Document: JAM-3-38-21

Coexistence of competitors: Evidence of a critical role of soil community dynamics.

BEVER, J.D.* and K.M.WESTOVER

University of California-Irvine, Irvine, CA 92697-2525 ¹

Abstract:
The coexistence of competitors may result from host-specific interations with other trophic levels. We have accumulated evidence of strong negative feedback on plant growth through changes in the soil community. Specifically, we have found that the presence of a plant results in a host-specific change in the composition of its soil community (in both the community of mutualists and pathogens) and this change in the soil community often decreases the growth rate of that plant species relative to that of a second species within our grassland study system. While this negative feedback suggests that the dynamics within the soil community can contribute to the maintenance of diversity within plant communities, it does not identify the importance of these feedbacks in plant species coexistence relative to other potential factors, such as interspecific competition. In this study, we tested the importance of the soil community feedbacks using a series of experiments on two co-occurring perennial grasses, Anthoxanthum odoratum and Panicum sphaerocarpon. Negative soil community feedbacks accumulate rapidly (within three months) between these two species and could therefore generate negative frequency dependence within a single growing season. We tested this prediction by planting mixtures of Anthoxanthum and Panicum at a range of densities and frequencies, and planting these mixtures into pots filled with sterile soil or pots inoculated with living soil. We found no measurable frequency dependence in the growth rates of Anthoxanthum and Panicum when grown in sterile soil, with Anthoxanthum always having a higher growth rate than Panicum. Therefore, Anthoxanthum is predicted to exclude Panicum in sterile soil. However, in living soil, the relative growth rate of Anthoxanthum and Panicum depended on their initial frequency, with Panicum having higher growth rates than Anthoxanthum when it was initially rare. This negative frequency dependence will contribute to the coexistence of these competitors. These results suggest that the dynamics within the soil community are critical to the coexistence of these two competitors.

Keywords: interspecific interaction, plant competition, soil community feedback, coexistence

This abstract is being presented at: 11:00 AM in session:
Oral Session #23: Soil Ecology.