School of Mathematics Science, Huaqiao University, Quanzhou, Fujian 362011, P.R.China;
Cele National Station of Observation & Research for Desert Grassland Ecosystem, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, P.R.China;
College of Mathematics and Systems Science, Xinjiang University, Urumqi, Xinjiang 830046, P.R.China;
College of Mathematics and Systems Science, Xinjiang University, Urumqi, Xinjiang 830046, P.R.China;