Theoretical work has identified the interaction between animal movement patterns and spatial habitat heterogeneity as potentially important for food web dynamics. Despite this theoretical importance, there is a dearth of empirical work to hone and verify proposed models. The fine-scale spatial structure within species geographic ranges may be regulated via combinations of donor-control, predation, and movement patterns. I am employing both theoretical and empirical studies to explore these mechanisms and their interactions in an insect food web composed of sagebrush (Artemisia tridentata), a fly (Tephritidae- Eutreta diana) that forms galls on sagebrush, two species of parasitoids (Chalcidoidea) that attack E. diana larvae, and insectivorous birds that peck open galls to consume inhabitants. In 2009, I censused plant characteristics and collected galls in 200 m² plots (1m² resolution) at Valentine Eastern Sierra Reserve, CA. Galls were dissected and their puparia used to identify healthy E. diana emergence or parasitism. I used spatial autoregressive models to estimate the strength of sagebrush characteristics as predictors of presence/absence and abundance patterns of E. diana galls and parasitioids. Finally, I compared observed distributions to expectations based on models I constructed for E. diana and parasitoid movement and oviposition behavior.

Results/Conclusions

Patches of galls and parasitiods formed small islands surrounded by large regions of unoccupied sagebrush. This pattern was also evident at a finer scale within occupied patches. Consequently, galls were significantly more clumped than expected under complete spatial randomness at spatial resolutions ranging from one to hundreds of square meters. Gall distribution also exhibited significant spatial autocorrelation. The number of galls in a plot increased significantly with sagebrush cover in a plot, but gall density on sagebrush did not vary significantly with sagebrush cover. Other sagebrush characteristics, including height and density of flowering stalks, exhibited high spatial variation at multiple spatial scales but were weak (yet significant) predictors of gall presence/absence and weaker predictors of gall density. This suggests that gall distribution is not merely donor-controlled. Levels of parasitism varied significantly from patch to patch, ranging from near zero to over 50% of galls parasitized. The observed gall and parasitoid distributions supported predictions from my hypothesized models of dispersal and oviposition behavior. These results suggest that there is more to the story of the E. diana food web than solely plant quality or dispersal limitation.