Authors' contributions
All authors contributed the design of this research. CY drafted the manuscript
and has been involved in the interpretation of the data. GX and RS performed
statistical analyses; TS, BP, XJ, XY, SL and MS played a major role in the field survey.
HH made a substantial contribution to the interpretation of the data and
has been involved in revision of the manuscript through all stages. All authors
read and approved the final manuscript