Ant-plant interactions at Mount St. Helens

Within the blowdown zone of Mount St. Helens, Formica pacifica is the most common ant species. From studies during the summer of 1991 patterns were observed in the spatial dispersion of F. pacifica nests and also in the plant species present on the middens of these nests. Formica pacifica is less active within species rich vegetation patches than in exposed areas. Studies in 1992 will investigate whether F. pacifica nests are more common along the edges of vegetation patches. Other studies will investigate the presence of Hypochaeris radicata (false dandelion) on the middens of these nests. The wind born seeds of H. radicata may have difficulty establishing on the nutrient poor tephra. Ant middens may provide suitable sites for these plants to establish. Soil brought to the surface during nest excavation and the collection of plant and insect parts may add nutrients to the substrate of ant middens. Once established above an ant nest, plants would begin to cause shading of the nest. The resulting decrease in temperature may cause the ants to move their nest location. By moving out away from the shade of a vegetation patch and creating more sites for plant establishment, ants of F. pacifica may be effecting the expansion of vegetation patches. Since primary succession at Mount St. Helens is a slow process, the seemingly insignificant effect of ants on this process may be relatively important.


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