the broad-scale management options of land can be elucidated. Clarifying the effects of disturbance by marmots on spatial heterogeneity of vegetation and soil among the disturbance regimes at the colony scale (Chapter 4) will reveal management options using marmots at the local scale.

Chapter 2. The effects of disturbance by marmot on vegetation among landscapes

Can an a priori evaluation of the impact of keystone species (ecosystem engineers) reveal the general susceptibility of ecosystems to disturbance at a broad scale, thereby revealing vulnerable areas and allowing managers to optimize the performance of the keystone species? To answer the question, I surveyed plants growing on and off the mounds created by marmots (Marmota sibirica) among 14 landscape positions and within a single mountain slope in Hustai National Park (HNP) in a forested steppe region of Mongolia. Significant interactions between landscape position and soil disturbance by marmots were seen in forb volume. The impact of soil disturbance was low in mountain areas and high on depositional plains. Soil disturbance may have changed microenvironments from xeric to more humid or from moist to more xeric, depending on the other site characteristics. Marmots enhanced newly established species in the landscape positions close to the forested mountain or river. Collectively, my results indicate that sedimentation, preexisting water conditions, slope angle, all modify the relationships between soil disturbance and landscape position. My a priori evaluation of the influence of keystone engineer on ecosystems at a broad scale could provide insights into how to optimize the performance of ecosystem engineering in a way that is beneficial to ecosystem management.

Chapter 3. Effects of disturbance by marmots on vegetation and soil by land use

In the Mongolian steppes, livestock and rodents are the main habitat modifiers. Although grazing lands and rodent habitats overlap, the combined effects of disturbance by livestock and rodents on spatial heterogeneity have rarely been evaluated. I established a 50-m × 50-m plot in each of four sites: heavily grazed by livestock with and without marmot burrows, and ungrazed with and without marmot burrows. I subdivided each plot into 4-m2 quadrats and surveyed the plant composition and soil nutrient properties in each quadrat. The spatial heterogeneity of vegetation and soil nutrient properties were calculated by using non-metric multidimensional scaling analysis and the coefficient of variance among soil samples at three spatial scales. The spatial heterogeneity of the vegetation did not differ between grazed by livestock and ungrazed plots; it was higher under marmot disturbance than in the absence of marmots at a fine scale, but lower under marmot disturbance than in the absence of marmots at a coarse scale irrespective of livestock grazing. At a fine scale, unique habitats were formed in each combination of grazing presence/absence and marmot presence/absence. In addition, the species composition in the grazed plots was distinct from that in the ungrazed plots at the coarser scale. The occurrence of degraded plant species depended on the presence of grazing rather than on the additive effect of both habitat modifiers. Marmots enhanced newly established species only in the ungrazed plots. These results show that the ecological roles of livestock and marmots are not functionally substitutable, but are complementary. That is, livestock modified the whole vegetation composition, and thereby spatial heterogeneity at the landscape scale, while marmots modified spatial heterogeneity at the local scale.

Chapter 4 The effects of disturbance by marmots on spatial heterogeneity at local scale

4-1 The effects of marmot density on spatial heterogeneity

I examined the effects of disturbance by Siberian marmots on the spatial heterogeneity of vegetation at three spatial scales (fine, intermediate, coarse) in a Mongolian grassland. I established three 50 m × 50 m plots according to burrow density and another plot in an area with no burrows. Each plot was subdivided into 625 adjacent 4-m2 square quadrats and the plants and soil nutrient properties in them were surveyed. Spatial heterogeneity was calculated for vegetation using the mean dissimilarity of species composition among sample quadrats and geostatistical analysis was used to calculate soil properties. The off-colony plant community was dominated by graminoids, but a variety of plants, including graminoids, forbs, and shrubs, were present in the on-colony plots. The greater fine-scale heterogeneity in the high-density burrows plot reflected the presence of either disturbed or undisturbed patch within a single measurement unit, whereas the lower coarse-scale heterogeneity in the on-colony plots reflected the presence of both disturbed and undisturbed patches within a single measurement unit. My results demonstrate the possibility of using the marmot as a conservation tool in my study site as far as a plant biodiversity is targeted.

4-2 The effects of spatial configuration of burrows on spatial heterogeneity

Disturbance frequency is key factor responsible for changes in spatial heterogeneity. My objective was to examine how the configuration of disturbance by burrowing rodents affected the spatial heterogeneity of vegetation and soil nutrient properties through the spatial pattern of disturbance frequency. I established three 2500-m2 (50 m × 50 m) isolated-burrows plots and three 2500-m2 clustered-burrows plots in a Mongolian grassland. Each plot was subdivided into 4-m2 quadrats, and the plant species richness, percent coverage and soil nutrient properties in the quadrats were surveyed. Spatial heterogeneity was calculated for vegetation using the mean dissimilarity of species composition among sample quadrats and geostatistical analysis was used to calculate soil properties. Heterogeneous patches of plants such as Achnatherum splendens and higher nutrient concentrations were found only near the clustered burrows. As a result, spatial heterogeneities of vegetation and soil nutrient properties were higher in the clustered colony than those in the isolated colony. I considered that Achnatherum splendens may have grown near the clustered burrows because such structures captured a great amount of temporal water flows. Patches of higher soil nutrient concentrations can be attributed to locally concentrated excretions by the rodents and a reduction of uptake by plants in highly disturbed areas. I concluded that the configuration of disturbance patches affected the spatial heterogeneity at the local scale through the spatial pattern of disturbance frequency.

4-3. The effects of spatial grazing pattern of marmot on spatial heterogeneity

Disturbance size and intensity are key factors responsible for changes in spatial heterogeneity. If I consider that herbivore behavior will affect their disturbance regime (size and intensity), then it will also change the associated spatial heterogeneity. In this study, I examined whether the pattern of spatial disturbance created by a herbivore, the Siberian marmot (Marmota sibirica), affects the spatial heterogeneity of vegetation and soils at a colony scale in the Mongolian steppes. I estimated the pattern of spatial disturbance based on the spatial allocation of marmot burrows and feces and by direct observation of their grazing behavior in protected and unprotected areas. I then surveyed plant communities and soil nutrients in these areas to assess the spatial heterogeneity. I found that disturbance was more concentrated near marmot burrows in the unprotected area, where the degrees of spatial heterogeneity of vegetation and soil nitrate-nitrogen were greater than in the protected area, where disturbance was more widely distributed. These results indicate that the spatial pattern of disturbance by herbivores affects the spatial heterogeneity of vegetation and soils through changes in the disturbance regime. This finding also supports the hypothesis that the intensity of disturbance is more important than its size in determining community structure in the Mongolian steppes, thereby providing empirical support for the importance of considering the behavior (spatial grazing pattern) of herbivores in future grazing studies.

Chapter 5. Synthesis and application

The combined results supported the following generalizations on the effects of disturbance by marmots on the Mongolian steppe.

At the landscape scale, the impact of disturbance by marmots on plant community structure differs more among landscape divisions than between land use divisions. Marmots enhanced plant species diversity at sites close to seed sources (e.g., forested mountains and rivers) or under low livestock grazing intensities, but not at sites far from seed sources or under heavy grazing intensities. Depositional plains showed some evidence of degradation by disturbance.

Marmot disturbance allowed, without loss of plant abundance, the persistence of fugitive forb species, resulted in increase of species richness and spatial heterogeneity of vegetation at fine grain size. However, the effect of disturbance by marmots on spatial heterogeneity of vegetation was scale-dependent; decreased at coarse grain size. Marmot also created the high soil nutrient concentration near their burrows by a combination of high deposition of dung and urine and modification of soil physical structures, resulted in increase of spatial heterogeneity of soil NO3-N at local scale.

At the local scale, spatial heterogeneity was increased when the plot included many burrows, clustered burrows, or marmots with a small activity range. Clustered marmot burrows especially yielded higher spatial heterogeneity, but high population densities caused marked land deterioration.

The density of herbivore, the configuration of their disturbance patches and spatial grazing pattern of herbivores affected the spatial heterogeneity through change in the disturbance regime. The frequency of disturbance is important in determining the spatial heterogeneity at the local scale.

On the basis of these generalizations, I consider that the following goals would be useful for managing ecosystems sustainably through the use of marmots in Mongolian steppes: (1) When the conservation target is ecosystem engineers, a priori evaluation of the influence of the ecosystem engineers on the ecosystems at broad scale could provide insights into how to optimize the performance of ecosystem engineering in a way that is beneficial to ecosystem management. (2) Because the landscapes and land use divisions can be classified clearly into those that received only positive influences and those that received only negative influence from the marmot disturbance, zoning becomes more meaningful. (3) When the aim is enhancement of plant species diversity by marmots, landscape positions located close to seed sources or areas under low livestock grazing may become priority marmot conservation areas. (4) Because landscape positions located far from seed sources or landscape positions under heavy livestock grazing exhibited low enhancement of species diversity by marmots, and actually depositional plains showed some evidence of degradation, these landscape positions cab be used for sustainable use of marmots. (5) When the aim is to maintain spatial heterogeneity at the local scale, clustered marmot burrows can be conserved, but density must be limited. (6) When the aim is to maintain biodiversity at the local scale, a higher level of spatial heterogeneity in plots with frequently disturbed but patchily distributed small disturbances (i.e., heterogeneous disturbance regime) is appropriate.

Further experimental research is needed to determine guidelines for managing marmots in the Mongolian steppes.

長い間草原は耕作や放牧など生産の場として利用されてきた。近年、草原は生物多様性に考慮した利用と保護の重要性が認識されるようになってきた。局所スケールの種多様性に影響を及ぼす主な環境要因は空間的異質性、生産性、および撹乱である。空間的異質性は種多様性と正の相関があるため、生物多様性保全の観点から、空間的異質性の維持された景観を管理するアプローチの有効性が高まってきた。

撹乱は空間的異質性を創出する主要因である。撹乱体制は景観内の各遷移段階の割合を変化させるので、撹乱後の空間的異質性を予測するためには撹乱体制を理解することが必要である。撹乱は空間的異質性を創出する主要因である。撹乱体制は景観内の各遷移段階の割合を変化させるので、撹乱後の空間的異質性を予測するためには撹乱体制を理解することが必要である。

モンゴル草原は、イネ科を中心とした比較的群落の単純な草原で、大型げっ歯類シベリアマーモットの巣穴周辺に裸地や双子葉類などの異質な群落が出現している。マーモットは、物理的・化学的に局所的な撹乱を通して巣穴を中心とした群落、土壌を改変する。あるスケールの種多様性の保全を目的とする場合、そのスケール内の空間的異質性を維持することができる撹乱体制を管理する必要がある。そこで、モンゴルでマーモットによる撹乱が果たしてきた役割を評価することで、草原の生態系管理の手法につながると考えられる。一方、特に半乾燥、乾燥草地では草食動物の撹乱による土地劣化が危惧されているため、土地劣化を抑えた上で種多様性を維持できるような管理が目標であると考えられる。マーモットのようなエコシステムエンジニアによる生態系への影響は、広域スケールで空間依存的であるため、その影響を広域スケールで事前に評価しておく必要がある。また、撹乱が空間的異質性に与える影響は空間スケール依存であるため、複数スケールによる評価が必要である。

以上から、景観生態学の理論(空間的異質性、撹乱体制)に基づきマーモットによるモンゴル草原生態系への影響を複数スケールで評価する。その結果に基づき、モンゴル草原の持続的利用に向けた管理手法としてのマーモットの撹乱の有効性を検討した。

2章では、マーモットによる生態系への影響を植生の種構成と多様度の観点から地形単位で評価することを目的とし、マーモットの撹乱による各地形単位の植生への影響を調べた。3章では、マーモットによる生態系への影響を、植生の種構成と土壌の空間的異質性の観点から土地利用区分(放牧)で評価することを目的とし、マーモットの撹乱による各土地利用区分の植生と土壌への影響を調べた。4章ではマーモットによる様々な空間利用パターン(密度、巣穴の空間配置、個体の撹乱範囲)を植物と土壌栄養塩の空間的異質性の点から評価することを目的とし、局所スケールでのマーモットによる撹乱が植生と土壌の空間的異質性に与える影響を調べた。

これらの結果から、局所スケールでのマーモットの撹乱の影響をまとめると、(1)植生への影響は地形区分間で大きく異なり、森林や河川に隣接した地形単位もしくは軽牧地では広域スケールの種多様性を高める。一方で、(2)それらのシードソースから離れた地形単位や重牧地ではその効果は低下する。(3)低平地では劣化指標種が増加する。

また、広域スケールでのマーモットの撹乱の影響をまとめると、(1)集中した巣穴では空間的異質性の増加が最も期待でき、高密度になると劣化指標種が増加する。(2)イネ科の優占する草原内に、撹乱依存的な双子葉類を空間内に共存させることで、特にfine スケールの空間的異質性を増加させる。(3) 撹乱による植生の空間的異質性への影響は空間スケール依存であり、coarse scaleでは低下する。(4)糞尿や土壌構造の改変等によって巣穴近くに高濃度の土壌栄養塩類パッチを形成する。その結果、土壌の空間的異質性を増加させる。(5)巣穴が高密度、集中分布、マーモットによる撹乱が局所的である場合、空間的異質性が高くなる。(6)モンゴル草原では、撹乱頻度は空間的異質性を決定する重要な要因である。

5章 では、マーモットによる撹乱の影響を負の面からも評価することで管理へ結びつけることを目的とした。この結果より、生態系への影響を加味した上で保護区と狩猟区に分割する方法(ゾーニング)が有効であると考えられる。また、空間内に撹乱頻度の高低を与えることで、土地劣化を抑えつつ高い空間的異質性を維持することができると考えられる。

以上要するに,本論は,モンゴル草原においてげっ歯類による撹乱の影響を局所スケールから広域スケールまでマルチスケールで評価し,さらには撹乱体制と植生や土壌の空間的異質性の関係を明らかにするなど,基礎生態学として高い学術性を持っている。さらには,現在のげっ歯類の管理計画に対する改善点も指摘しており,本研究の今後の展開により,その成果は実際のモンゴル草原の持続的管理に応用可能である。本研究は,乾燥草原生態系の保全管理にふさわしい野生動物の管理計画の構築に向けてその端緒を拓いたといえ,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