Seasonality in community composition

To characterize how seasonality in community composition is dependent on climate we computed for each site an index of seasonality in community composition and then fitted a linear model in which we regressed this index against the MAT of the site. To describe seasonality in community composition we examined how much more similar pairs of samples were in terms of their community composition if they were sampled from the same season compared with whether they were sampled from different seasons. We considered a pair of samples as belonging to the same season if they were taken at most 1 month apart, whereas we considered them as belonging to a different season if they were taken 3 months (plus or minus half a month) from each other. As a measure of community similarity we used the Jaccard similarity index, which we averaged over those pairs of samples that contained at least five species. We then used an index of seasonality in community composition calculated as the average Jaccard similarity index for pairs of samples that were taken in the same season, minus the average Jaccard similarity index for pairs of samples taken in a different season. We accounted for the Jaccard similarity index for pairs of samples that were taken in the same season to control for possible variation in the baseline turnover and thus to extract the sole effect of seasonality.