THE SIZE OF THE ECOLOGICAL NICHES AND THE OVERLAPPING OF NICHES IN RESPECT TO THE SELECTION OF FOOD PLANTS BY FALLOW DEER (CERVUS-DAMA LINNE, 1758) AND ROE DEER (CAPREOLUS-CAPREOLUS LINNE, 1758)

The size of the niche as a measure for the extent of the nutrient demands on an actively used resource spectrum ("reciprocal of specialization") and the overlapping of niches in respect to nutrient selection were determined for the first time for two free living wild populations. The results of the observations of 7302 browsing fallow deer and 4141 browsing roe deer within a 25 km2 investigation area in Lower Saxony were compiled into a resource matrix (Tabs. 1 and 2) on the basis of a phenologically based chronological reference system as well as on a spatial reference system based on plant associations. Annual changes in the niche sizes of fallow deer (Fig. 1) and roe deer (Fig. 2) are closely correlated to the yearly fluctuations in nutrient availability and seasonal changes in behaviour. The portion of the resource spectrum utilized to satisfy nutrient demands is inversely proportional to the supply. The increase in niche size to a maximum of 2.1216 for fallow deer in the winter and of 2.7668 for roe deer in the early spring (Tab. 3) signifies an expansion in the use of the resource spectrum to satisfy nutrient demands or conversely a decrease in the selectivity in the choice of plant associations with a corresponding decrease in available browse. The minimum niche size corresponds to the time periods of optimum nutrient availability. The minimum for roe deer (0.6283) in early summer reflects this species relationship to concentrate selectors; the minimum for fallow deer (0.8895) in early spring when grass growth sets in, reflects the affinity to the ruminants of intermediate type. A second maximum in niche size for roe deer occurs during the mating season, at which time the fallow deer demonstrate a second minimum. The values of 0.1505 and 0.8750 show a low overlapping of niches (Fig. 3). The complementary use of the resource spectrum by these two species is conductive to the carrying capacity of the habitat.