Effects And The Underlying Mechanisms Of Grazing On Plant Diversity And Productivity In A Songnen Grassland

Herbivores strongly influence plant communities in grassland ecosystem, often changing vegetation composition, species diversity, and productivity. Therefore, such herbivore-mediated processes are inextricably linked to the issue of how herbivores influence ecosystem function and stability. Despite the complexity generated by plant-herbivore interactions, plant diversity should still act as a buffer against these interactive effects and regulate ecosystem responses to herbivore grazing disturbance.Additionally, the composition and traits of plants and herbivores could influence how herbivores affect plant communities. Understanding the interactive effects of plant and herbivore characteristics on plant community dynamics is of fundamental and practical importance. Although the effects of different herbivores and plant species on plant community characteristics have been analyzed separately, their effects in nature may be interactive. Whether plant diversity alters the effects of different-sized herbivores has never been tested. Therefore, we contrasted the impacts of sheep and cattle grazing separately and in mixed herds in Songnen grasslands that differ in plant diversity level.Our aim was to determine patterns of potentially interactive relationships between large herbivore grazing and plant diversity at a community level that can provide insights for sustainable management of grazed grasslands. This research has potential importance in greatly enhancing our knowledge of the effects of grazing on plant communities in grassland ecosystem. Moreover, understanding plant-herbivore interactions is essential for maintenance of grassland biodiversity, and sustaining the stability of grazing ecosystems,as well as effectively managing the grasslands.The study was conducted in meadow steppe where lies in the eastern region of the Eurasian Steppe Zone. We adopted a series of manipulated grazing experiments, in which the herbivore assemblages and plant species diversity context were rigorously controlled.We finally obtained the important results and conclusions as follows:(1) Based on the body sizes and foraging strategies, large herbivore have evolved different requirements of the plant resources. When herbivore grazing on different plant species, their impacts on plant communities were significantly different. Moreover, the impacts of large herbivores on plant species diversity were closely relatived with plant diversity levels. Plant diversity could mediated foraging behavior of herbivore. When herbivore grazing at different plant diversity grasslands, their foraging selective patterns would changed and leded to difference effects on plant communites. Mixed grazing by large herbivore increased plant diversity at both low and high diversity grasslands, but theircombined effects were different. Mixed grazing by different foraging selectivity herbivores can be expected to operate through potential additive or compensatory effects at different plant diversity levels. At low plant diversity grassland, diet overlap is high and the presence of cattle and sheep can lead to overutilization of dominant grasses, and underutilization of other species. Alternatively, at high plant divetsity grassland, cattle and sheep partition the available resources, it can lead to a more even usage across all plant species compared to herbivory by a single species. Therefore, both diversity of the plant species and the degree of complementarity in diet between herbivores had important influences on the mixed grazing impacts. Our data suggest that multiple-species mixed grazing is more advantageous to maintain or improve plant species diversity in grassland.The loss of species diversity, to a certain extent, can be related to the decrease of wild and domestic herbivore species in most natural and managed grasslands.(2) There were significantly different impacts of sheep and cattle grazing separately and in mixed herds on plant productivity in grasslands that differ in plant diversity. The foraging patterns of cattle and sheep were main determining factors to influencing herbivore grazing effects. At low diversity grassland, cattle significantly decreased plant above-ground biomass, but there was no significant impact on plant biomass from sheep grazing. Compared with cattle grazing, mixed grazing led to accentuated effects on dominant grasses at low plant diversity conditions. The impact of mixed grazing on plant above-ground biomass can be expected to operate through potential additive effects on dominant species. Generally, plant community dynamics can depend much more on changes of dominant species at low plant diversity levels. Here we show that cattle had more crucial effects on plant productivity of dominant species than expected at the low plant diversity level, with 20% reduction in plant biomass induced by cattle. Cattle are a less selective herbivore and can tolerate low plant nutrient content, which results in greater foraging on grasses. At low plant diversity grassland, both cattle and sheep tended to consume similar plant species, and feed more on dominant species such as Leymus chinensis and Phragmites australis. However, higher plant diversity reduces herbivore selectivity and promotes more uniform use of different plant species. Both grazing by a single species or mixed grazing by large herbivore had insignificant effects on the plant above-ground biomass at high plant diversity grassland. Therefore, our data suggest that multiple-species mixed grazing regimes in grassland systems with high plant diversity could represent the optimal protocol for grazing management, which can be beneficial to improve plant species diversity and to maintain plant communities with high productivity.(3) Grazing by cattle or sheep alone have similar effects on plant communities structure and Whittaker’s similarity index at both low and high plant diversity grassland.There were significantly different impacts of sheep and cattle grazing separately and in mixed herds on plant community structure. The effects of herbivores grazing on plant community structure and composition can largely mediated by plant diversity levels.Firstly, plant community dynamics can depend much more on changes in dominant species at low plant diversity levels. Thus, the Whittaker’s similarity index of plant community was higher at low plant diversity than at high diversity grassland. With rich species composition at high diversity grassland, relatively strong competition between plants species promoted low relatively proportion of each species. Community composition was easy to be changed. Secondly, plant diversity levels were the main determining factors to influencing herbivore grazing effects on plant community structure. Larger herbivores mainly influenced dominant species with high productivity or plant height at low plant diversity context. Thus, larger herbivore induced significant changes in structure of plant community. At high plant diversity grassland, the complicated relationship between plant species and herbivore foraging selective patterns have a joint effect. Large herbivores tended to be foraging diverse plant species. Variety of plant species can quickly complement the ecological space after grazing. Due to a wider and uniform use of plant species, large herbivores have no significant effects on plant community structure. Finally,the combined effects of mixed grazing by different large herbivores on plant community that was not as same as simple superposed each species effect. Mixed grazing increased plant density at high plant diversity grassland. Due to mixed grazing promoting the compensation growth of plant community, more seedling emerge increased the plant number per unit area of plant community. Without affecting the plant community structure,mixed grazing improved the density of plant community, plant community reproduction,and speeded up the material circulation and energy flow at high plant diversity level.Accordingly, herbivore grazing might contribute more to the maintenance of grassland structure and ecosystem functioning under high plant diversity compared to low plant diversity.(4) Both theoretical and empirical studies have emphasized that consumers regulated plant community structure and diversity-productivity relationships of many ecosystems.Indeed, our results also indicated that grazing by large herbivores mediated such relationship through reducing plant above-ground biomass with increasing plant species richness at sites of low plant diversity. Large herbivores are implicated in accelerating expansion processes of forbs and legumes by consuming dominant grasses, and thereby releasing subordinate species from competition. The relative biomass of grasses declined by 38%, but increased by 33% for forbs in cattle grazing treatment. While herbivore effects on plant biomass should be weakened by high plant diversity. A decline in the effect ofherbivores on above-ground biomass with increasing species richness of plant communities,with 14% of the variance in plant biomass response to mixed grazing could be attributed to68% of the variance in plant species richness at high plant diversity sites. Correspondingly,plant community productivity tended to increase with increasing plant density. Thus, our study suggested that large herbivores can modify or even reverse the directionality of diversity-productivity relationships, and these relationships tended to be negative at sites of low plant diversity but positive at sites of high plant diversity.Based on the study about the interactive effects between plant diversity levels and herbivore assemblages on plant communities, we obtain more understanding and cognition of the interactions among adjacent trophic diversity. This study suggests that the diversity of large herbivores and plants may mediate the effects of herbivore grazing in grasslands.Different large herbivores may exert distinctive(based on body size and foraging strategies), interactive effects to influence grassland plant communities. Their effects,however, were not consistent at different plant diversity levels. Herbivore grazing under high plant diversity might contribute more to the maintenance of grassland structure and ecosystem function than under low plant diversity. Furthermore, this study indicates that multiple-species mixed grazing regime in grassland ecosystems with high plant diversity could be the optimal grazing management mode. Such comprehensive interactions among adjacent trophic levels may be more prevalent and complex than previously thought, and may require attention at multiple spatial-temporal scales. There is a need to integrate the behavioural ecology of grazing animals with that of plant community composition and the impacts of their interactions on other trophic levels in the regional-scale grassland ecosystems. From a conservation perspective, maintaining species diversity of plants and herbivores should be a high priority objective of grazing or grassland management in the eastern Eurasian steppe.