Testing the effect of plant chemical diversity on herbivore damage and community assembly

Background/Question/Methods Ecologists have strived to find mechanisms that govern the assembly of natural communities. Particularly challenging to explain are “species swarms” of understory herbs, shrubs, and trees found in tropical wet forests. Classical ecological theory predicts that in order for coexistence to occur, species differences need to be maximized across biologically important niche dimensions. New evidence suggests that natural enemies and chemical defenses could play an important role in the assembly of hyper-diverse tropical plant communities. It has been recently suggested that, within a particular plant community, species that maximize the difference in chemical defense profiles compared to neighboring taxa will have a relative competitive advantage. Here we experimentally tested the relationship between chemical community diversity, phylogenetic diversity, plant–herbivore interactions, and plant community assembly. We used an array of 60 experimental multi-species Piper (Piperaceae) communities (~1500 individuals from 16 Piperspecies) planted into the native undisturbed, lowland wet forest of the La Selva Biological Station, Sarapiqui, Costa Rica. We purposely manipulated species composition to create significantly different chemical and phylogenetic community compositions for each plot.

Results/Conclusions After one year of exposure to herbivores, plots with high chemical diversity suffered significantly less herbivore damage, lower plant mortality, and less local extinction than low chemical diversity plots. Although chemical diversity had the strongest effect on local mortality and extinction, phylogenetic diversity had the strongest effect on total herbivore damage. Average total herbivory was 17.5% leaf area missing, approximately half of which was produced by specialist herbivores (8.0%) and half by generalist herbivores (9.5%). Chemical diversity was negatively correlated with damage by specialist herbivores and not by generalist herbivores, suggesting that specialist herbivores are the main driving agent influencing local Piper diversity. Our results support the idea that both natural enemies and plant defenses likely play a significant part in the structuring of local communities and the maintenance of rich tropical biota.