Taxonomic divergence and functional convergence in Iberian spider forest communities: Insights from beta diversity partitioning

Aim The main aims were to determine: (a) the relative contribution of species replacement and richness difference from components to overall taxonomic (TD beta) and functional (FD beta) beta diversity of spider communities; (b) the degree to which TD beta and FD beta components can be explained by the environmental or geographic predictors; (c) whether FD beta components were lower than expected given the underlying TD beta variation. Location This study was carried out in 22 oak forest sites across the Iberian Peninsula. The area comprises two biogeographic regions, Eurosiberian (North) and Mediterranean (Centre and South). Methods Spiders were sampled using a standardized protocol. A species x traits matrix was constructed. Total taxonomic (TD beta(total)) and total functional (FD beta(total)) beta diversity were calculated, by pairwise comparisons, and partitioned into their replacement (beta(repl)) and richness difference (beta(rich)) components. Mantel tests were used to relate taxonomic and functional dissimilarity with environmental and geographic distances. A spatial eigenfunction model was constructed and the variation in TD beta and FD beta explained by environment and geographic predictors was quantified. Null models were used to test if FD beta was higher or lower than expected given TD beta. Results beta(repl) was the dominant component contributing to 84.2% and 72.8% for TD beta(total) and FD beta(total), respectively. TD beta(total) and FD beta(total) (and their replacement components) were higher between- than within-biogeographic regions. TD beta(total) and TD beta(repl) were positively correlated with environmental and geographic distances, even when controlling for a biogeographic effect, but their functional counterparts were only correlated with environmental distance. Variation partitioning showed that pure environmental and spatially structured environmental effects had a small contribution to beta diversity, except for TD beta(rich). The observed slopes of the regressions of FD beta(total) and FD beta(repl) in relation to environmental distance were slower than the null model expectations. Main Conclusions Spider assemblage variation was mainly determined by the replacement, and not the net loss, of species and traits. TD beta was influenced by niche filtering and dispersal limitation, whereas FD beta was mainly generated by niche filtering. A high level of functional convergence among spider communities, despite the high taxonomic divergence, revealed the signal of replacement of species performing similar functions across sites.