Endogenous and Exogenous Spatial Dynamics of the Keystone Ant Azteca Sericeasur in an Intensifying Coffee Agroecosystem

Abstract

Azteca sericeasur is a keystone tropical arboreal ant species whose ecological interactions with mutualists and natural enemies are believed to determine a self-regulating (endogenous) spatial process. We inferred ecological processes from the spatial patterns of A. sericeasur in a 45-ha coffee farm plot from 2004 to 2012 using three types of analysis: (1) clustering statistics describing the range of significant spatial clustering; (2) stochastic spatial process models of cluster formation; and (3) tree-level logistic regressions predicting ant nest formation and disappearance. We fitted the latter two models with endogenous and exogenous (habitat and environmental) variables to investigate whether endogenous processes sufficiently explained nest clustering, or whether other exogenous variables also played a role. We found that nest clusters were predicted by nest density from the previous year and not consistently by any exogenous variables except tree density in later years. Nests were clustered to a scale of about 40m and new nests occurred within clusters of existing nests to a scale of approximately 30m. These patterns most likely reflected budding colonies, suggesting the ant’s preferred maximum dispersal range. Nest formation was positively related to measures of nest density and proximity, but nest disappearances could not be predicted by any combination of variables. Nest disappearances also had no significant clustering pattern with other nests. These results support our hypothesis that the A. sericeasur system is endogenously driven, although mainly through effects on new nests rather than nest disappearance. Tree density became a consistently significant predictor of nest clustering after the farm shifted toward more intensive agricultural management. Intensification resulted in thinning of host trees, which caused new nests to be spaced farther apart. This would have counteracted negative density-dependent interactions with natural enemy species if those species had a shorter dispersal range than A. sericeasur. These results have implications in coffee agroecological management, as this system provides important biocontrol ecosystem services.