336 G. O. Batzli et al. 



Herbivore- based 

 trophic system 



Detritus-based 

 trophic system 



FIGURE 10-1. A generalized trophic structure for terrestrial 

 ecosystems, showing the distinction between herbivore- 

 based and detritus-based trophic systems. Arrows represent 

 the flow of energy and materials; R represents respiratory 

 loss of energy. (After Heal and Mac Lean 1975.) 



(fixed carbon) into the ecosystem. The detritus-based system is based 

 upon the consumption of dead organic matter. Saprovores and microor- 

 ganisms influence the ecosystem through their control of the rate of 

 decomposition and cychng of mineral nutrients. There is ample reason to 

 believe that, in tundra, this is particularly important (Chapter 5). Other 

 distinctions between the herbivore- and detritus-based trophic systems 

 are discussed by Heal and MacLean (1975). 



Herbivore-based food chains in arctic regions contain relatively few 

 taxa. Whole groups of invertebrates that are common in grasslands at 

 lower latitudes, e.g. insects and mollusks, have few representatives in the 

 Arctic (MacLean 1975a). The most abundant herbivores are homeo- 

 therms, probably because they can maintain high rates of activity and 

 growth at low temperatures. 



Herbivorous birds, especially ptarmigan {Lagopus spp.) and geese 

 {Anser albifrons, Branta canadensis and Chen hyperborea), use the 

 North American tundra as a breeding ground during summer, but their 

 occurrence and impact appear to be patchy. While avian herbivores gen- 

 erally migrate south in late summer, some ptarmigan do overwinter on 

 inland tundra where they consume mostly willow buds and twigs (West 

 and Meng 1966). 



