PARENT SESSION
Oral Session #96: Plant Competition II.
Presiding: K. Suding
Thursday, August 8. 1:00 PM to 3:45 PM. Mesquite Room, Radisson.
Quantifying the contribution of coexistence processes in the Arizona desert.
Sears, Anna*,1, Chesson, Peter1, 1 University of California, Davis, Davis, CA
ABSTRACT- We quantified two processes that may contribute to the coexistence of two annual plant species in the Chihuahuan desert, Arizona. Plants coexist if their population-level intraspecific competitive effect is greater than their interspecific competitive effect. However, there are many different processes that can contribute to the relative strengths of intraspecific and interspecific competition, and these can occur on different spatial scales. Here, we compared the relative effects of direct neighborhood competition, which reflects local resource consumption on a small spatial scale, with covariance between plant response to the environment and competition, which arises when there is environmental heterogeneity in space, and is only apparent at larger spatial scales. We used neighborhood competitor removal experiments in an area without noticeable environmental gradients to compare the importance of these processes. Erodium cicutarium, a high-density exotic species had little direct effect on its own growth and inflorescence number, and a strong negative effect on the growth and inflorescence number of Phacelia popeii, a sparse native species. However, covariance between response to the environment and competition strongly increased Erodium's intraspecific competitive effect, without affecting its interspecific competitive effect on Phacelia. Although there was no indication that neighborhood intraspecific competition was greater than interspecific competition, our data imply that covariance between plant response to the environment and competition elevated intraspecific competition relative to interspecific competition on the larger spatial scale, and thus reduced the ability of Erodium to exclude Phacelia. We did not test the reciprocal effect of Phacelia on Erodium because Phacelia was at very low density. This study illustrates the importance of using population models in conjunction with field experiments, in order to partition out the effects of scale-dependent processes.
KEY WORDS: coexistence, environmental heterogeneity, neighborhood competition, spatial storage effect