Maintenance of Functional Ant Diversity in Coffee Agroecosystems: Disturbance, Habitat Complexity, and Implications for Spatial Pattern

Abstract

Numerous mechanisms promoting assembly and maintenance of diversity have been identified, from disturbance to habitat heterogeneity to structure of species interaction networks. Understanding which mechanisms are operating and how they are interacting in the field is crucial for conserving functional diversity and important ecosystem services. This is particularly true in managed ecosystems, such as agroecosystems, where effective biocontrol often relies on associated biodiversity such as insect predators and parasitoids. Here we examined the impact of habitat complexity on the response of an arboreal foraging ant community to disturbance in a coffee agroecosystem in Chiapas, Mexico. The primary disturbance in this system is driven by an entomopathogenic fungus (Lecanicillium lecanii) infecting the green coffee scale (Coccus viridis), an important food resource for its mutualist partner, Azteca instabilis, a dominant ant species. We hypothesize the disruption of this mutualism forces a shift in foraging of the dominant competitor and thus has cascading effects on the arboreal foraging ant community. Furthermore, we hypothesize that increasing habitat complexity, in this case shade tree density, provides refugia and alternative foraging resources for the keystone dominant thereby encouraging a resource shift, which in turn facilitates transmission of the disturbance to the arboreal ant community. To test these hypotheses, we induced an artificial fungal epizootic in four experimental sites by spraying a suspended L. lecanii spore mixture on coffee bushes surrounding A. instabilis nest-sites in which the ant/scale mutualism was strong. Surveys of activity of all arboreal foraging ant species present were undertaken before and after the epizootic. These surveys were undertaken in both coffee bushes and shade trees within the experimental plots and were compared to 4 identical analyses undertaken in a control site. We found a significant shift in foraging activity of A. instabilis in two of four experimental sites after the artificial epizootic. This response was correlated with shade tree density; at high tree density, we found a significant decrease in A. instabilis foraging activity in coffee bushes and corresponding increase in foraging activity in shade trees. Additionally, we found an increase in foraging activity of other species of ants correlated with the shift in A. instabilis foraging. These results suggest that increasing habitat complexity allowed A. instabilis to respond to disturbance and the resulting change in foraging location of the dominant competitor opened niche space for other arboreal foraging ants, promoting maintenance of functional ant diversity. These results provide insight into how complex interactions can drive spatial patterns of species distribution, and have implications for shade management as a means of promoting predatory ant diversity and thus biocontrol of coffee pests.