The effect of tree identity and aggressive ants on arboreal spider diversity in coffee agroecosystems

Background/Question/Methods

Biotic factors affect species distribution and abundance therefore the overall impact of organisms on ecosystems. Indeed, some species have a strong effect on the distribution of other species because they act as keystone species that cascade (up or down) their effects to other trophic levels.  In addition to their importance for biodiversity conservation in the tropics, coffee agroeocosystems are an excellent model where to study the effect of keystone players on species distribution and abundance. In organic coffee farms the diversity of trees that provide shade to the coffee plants is high whereas in farms tending towards intensification most of the shade trees belong to the Inga genus. In addition, in these agroecosystems the arboreal ant Azteca instabilis is a keystone species regarding biological control. Here we examine the spider diversity that inhabits the trunks of shade trees of coffee agroecosytems in relation to two keystone players: A. instabilis and tree identity.  In 2012 we sampled arboreal spiders in 18 ha plots in Southern Mexico. In each plot we selected four trees, two from the Inga genus and two belonging to a different genus. In each pair tree, one of the trees had an active nest of the ant A. instabilis.

Results/Conclusions

Results show spider diversity is affected by both Azteca instabilis and tree identity.  Average spider richness was significantly lower in the presence of the ant (p<0.001). In addition, spider richness significantly decreased when the sampled trees belonged to the genus Inga (p<0.001). Indeed Inga trees had fewer trunk depressions than any other tree (p<0.05) and the size of the depressions was smaller (p<0.0001). The effects of A. instabilis but not  Inga trees were completely different regarding spider abundance. Spider abundance and biomass increased significantly in the presence of the ant (p<0.0001).  Species composition analysis based on the Bray-Curtis similarity index shows that spider composition in the presence of A. instabilis is different from the spider composition in the absence of the ant.  Feeding trials showed that most of the spiders recorded in the presence of the ant were feeding on A. instabilis. Results suggest that A. instabilis and Inga trees shrink the arboreal spider community. Indeed, the fact that spider diversity is greater on other trees different from Inga indicates that organic farms harbors a more diverse set of spiders at the tree layer.