Vegetation community selection by ungulates on the Isle of Rhum. III. Determinants of vegetation community selection.
Possible determinants of seasonal vegetation community selection by cattle, red deer, goats and ponies on the Isle of Rhum, Scotland are described and discussed. The feeding strategies of each species are discussed with respect to a theoretical framework of constraints on diet selection in ungulates based on mouth size, body size and the digestive system within species of ruminant, their rumen morphology and digestive function. Throughout the year the goats fed on vegetation communities containing high biomasses of live dwarf shrubs and forbs. The other species all fed predominantly on the grassland communities. Cattle, having a large body size and rumen, a long passage time of food material through their digestive tract and small incisor breadth : metabolic requirements ratios, fed on communities containing the highly digestible live mesotrophic graminoids and forbs when these were abundant and on communities containing live oligotrophic graminoids when the former declined in abundance. Red deer stags and hinds, which are smaller and have larger incisor breadth : metabolic requirements ratios than cattle, selected vegetation communities containing live mesotrophic graminoids and forbs throughout the year. Goats, having digestive systems adapted for fast passage rates, fed predominantly on communities containing live dwarf shrubs and forbs. Because of their high cell content concn, this type of plant material will release most nutrients in the shortest time. Ponies, being hindgut digestors, have relatively low digestive abilities and, as a result, high intake requirements. Consequently, they fed on communities with high biomasses of live mesotrophic graminoids and forbs when these were abundant and on communities with high biomasses of both live and dead mesotrophic graminoids and forbs when the biomass of the live component declined.