| Grain is the material basis for human survival and development.With the rapid growth of the world population,the contradiction between grain supply and demand has become increasingly prominent.Grain trade has played an important role in adjusting the balance of international grain supply and solving the problem of international grain security.Since the 21 st century,globalization has deepened,the demand for grain and the scale of grain trade in various countries have expanded,and the international grain trade pattern has undergone profound changes.Grain trade has become an important factor affecting national grain security.Research on the structural evolution and price transmission characteristics of international grain trade can help to comprehensively understand international grain supply,demand and potential risks,to lay a scientific foundation for ensuring national grain security.Based on this,the research of thesis is carried out in the following three aspects:(1)Construct an international grain trade network and analyze the evolutionary characteristics of international grain trade patterns.Based on the complex network theory a directional weighted network of international grain trade from 1987 to 2019 is constructed,while the spatial structure of the global grain trade network and its evolutionary characteristics are explored from three scales: global,regional and national.The number of grain trade cooperating countries,import and export trade volumes are analyzed,and the evolutionary characteristics of the overall structure of the international grain trade network are analyzed using relevant indicators such as aggregation coefficient,average path length and entropy in the complex network.The results show that the international grain trade network satisfies the scale-free characteristic,the global trade volume is increasing year by year,and the trade volume of 20% of the countries in the trade network accounts for 80%.In recent years,the position of countries of the Asian region in the grain trade network has gradually increased,the decline of the original major hub countries in the trade network,and the importance gap of node connections is narrowing,showing the trend of multilateralization of global trade.The world’s grain market exports are highly concentrated in a few major exporting countries such as the United States,Canada,Brazil,Argentina and Russia.(2)An international wheat trade network is constructed to study the robustness of the wheat trade network.Taking wheat as an example,the international wheat trade network is constructed and the robustness of the trade network is analyzed by both random and deliberate attacks.Comparing the feedback of the network under different failure nodes,it is found that the trade network has higher robustness in response to random attacks.If attacked deliberately,the network vulnerability is strong,and the network has been paralyzed when the node failure rate reaches 40%.Locally,the division of the community structure of the international wheat trade network has become clearer over time,and different communities have a higher degree of satisfaction of wheat trade demand within the communities.Among the top five global wheat importers in 2020,Italy and Turkey have relatively concentrated trade imports.When trade risks occur,these two countries are greatly affected.(3)Construct a wheat price linkage network and study the linkage pattern of domestic and international wheat prices.Changes in grain prices are inevitably caused by changes in grain trade patterns.The volatility states of international and domestic wheat futures prices are transformed into price linkage volatility modes by coarse-grained method,and then the international wheat price linkage network is constructed to analyze the linkage patterns of international and domestic wheat prices.The results show that there is linkage between domestic and international wheat prices,and their linkage fluctuation patterns are power-law,cluster,and cyclical. |
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