Feeding by vertebrate herbivores in a chemically heterogeneous environment

We hare studied the constraints that food quality imposes on feeding decisions in three free-ranging herbivores feeding on winter twigs of the mountain birch (Betula pubescens ssp. tortuosa) in the Abisko Mountains (Sweden). Comparison was made between two sites, one at the treeline (600 m above sea level) and the other in a birch wood (380 m above sea level). The concentrations of secondary compounds (phenols) and N varied with respect to twig diameter, among individual trees and between sites, higher phenol and N concentrations recorded at the treeline site and in thinner twigs. Farces and urine of the mountain hare: (Lepus timidus) and farces of the willow grouse (Lagopus lagopus) showed the same site difference in phenol and N concentrations as found in the birch, with higher concentrations al the treeline. Faeces of the moose (Alces alces) did not show such a pattern. We discuss these differences in relation to size of eaten twigs, digestive strategies, and mobility of the species studied. According to our data, spatial variation in quality might be of greater importance than variation in quantity for small herbivores such as the willow grouse and mountain hare, whereas larger animals such as the moose may be less responsive to variation in quality.