Spatial pattern as a consequence of multispecies interactions: Lessons from the coffee agroecosystem

A substantial body of ecological theory shows that 1) spatial pattern can spontaneously arise from the underlying rules of the biological interactions involved and 2) spatial pattern can determine the consequences of biological interactions. Furthermore, spatial patchiness is well-known to be a potential stabilizing mechanism for biological interactions that are unstable at a local level. However, encountering field data that speak to these theoretical ideas has been difficult. Cases in which a spatial pattern observed in nature can unequivocally be shown to be a consequence of biological interactions are rare. At least one reason that such biologically determined spatial pattern is not frequently observed is the confounding factor that nearly all habitats are patchy. Agroforestry systems, in contrast, are relatively homogenous environments that provide an excellent model system for studying the distribution patterns of organisms in space, the forces that generate them, and the consequences to the organisms involved. Here, we present an ecological system consisting of an aggressive arboreal ant that nests in the shade trees and forms clumps of varying sizes, despite the relatively uniform distribution of the host trees. The ant has a mutualistic relationship with a scale insect, which protects it from a predatory beetle and several species of parasitoids. Additionally, the ant has a phorid parasitoid that reduces its foraging efficiency and consequently the efficiency of its protection of the scale insect. The distribution patterns are striking, and clearly the consequence of biological interactions. Thus, the particular status of this agroforestry system in which the shade trees are intentionally uniformly planted over a large area enables us to demonstrate 1) the spontaneous generation of spatial pattern through biological dynamics and 2) the stability consequences of that spatial pattern in a different organism.