Resource limitation in a generalist herbivore, the moose Alces alces: ecological constraints on behavioural decisions

The relationship between foraging behaviour and the time-energy budget of adult moose (Alces alces) cows was studied during the winter in two Norwegian populations. In the study area with lowest food availability, the moose foraged almost exclusively on birch (Betula pubescens) of poor quality (expressed as digestibility). In the other study area with higher food availability, a much more diversified species composition of the diet was found, including several Salix spp., rowan (Sorbus aucuparia), aspen (Populus tremula), and bird cherry (Prunus padus). Large differences in the daily activity of moose were found between the study areas. In the poor food availability area, moose browsed twigs of poor quality and were inactive a greater proportion of the time. They used a smaller area for foraging, but removed a much larger fraction of the available biomass from each browsing site than in the higher food quality study area. No significant difference was found between the study areas in the gross daily dry matter intake rate. A larger amount of feces was defecated in the low quality study area, resulting in a significantly higher net intake of digestible dry matter in the area with high food quality. Thus, because of digestive constraints, the quality of the forage determined the rate of energy gain from foraging. The interaction between moose and their food is therefore strongly dependent on factors that influence the digestibility of forage.