As an implicit justification for the importance of conserving biological diversity it is postulated that biodiversity confers benefits to ecosystems. However, how does it promote ecosystem function and stability is not fully understood. Natural pest control is an example of an ecosystem service thought to be enhanced by the high biodiversity and embedded biocomplexity of diverse agroecosystems. My PhD. research addresses how the biocomplexity of an agroecosystem allows the population persistence of important natural enemies, which in turn, help to maintain the populations of potentially harmful pests below damaging levels. I focused on a subsystem of a complex ecological insect web in an organic coffee farm in Chiapas, Mexico, where I studied the interactions between a tree-nesting aggressive ant (Azteca instabilis), a potential coffee pest (the green coffee scale, Coccus viridis), and a predatory ladybeetle (Azya orbigera). This voracious predator of green scales is very abundant in the farm and, therefore, a potential natural controller of green scale populations. I used a combination of laboratory and field experiments to understand the local interactions between all the organisms involved; large scale field surveys to determine the spatial distribution and persistence of the organisms; and computer simulations to experiment with different hypothetical scenarios. I show that, contrary to previous reports, the mutualism between A. instabilis and green scales can be beneficial to the ladybeetle populations. I showed how the complex interactions associated with the this mutualism influence the persistence, spatial distribution, and dispersal of the voracious A. orbigera, which in turn, is imposing the control that is likely preventing green scale population outbreaks. The result is a healthy agroecosystem with little necessity for external inputs for green scale management. Since green scales are persistent pests in many coffee producing areas in the world, its maintenance below damaging levels in this particular farm may be an example of an important ecosystem service provided by complex local and spatial dynamics characteristic of diverse agroecosystems
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