Compensatory growth of Festuca rubra after grazing: can migratory herbivores increase their own harvest during staging?

Abstract

Summary The grazing optimization hypothesis predicts increased production and quality of plants grazed at intermediate grazing pressures. Following this hypothesis, herbivores will be able to increase their own harvest by repeated grazing. We tested the predictions of this hypothesis for Barnacle Geese, Branta leucopsis. We manipulated the grazing intensity of Festuca rubra swards through trials with captive geese in early spring. Levels on experimental grazing matched levels of natural grazing pressure. The growth response of individually marked tillers was measured over 6 weeks. Above‐ground biomass production of individual tillers was not different among different grazing intensities. Lost biomass in grazed tillers was compensated by a lower rate of senescence. Grazing affected sward characteristics significantly: the proportion of dead biomass in the vegetation was reduced, and production of additional axillary tillers increased. When extrapolating the experimental findings to foraging opportunities for staging geese, we calculate an increase in potential harvest for grazed compared with ungrazed swards at levels of natural spring grazing. This experiment demonstrates an increase in the carrying capacity of the staging site for migratory geese through grazing. When comparing the experiment with grazing levels of wild Barnacle Geese, it is clear that current goose densities maximize potential harvest.