PS 80-74 - What predicts dominant ant foraging in a coffee agroecosystem? Insights from regression analysis

Friday, August 12, 2011
Exhibit Hall 3, Austin Convention Center
Andrew J. MacDonald, Ecology, Evolution and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA and Ashley E. Larsen, Ecology, Evolution & Marine Biology, University of California, Santa Barbara, CA

Ants provide numerous important services ranging from nutrient cycling to pest control in both natural and managed ecosystems.
Here we investigate the foraging behavior of a dominant arboreal ant, Azteca instabilis, which forms a ‘keystone’ mutualism with the green coffee scale, Coccus virdis. In a 300ha organic, shade coffee farm in Chiapas, Mexico, this mutualistic association is the epicenter of numerous trophic interactions involving coffee pests, beneficial insects, a suite of other ants, a scale-killing fungus, and an Azteca-specific phorid fly parasitoid.

Given these various interactions, especially scale availability and parasitoid risk, it is not surprising Azteca foraging behavior is not described by a simple radius from the nest. While the boundary of Azteca foraging is apparent from the lack of ant-activity and scale insects, the choice of which bushes to tend and how heavily to tend within the larger foraging radius is not clear. We use a linear probability model, a probit model, and interval regression to investigate how number of scales, distance from the nearest nest, and proportion of scales infected with the scale-killing fungus, Lecanicillium lecanii, predict the probability of Azteca instabilis presence in a coffee bush and the number of Azteca instabilis individuals present.


In all models, we find the number of scales to be an important predictor of Azteca, but surprisingly we find distance from the Azteca nest has no influence on either the presence or the number of Azteca foragers. In addition, when one suspect site is dropped, distance becomes significantly positively related to the probability of Azteca being present.  We find fungus incidence is not an important correlate to Azteca being present, but does have a significant negative effect on the number of Azteca individuals on a bush. Site effects were important in all models. We build on the original model parameters (number of scales, distance and fungal infection) to increase the variance explained by the biological variables compared to the site-specific effects.

The foraging and patrolling behavior of the dominant ant has important implications for pest incidence and ant diversity in the surrounding coffee bushes. We discuss our findings in relation to several trophic interactions related to the Azteca-scale mutualism, previous research conducted in this site, and more broadly to how foraging behavior might alter the expected benefits of ant-scale mutualisms in agricultural systems.

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