Experimental evidence that ptarmigan regulate willow bud production to their own advantage.

ABSTRACT

In some ecosystems, vertebrate herbivores increase the nutritional quality and biomass of their food source through repeated grazing, thereby manipulating their environment to support higher densities of animals. We tested whether ptarmigan (Lagopus lagopus and L. muta) are capable of regulating the nutritional quality, abundance, and availability of feltleaf willow (Salix alaxensis) buds using a simulated browsing experiment and a feeding preference study with wild birds. Simulated ptarmigan browsing resulted in smaller buds, but greater numbers of buds per shoot. Furthermore, browsing altered the morphology of willow branches such that buds were at higher densities and closer to snow level compared to unbrowsed controls. Browsing increased the number of willows with accessible buds (buds within 50 cm of snow level) from 55 to 89%, and increased total accessible bud biomass from 113 ± 30 to 129 ± 50 mg/ramet. Browsing did not affect bud nitrogen or carbon concentration and slightly reduced protein precipitation capacity (tannins) in buds the following winter, indicating that ptarmigan browsing does not induce a defensive response in this species. When branches of broomed (previously browsed) and unbroomed willows were placed in the snow at equal heights, ptarmigan showed no preference for either type; however, they obtained more buds from broomed willows. Increased accessibility and density of willow buds caused by browsing has the potential to increase habitat carrying capacity, thereby supporting higher densities of ptarmigan.