Good (de)fences make good neighbors: Diffuse coevolution and community assembly in tropical forests

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Eric Sodja, School of Biological Sciences, University of Utah, Salt Lake City, UT and Frederick Adler, School of Biological Sciences and Department of Mathematics, University of Utah, Salt Lake City, UT

Background/Question/Methods
Whether communities are mere assemblages of species or a coevolved network of interactions depends on the mechanisms that shape the arrival, loss, and change of component species. Aspects of assembly that operate on large spatial and temporal scales (colonization, speciation, extinction and evolution) are often regarded as background processes that only provide the “raw materials” for local ecological processes to filter out or competitively exclude. However, recent evidence shows that local and fast processes, such as competition and consumption, interact with the processes that operate at larger scales. In tropical forest systems, for example, several plant genera have shown a pattern of a high diversity of rapidly diverged anti-herbivore defenses with a flagrant lack of phylogenetic structuring behind these patterns. Microevolution of plant defense traits, especially of secondary metabolites that reduce herbivory, is answered by phytophagous insects with consumption adaptations specialized to overcome host plant defenses. Plant defenses also show negative density dependence. Coevolution, specifically diffuse coevolution, may explain this pattern, although conflicting forces of host switching and ecological fitting may limit its influence. To understand and quantify the effects of diffuse coevolution and these conflicting forces on community assembly, we have developed a mathematical model that includes lineage divergence in response to mutation, host shifting of herbivores in response to changes in plant defense traits, and ecological fitting via migration between interconnected communities.
Results/Conclusions
Preliminary results indicate that neither host shifting nor ecological fitting preclude coevolution as a structuring factor in plant-herbivore communities, although high frequencies of migration and host switching reduce its magnitude. In particular, diffuse coevolution can control trait shifts in response to novel interactions and impose selective pressure in spite of host switching. This work provides new light on the role of evolution in community assembly at the local scale, and suggests that understanding the structure of communities requires the inclusion of diffuse coevolutionary processes.