Effects of resource distribution patterns on ungulate foraging behaviour: a modelling approach

The food resources of forest ungulates typically are patchily distributed. Research on foraging behaviour has often focused on habitat selection but has rarely taken into account the influence of the spatial distribution of different food patches in two dimensions. However, especially when travelling costs become significant, the pattern of resource distribution is likely to be a major factor in determining foraging behaviour and, hence, ungulate distribution. Using a modelling approach I examine the effects of group size and the effects of increasing inter-patch distance for various resource distribution patterns (random, uniform and aggregated) on the spatial distribution of foraging time for an ungulate in an hypothetical environment. The animal is faced with two patch types: food and non-food. Food patches are loaded with a similar initial forage quantity and productivity. They may be depleted but regenerate by a constant production rate. Forage quality is assumed to decline with increasing standing crop. Young regrowth therefore has the potential to attract ungulates by virtue of its high quality. The foraging ungulate, represented by a large-bodied ruminant, is assumed to follow an energy maximising strategy. It decides at regular intervals to stay or to leave for another patch by balancing potential energy intake and travel costs. The results of the simulations reveal that travel costs can be an important factor in foraging decisions, even though they constitute less than 10% of the daily energy intake. The total number of exploited patches over a 100-day period was mainly determined by forage depletion rate. The number of patches visited daily was a function of both inter-patch distance and forage depletion rate. By differences in the variation of inter-patch distance, the distribution mode has important implications for the foraging route and the number of daily patch visits. With respect to grazing management the important implication is that, aside from the relevance of forage quality and abundance, the spatial distribution of food resources is another factor determining concentrations of ungulates and, hence, their impact on the vegetation. Determination of the appropriate time scale of foraging decisions needs further study, as the role of travel costs increases with a shorter time horizon and leads to a higher ungulate aggregation.