Physiological responses of plant populations to herbivory and their consequences for ecosystem nutrient flow.

We explored how responses of two populations variable in grazing tolerance provide feedbacks to nutrient supply by controlling carbon supply to soil heterotrophs. The study focused on differences in production and carbon and nitrogen allocation patterns between the two populations. The grazing-tolerant population, or on-colony population, is found on intensively grazed prairie dog colonies, and a grazing-intolerant population, the off-colony population, is found in uncolonized grasslands. Equations describing the production and allocation responses to defoliation for the two ecotypes described were incorporated into CENTURY, a nutrientcycling simulation model. Simulations showed an increase in plant production that paralleled increases in net nitrogen mineralization. Production was greater with grazing and was maintained at higher grazing intensities for the on-colony than the off-colony population. Differences between the populations provided important controls over nitrogen losses. Feedbacks between plant responses to grazing and nitrogen cycling accounted for increased nitrogen availability with grazing. These feedbacks were more important determinants of ecosystem function than were fertilization effects of urine and feces deposition. The simulation results suggest that ecosystem function may be sensitive to physiological differences in population responses to periodic disturbances like herbivory.