PARENT SESSION
Oral Session # 33: Pathogens, Toxins, and Disease II: Plants.
Presiding: A Davelos
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 201.

Assessing interaction strengths in a plant - arthropod - plant pathogen interaction web: Do factorial manipulations predict real world patterns?

Norton, Andrew^*,1, English-Loeb, Greg², Hesler, Steve², ¹ Colorado State University, Fort Collins, CO² Cornell University, Geneva, NY

ABSTRACT- Determining interaction strengths in multi-trophic systems has been a challenge for applied and basic ecologists for decades. In multi-trophic systems both indirect effects (i.e. species A alters the density of species B which in turn alters the density of species C) and interaction modifications (i.e. species A alters the per-capita impact of B on C) may be common. Factorial experimental manipulations and analysis with a combination of ANOVA and path analysis are powerful tools for the examination of interaction webs. However, practical considerations often limit the spatial and temporal scale over which we are able to manipulate organism abundance, potentially limiting the predictive ability of experimental results. We used a combination of three years of short term factorial manipulation experiments with a large scale, multiple year field trial to examine interaction webs in a plant - mite - plant pathogen system. The riverbank grape, Vitis riparia, possesses acarodomatia which separately increase the densities of two taxa of beneficial mites: the mycophagous Orthotydeus lambi and the predaceous Amblyseius andersoni. These mites reduce the abundance of the plant pathogen grape powdery mildew and plant feeding spider mites respectively. However, these two mites negatively effect each other, potentially limiting the value of acarodomatia to the plant. ANOVA and path analysis of factorial manipulations of acarodomatia and the mites indicate that indirect effects are common in this system but that there is little evidence for interaction modification. Further, interaction strengths in this web were largely consistent between the different years the experiments were run. Comparison of these results with a large scale multi-year field trial indicates that the results from factorial manipulations are a good predictor of organism abundances at a much larger scale.

Key words: path analysis, mutualism, food web, acarodomatia