RADF 



The Herbivore-Based Trophic System 393 



Relative Degree 



of 



Insect Relief 



Availability 

 % of area 



In Vitro 



Digestibility 



% 



Peak Biomass 

 gdw m"2 



Biomass Ratio 

 (liverdead) 



Sand 

 Dunes ^ , , ^ 



I. Or Dryos Heatti/ 



Snowbed 



Dupontia 

 Stream Meadow 



60r 







60r 





 80 



O 



I 5r 







I2(- 







Enophorum Corex 



Meadow Meadow 

 M E3 



O 



_Ea_ 



j^_ 



Increasing Wetness of Habitat 



FIGURE 10-21. Characteristics of the vegetation types in the Prudhoe 

 Bay region. The relative degree of insect relief is an index scaled to the 

 wetness of the habitat. RADF (relative availability of digestible forage) 

 = availability x digestibility x biomass x biomass ratio. (After White et al. 

 1975.) 



types highest in green biomass caribou could maintain a period of maxi- 

 mum food intake from late June to mid-August. 



Each vegetational type has different attributes with respect to cari- 

 bou grazing in the Prudhoe Bay region (Figure 10-21). The product of 

 four measured parameters — availability of habitat, digestibility of for- 

 age, peak green biomass and ratio of live-to-dead material — was used as 

 a summary index of the relative availability of digestible forage for each 

 habitat. The relative availability of digestible forage appeared to be posi- 

 tively correlated with the distribution of caribou on days without insect 

 harassment, particularly in groups of ten or fewer individuals (Figure 

 10-22a). Caribou group size was generally between one and ten individu- 

 als following severe harassment as herds moved from relief areas to 

 preferred grazing areas. On days with harassment, distributions changed 

 noticeably, and there was no relationship to the relative availability of 

 digestible forage (Figure 10-22b). 



