Declines in biodiversity generated by anthropogenic stressors at both species and population levels can alter emergent processes instrumental to ecosystem function and resilience. As such, understanding the role of biodiversity in ecosystem function and its response to climate perturbation is increasingly important, especially in tropical systems where responses to changes in biodiversity are less predictable and more challenging to assess experimentally. Using large scale transplant experiments conducted at five neotropical sites, we documented the impacts of changes in intraspecific and interspecific plant richness in the genus Piper on insect herbivory, insect richness, and ecosystem resilience to perturbations in water availability. We found that reductions of both intraspecific and interspecific Piper diversity had dramatic and site specific effects on herbivory, herbivorous insect richness, and plant mortality. Ecosystem responses to reduced intraspecific richness were often similar in magnitude to responses to reduced interspecific richness. Increased water availability reduced herbivory by 4.2% overall, and the response of herbivorous insect richness and herbivory to water availability was altered by both intra and interspecific richness in a site contingent manner. Our results underscore the role of intraspecific and interspecific richness as foundations of ecosystem function, and the importance of community specific contingencies in controlling function in complex tropical systems.