Contrasting species and phylogenetic composition of harvestmen between different sides of a geographic barrier in the atlantic forest hotspot

Background/Question/Methods

Harvestmen communities have an astonishing level of endemism in the Atlantic Forest hotspot (~97%), which is probably related to their poor dispersal capability. Therefore, those arachnids have been considered key organisms to test how environmental variation (from fine to broad scales) affects community structure. However, it is surprising that such highly endemic organisms with limited dispersal are still poorly studied in Atlantic Forest. We investigated the current spatial distribution of harvestmen by contrasting species and phylogenetic composition of communities separated by a large geographic barrier (Doce River). Also, we tested whether climate (mainly temperature and precipitation) explains that composition. We use the spatial distribution of harvestmen – at specific level - from 19 sites in the Atlantic Forest (database obtained in the Museu Nacional - UFRJ collection and the literature). We selected sites localized 612 Km from north and south Doce River. Importantly, we chose Itapebi – BA as the northernmost site and Apiacá – ES as the southernmost place as limit sites before any other large rivers.  We extracted 19 bioclimatic variables from worldclim to be used as predictors of species and phylogenetic composition.

Results/Conclusions

Species composition differs markedly between river sides, and were mainly affected by annual mean temperature and precipitation. However, climate has predominantly affected recent changes in the phylogeny, i.e., the ones close to the tips of the phylogeny (mainly at genus level). Conversely, we found the deepest phylogenetic gradient separating Sclerosomatidae (predominant clade in the southern region) from Laniatores was only affected by spatial variables. The main diversification of harvestmen occurred in the Permo-Cretaceous (290 – 65 MYA), whereas the appearance of the river dates from 1.8 MYA. Thus, climatic differences between the sides of the river are probably the main drivers of recent compositional and phylogenetic changes by limiting the spread of several genera. For instance, 32 genera (~50% of all genera found) is known only from one locality. Previous studies suggested that Doce River may be a strong vicariant barrier. By contrast, our findings suggest that the main diversification of harvestmen phylogenetic composition occurred prior to the appearance of this river. Also, even recent phylogenetic changes (i.e., at genus and specific level) were primarily explained by climate, reinforcing the limited spatial distribution of harvestmen in the Atlantic Forest hotspot and their dependency on specific ranges of temperature and moisture.