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Size segregation will lead to stratification of a particle system. At present, people have not fully understood the segregation mechanism. In this work, we have studied the size segregation behavior of two-component disk particles in chute flows. The effects of particle size ratio η, particle density ρ, static friction coefficient μ and chute angle α on size segregation are discussed. We use the discrete element method to simulate and calculate the force of disk large particles during segregation. Results show that the 'squeeze expulsion' mechanism plays a key role in the size segregation of a disk particle flow. We establish a physical model of 'squeeze expulsion' of disk particles and obtain the conditions for the formation of 'squeeze expulsion' mechanism. [ABSTRACT FROM AUTHOR]

The four kinds of sites under different kinds of human disturbances, i.e. a farmland area, a rotational-grazing area, an over-grazing area and a prohibited-grazing area, were selected in the typical region of the Alashan Desert of Inner Mongolia from April to October in 2002, 2003 and 2004 respectively. The fluctuating tendency of rodent communities and the sensitive response of their populations were studied using mark-recapture and trap-day methods. The four kinds of mark-recapture samples and line samples were taken in different human disturbance areas. The area of the mark-recapturing sample was 0.95 hm2 and that of the line sample was 10 km2. The samples were collected every month in the mark-recapturing sites for four consecutive days and in line sites in April, July and October every year. The variable matrix was composed of the numbers of captured rodents of the rodent communities. The sensitive response of the populations in the communities was analyzed with principal component analysis (PCA). Results showed that there were different types of rodent communities under different kinds of human disturbance on two scales in three years. In each scale, there were large differences in both the number of species and the number of main population in the rodent communities under different kinds of human disturbance on the farmland area and over-grazing area especially. The results of PCA showed that the sensitive response of the populations of the commu nities was significantly different under different kinds of human disturbance on the two scales in the three years. Cricetulus barabensis was the most sensitive on the farmland area, there were no differences in sensitive response of the various rodents on the rotational-grazing area, and Dipus sagitta and Phodopus roborovskii were the most sensitive in the over-grazing area. [ABSTRACT FROM AUTHOR]

Four diverse disturbance types, namely, farmland, rotationally grazed grassland, overgrazed grassland and forbidden grazing grassland, were identified in the Alashan desert region of Inner Mongolia. Rodents were sampled in April, July and October in 2002 and 2003 using the trap-day method. Their spatial and temporal niches among the four disturbance types were assessed using the Shannon-Wiener niche breadth index and the Pianka niche overlap index. The Shannon-Wiener niche breadth index was the highest for Allactaga sibirica and Meriones meridianus (0.925 and 0.908, respectively), intermediate for Cricetulus eversmanni, Phodopus roborovskii and Citellus dauricus (0.789, 0.782 and 0.711, respectively), and lowest for Cardiocranius paradoxus (0.003). The Pianka niche overlap indices of six-paired species were the highest and varied from 0.900 to 1.000, suggesting their spatial niche overlapped almost completely. Conversely, the spatial niche overlap index for Meriones unguiculatus, Cardiocranius paradoxus, and Salpingotus kozlovi was zero, thus indicating a complete absence of competition among them. In addition, the temporal niche breadth and overlap indices varied greatly in relation to disturbance types and season. The level of temporal niche overlap in spring and autumn was low while in summer it was high. For all disturbance types and for each season, the temporal niche overlap index of Meriones meridianus and Cricetulus barabansis was always highest but for Meriones unguiculatus and Salpingotus kozlovi always lowest. [ABSTRACT FROM AUTHOR]