The Herbivore-Based Trophic System 391 



c 



UJ 



60 



Q 50 



H 40 



a. 



20 



40 



60 



Live Biomass, gdw m ' 



a. 



Live Biomoss, gdw m 



80 



-2 



100 



FIGURE 10-19. Consumption of forage by esophageal-fistulated rein- 

 deer (a) and the percentage of the day spent eating (b) in relation to the 

 available green forage. The dashed line represents sheep grazing on range 

 similar in available biomass to that of the Prudhoe Bay region. The solid 

 lines represent the extrapolation of the relationship to caribou. The 

 shaded column represents available biomass of vascular plants in the 

 Prudhoe Bay region in July. (After White et al. 1975.) 



gestibilities estimated for reindeer may reflect the effects of confining 

 them to specific vegetation types. 



The potential digestibility of all material from the vegetation types 

 in the Prudhoe Bay region varies considerably, but selection of green 

 material would provide increased digestive efficiency. By following the 

 phenological progression in vegetational development, caribou may be 

 able to maintain maximum digestibilities of 57 to 63*^o throughout the 

 summer (Person et al. 1975, White et al. 1975, White 1979). 



Whether or not caribou can select individual plants for digestibility 

 is not known, but Klein (1970) suggested that they select for protein and 

 minerals, particularly phosphorus. In vitro digestibility is inversely re- 

 lated to nondigestible components such as lignin (White et al. 1975, Per- 

 son et al. 1975). And selection for high digestibility should also provide 

 higher intake of cell contents that contain most of the soluble protein and 

 phosphorus. It seems likely that caribou select vegetational types and the 

 plant species and parts within those types that are highest in general nu- 

 tritional quality, but they avoid plant species and parts that contain toxic 

 secondary compounds (White and Trudell 1980). 



Selection of vegetational types may also maximize the quantity of 

 food eaten. Non-lactating female reindeer grazing on Carex-Eriophorum 

 meadows were used by White et al. (1975) to estimate food intake. The 

 availability of green vascular plants was an important factor controlling 

 the rate of consumption of food (Figure 10-19a). Studies with grazing 



