Friday, August 7, 2009

PS 78-31: Are ants enemies of spiders? Spiders and ants in coffee agroecosystems in Southern Mexico

Linda Rivera-Marin, University of Michigan

Background/Question/Methods Coffee agroecosystem management influences the biodiversity present in coffee agroecosystems, for example, coffee plantations under an organic management harbor higher ant diversity than conventional coffee plantations. On the other hand, it has been shown that this biodiversity plays an important role in the biological control of herbivorous insects present in coffee plants. However, little information on the role that spiders, a very important predatory group, play in coffee agroecosystems is available. Furthermore, because in the coffee agroecosystems from Chiapas Mexico, the ant Azteca instabilis forms large colonies of aggressive workers, this ant could have an antagonistic effect on spider diversity. Two were the questions of this research. Does coffee agroecosystem management affect spider diversity? Does Azteca instabilis affect spider diversity? In order to answer these questions, during June–July of 2008 I sampled spiders in two coffee agroecosystems, one polyculture and one monoculture. I collected spiders that were in the nearest four coffee plants around a shade tree with an Azteca nest, simultaneously I collected spiders that were in the nearest four coffee plants around a shade tree without an Azteca nest. In the polyculture I sampled spiders around 18 trees, while in the monoculture I sampled spiders around 14 trees. Results/Conclusions In total 63 morphospecies were found in coffee agroecosystems. Accumulation curves showed that neither coffee agroecosystem management nor Azteca presence affected spider species richness. In the monoculture 48 spider species were recorded in comparison to 52 spider species recorded in the polyculture. In Azteca presence 53 spider species were recorded while 50 spider species were recorded in absence of Azteca. In addition, a generalized linear model (GLM) did not find any effect of coffee agroecosystem management, Azteca presence or any significant interaction between these variables on spider richness. However, a GLM showed that spider abundance was significant higher in the monoculture (p<0.001). Furthermore, Azteca presence had a positive effect on a specific spider species. These results suggest that coffee agroecosystem management seem do not have an effect on spider diversity, thus representing a case in which biodiversity is not influenced by agroecosystem management, however alternative explanations are discussed. Finally I conclude that the dominant ant Azteca instabilis does not have a negative effect on spider diversity, at least at the studied scale, therefore both predators could have additive or synergetic effects in the control of herbivorous populations in coffee agroecosystems.