Authors’ Contributions
LGC provided funds, LGC, WL and ZYM originally formulated the idea, WL and ZYM developed methodology, ZYM, TWZ, ZQ, THT and JYF conducted fieldwork and data collection, WL and ZYM performed statistical analyses, ZYM wrote the first version of the manuscript, WL and LGC revised the manuscript. All authors contributed to the final version of the manuscript.