地上地下食草动物对高寒草甸植物多样性的影响和作用机制

Most of our understanding of what controls plant species coexistence and limits diversity focusses on either aboveground light limitation or belowground nutrient competition, and factors such as herbivory or soil disturbance that switches or modify the degree of competition. Both large aboveground grazers and small belowground herbivores are common in most grassland, and we hypothesize that both are essential to maintain high plant diversity and a sustainable grazing system. Aboveground grazers preferentially consume grasses causing a vegetation shift to unpalatable forbs with large root systems. These forbs are superior nutrient competitors. Belowground grazers preferentially consume forbs with storage organs and cause an increase in grasses. These grasses are superior light competitors. The presence of both aboveground and belowground herbivory is essential to prevent dominance by either grasses or forbs. We challenge the notion that plant diversity in any specific grassland is controlled either by light or nutrient competition. We propose that the absence of different herbivore groups can cause functional plant group changes that can lead to either light or nutrient competition. Only a balance in plant functional groups leads to the high plant diversity observed in Tibetan grasslands. And because of the dynamic nature of grasslands, the size and scale of plant and animals, we can conduct experiments over meaningful scales and time frames to experimentally examine what controls plant diversity. We hypothesize that this functional plant group balance not only prevents qualitative changes in forage quality, but also quantitative changes. Intense aboveground and belowground herbivory drive fast nutrient cycling, no litter buildup and thereby high NPP that can sustain intense grazing. The elimination of either herbivore functional group leads to a drastic ecosystem change towards a litter-dominated nutrient cycling, with lowering nutrient cycling, lower NPP and the collapse of the grazing system. Maintaining plant diversity and sustainable livestock grazing are critically important for the livelihood of pastoralists who dominate the Tibetan Plateau. More broadly, belowground herbivores, such as zokors, gophers and prairie dogs, are often targeted with eradication programs, because of the belief that they consume NPP that would otherwise be available for livestock. However, rodent eradication might have the opposite effect, lowering NPP, desirable forage species, and plant diversity. Outreach from this project will inform land management on how different grazing functional groups influence NPP and vegetation, and potentially cause feedbacks that can have unintended consequences.

食草动物能否增加草地植物多样性问题备受争议，大多数研究仅考虑地上食草动物的影响，忽略地下食草动物的影响，且对其影响作用过程和机制尚不明确。本项目以青藏高原东部高寒草甸为研究对象，通过地上地下食草动物去除实验，系统研究地上/地下食草动物对高寒草甸植物群落特征、微生境特征、土壤特征、无性系更新（无性系分株）特征和有性更新（种子实生幼苗）特征的影响；重点分析植物功能群优势度与植物多样性和生产力的关系，以及微生境特征（土壤温度和水分、光照强度）、土壤特征和植物多样性的关系。同时通过人工去除不同植物功能群模拟地上地下食草动物的采食实验进一步检验上述影响过程和关系，从而揭示地上地下食草动物对高寒草甸植物多样性影响过程和作用机理。通过以上研究，不仅为"过牧和鼠害是引起青藏高原草地退化的主要因素"的说法提供了国际认可科学证据，更重要地是为青藏高原植物多样性的维持和可持续放牧提供科学依据。

在全球范围内，人类活动和气候变化导致动物种群数量持续下降，进而食草动物对植物的影响也发生了巨大的变化。食草动物对草地植物多样性问题备受争议，大多数研究仅考虑地上食草动物的影响，忽略地下食草动物的影响，且对其影响作用过程和机制尚不明确。本项目通过在高寒草甸布设地上高原牦牛和地下高原鼢鼠隔离定位实验，旨在揭示地上家畜与地下高原鼢鼠干扰对植物多样性的影响；阐述啮齿类动物在高寒草甸生态系统中的“关键种”作用；分析高原鼢鼠与家畜是否存在对草地资源的竞争关系。.经过四年的研究取得了以下主要结论：.第一，无论是物种丰富度，还是香浓维纳多样性以及均匀度指数均不响应地上高原牦牛和地下高原鼢鼠的短期排除，然而，随着高原鼢鼠的干扰强度的增加，物种丰富度，香浓维纳指数以及均匀度指数均呈现出单峰模式，这支持了中度干扰假说。.第二，高原牦牛的短期排除能显著地增加地上植物生物量，而高原鼢鼠的短期排除则降低了地下植物根系生物量；随着高原鼢鼠土丘盖度的增加，地上植物生物量呈现显著下降的趋势，而地下生物量却表现为显著增加的趋势。这说明了尽管高原鼢鼠的干扰会降低地上植物生物量，但它们的干扰有利于刺激高寒草甸植物根系的生长，这样相对于对地上植物生物量的负影响而言，它们对植物地下根生物量和总生物量有显著的正效应。.第三，土丘盖度，透光率，土壤水分，土壤有机碳与全氮植物地下根生物量以及植物根冠比与物种丰富度和香浓维纳多样性有显著地正相关关系，而植物地上生物量，土壤容重以及植物群落高度呈显著地负相关关系。这些说明高原牦牛的地上干扰和高原鼢鼠的造丘活动直接和间接影响植物群落的透光率，而高原鼢鼠的地下活动直接影响植物的根系生物量，进而影响植物对土壤养分的吸收利用，从而通过调节地上光资源和地下水分和养分资源影响植物多样性的维持。