A Study On Effects Of Species Coexistence Mechanisms On Diversity-stability Relationships

Grassland is the biggest terrestrial ecosystem type on the Earth’s surface,with essential production and ecological functions.However,under the combined influence of overgrazing and global changes（such as climate warming and nitrogen deposition,etc.）,the biodiversity and stability of grassland ecosystems have significantly been impaired,which directly threatens the living environment of human beings and the sustainable development of the social economy.In order to better understand the causes and consequences of the grassland biodiversity and stability loss,it is of great scientific significance to study the relationship between biodiversity and ecosystem stability.A long-standing unsolved classical challenge in ecology is whether and how biodiversity determines ecosystem stability.Many theoretical and grassland experimental studies have found that diversity begets community stability,but the underlying mechanisms remain very controversial.In essence,community stability,as a measure of the temporal invariability of community productivity（or broader ecosystem functions）,should ultimately emerge from the complex interactions of locally coexisting species in communities.Therefore,to gain a deeper understanding of the underlying mechanisms for the positive diversity-stability relationships,it is essential to develop a theoretical framework to bridge diversity-stability relationships and species coexistence.In this thesis,I use theoretical analyses and numerical simulations with the Lotka-Volterra competition model under modern coexistence theory to systematically and thoroughly explore the general rules of how coexistence mechanisms drive diversity-stability relationships.Moreover,to better explore the mechanisms,I for the first time further decompose the asynchrony（into actual correlation coefficient and SD ratio）and species stability（into new species stability and 2）)determining community stability.The results show that the increase of niche differences（stabilizing mechanisms）and the approaching of competitive differences towards 1（more symmetrical competition,equalizing mechanisms）both can improve asynchrony and species stability and thus increase community stability,and both can strengthen the diversity-stability positive relationship through their interactions with diversity.Thus,niche differences（stabilizing mechanisms）and competitive differences（equalizing mechanisms）,which determine species coexistence,also determine the diversity-stability relationships.This profoundly indicates the internal consistency and connectivity of the mechanisms behind the two.These new important theoretical findings significantly deepen our understanding of the diversity-stability relationships and also help us to interpret,analyze and discriminate the traditional mechanisms（insurance effects,statistical averaging effects,compensatory dynamics,and overyielding effects）.Finally,the results of this thesis would be helpful for us to better understand,predict and deal with the effects of overgrazing and global changes（such as climate warming and nitrogen deposition,etc.）on the structure,function and stability of grassland ecosystems,thus providing some application value for better utilization,management and restoration of grassland ecosystems.