Plant secondary chemistry plays an important role in modulating species interactions. Nevertheless, plant chemical diversity remains an understudied component of the biological complexity of natural systems. Recent studies have shown the importance of chemical diversity on local community assembly and species composition. In contrast, little is known about how plant chemical diversity could influence biological systems across large geographical scales. Here we propose multiple mechanisms by which plant chemical diversity could help forge patterns of rarity and commonness across Amazonian tree species. Furthermore, we use a combination of field herbivore surveys, metabolomic approaches, and data from large scale tree surveys to assess the relationship between plant chemistry, species herbivore load, and plant abundance. We surveyed the secondary metabolites and insect herbivores of 29 sympatric Protium Amazonian taxa at two distantly related sites (Iquitos, Peru, and Manaus, Brazil). For each species, secondary metabolites were assessed both from leaf and from root tissues using a combination of GC-MS and HPLC-MS. Herbivore loads where assessed on more than 1100 individuals. Each individual was surveyed once a week for 64 weeks yielding over 7300 feeding records from 327 morphospecies of insect herbivores (confirmed by DNA barcoding). Finally, we extracted Amazonian wide abundance data for our 29 Protium species from the available ATDN dataset.
Results/Conclusions
Despite the large geographical distance, we found a very high consistency within species secondary chemical composition across our two sites (>98% chemical similarity). Contrastingly, we found almost 100% turnover in herbivore species composition between the two sites. Finally, we found strong associations between the local Protium secondary chemistry, local herbivore diversity, and regional species abundances (r2=0.42, p<000.1 and r2=0.47,p<000.1 respectively). Despite multiple caveats associated with the ATDN and our dataset, these results fit the prediction of one of our proposed mechanisms. In this mechanism, plants species with a high diversity of secondary metabolites could not only have a higher chance of expanding their geographical range but also have a strong competitive advantage at the local scale. In concert, our results suggest that plant chemistry, as well as plant-enemy interactions might play an important role in the distribution and abundance of tropical plants.