Effects of density, season and weather on use of an altitudinal gradient by sheep

The distribution of high-quality forage is an important determinant for foraging patterns of ruminants. It has been suggested that herbivores should gradually increase the use of higher altitudes as new, high-quality forage emerges. Further, the ideal-free distribution (IFD) theory predicts a more even use of habitats (i.e., altitudes) at high population (stocking) density. We tested these predictions by following individually marked sheep within a landscape-scale experiment with high (80 km2) and low (25 km2) sheep population density, covering altitudes from 1050 to 1300 m a.s.l. on alpine ranges of Norway. Sheep used all altitudes from the beginning to the end of the summer grazing season. Contrary to prediction from the IFD theory, there was no effect of population density on the use of the altitudinal gradient. As predicted, sheep used lower altitudes more often during the early season than during the middle of the grazing season. However, contrary to prediction, there was a decline in average altitude used during the last part of the season. There was also a strong effect of prevailing weather. Early in the season, sheep used higher altitudes more during warm than cold weather both during grazing and resting. As the season progressed, the effect of temperature decreased and the altitude difference between resting and grazing increased. Sheep used higher altitudes more during clear weather than during cloudy weather, and used the lowest altitudes when there were clouds and no wind. Sheep were more often at high altitude when there was wind, especially during the first part of the grazing season, and tended to be at a lower altitude during grazing than resting when there was no wind. We conclude that movements in the altitudinal gradient were to a large degree determined by factors other than population density.