Measurement of species turnover rate
Species turnover rate is the rate of dissimilarity among species composition across all possible plot pairs along the spatial or environmental gradient. The slope of the relationship between the species turnover and environmental divergence measures species turnover rate. Jaccard’s index (βj; Jaccard et al., 1912) and SØrenson’s index (βs; SØrenson et al., 1948) were used to measure turnover rate of species composition. βj and βs are two widely employed indices, which only consider presence/non-presence of species, are independent of α-diversity (Jost et al., 2007). βj and βs are calculated following the equations:
\begin{equation} \beta_{j}=1-c/(a+b+c)=(a+b)/(a+b+c)\nonumber \\ \end{equation}\begin{equation} \beta_{s}=1-2c/(a+b+2c)=(a+b)/(a+b+2c)\nonumber \\ \end{equation}
where a and b are the numbers of species only occurring in the focal and neighboring plots, respectively, and c is the number occurring in both.