COS 180-5 - A Jack of all trades in the land of diversity: Investigating the link between herbivore diet breadth and plant abundance in the Amazon understory
The mechanisms that forge and maintain complex tropical communities are not well understood. The exceptional species diversity of tropical ecosystems is expected to channel species' evolutionary trajectories towards higher levels of specialization, reducing negative intraspecific interactions. For herbivores specifically, higher specialization levels are often linked to narrow diet breadths. However, empirical evidence suggests that most tropical plant species are likely to be rare. As hosts become locally rare, resources invested in host-tracking could offset host specialization's advantages. Therefore, the competitive advantage granted by a specialized diet is likely negatively correlated with plant host abundance. Under this scenario, herbivores with a wider diet breadth comprised of less common plant hosts could gain a distinctive competitive advantage while reducing species interactions. Here, we use a set of 31 closely related Amazonian tree species (Protium; Burceraceae) to examine the relationship between herbivore diet breadth and plant host abundance. We surveyed foliar insect herbivores on 25 individuals per species for 64 weeks and used DNA barcoding to cross-reference species identities. We also assessed host species' local abundance using randomly placed 100m transects. We predicted that the herbivore communities of rare plant hosts would be composed of herbivores with a wider diet breadth.
Results/Conclusions
Our herbivore surveys gather data on more than 4000 plant-species interactions and 250 herbivore species. Most of the herbivores found were from the orders Lepidoptera, Coleoptera, and Hemiptera. Lepidopteran herbivores had the lowest number of plant hosts (around 5 hosts per species), and Coleopterans showed the widest diet (about 13 hosts per species). After removing species with low total abundances, we found a very strong association between an herbivore's diet breadth and the herbivore's abundance within the community. As expected, locally abundant plant species hosted the largest taxonomic and phylogenetic diversity and abundance of herbivores. Notably, herbivores feeding on rare host species had, on average, wider diet breadths than herbivores using locally abundant hosts. Logically, plant hosts with larger populations represent a stable and predictable resource that could sustain larger populations of herbivore species. Nevertheless, our results suggest that the less abundant plant host could represent a resource that could make wider diet breaths a viable ecological strategy in tropical forests. We propose that despite the evolutionary and metabolic challenges related to host-expansion (e.g., a new set of chemical defenses), using rare hosts might also reduce intraguild competition and grant release from predators.
