Ant-aphid relationship: protocooperation or commensalism?

Our study was done in a power line cut just south of the University of Michigan Biological Station. Our purpose was to explore the relationship between ants and aphids and to determine if this relationship is obligate mutualism or some other form of symbiosis including predation, commensalism, or protocooperation. Twelve Populus grandidentata saplings were found and randomly assigned to one of three treatments; A-aphids without ants and without predators, B-aphids, ants and natural predators, and C-aphids without ants, but with applied predators. Ladybird beetles from the Coccinellidae family were used as predators. Predators were added as necessary. Populations estimations were made every day for five days for each sapling. Our results showed a significant decrease in the population size of those aphids not being tended by ants.http://deepblue.lib.umich.edu/bitstream/2027.42/54278/1/2714.pdfDescription of 2714.pdf : Access restricted to on-site users at the U-M Biological Station