10 



The Herbivore-Based 

 Trophic System 



G. O. Batzli, R. G. White, S. F. Maclean, Jr., 

 F. A. Pitelka, and B. D. Collier 



INTRODUCTION 



The tundra is well known for its conspicuous and abundant animal 

 populations. Indeed, tundra may be better characterized by caribou, 

 wolves, lemmings, snowy owls, ptarmigan and hordes of flies than by 

 any other feature, at least in popular literature. The next two chapters 

 consider the composition and organization of animal communities, and 

 their participation in the energy and nutrient dynamics of the coastal tun- 

 dra ecosystem. 



Ultimately, all heterotrophic activity, animal and microbial, de- 

 pends upon the energy and nutrients fixed by green plants in net primary 

 production. The amount of net primary production (Chapter 3) sets a 

 limit upon the abundance and production of heterotrophic organisms. 

 Two more or less distinct trophic systems based upon this net primary 

 production may be recognized in virtually all ecosystems — a herbivore- 

 based system that begins with the consumption of living autotroph tis- 

 sue, and a detritus-based system that begins with the consumption of 

 dead organic matter (Figure 10-1) (BatzH 1974, Heal and MacLean 1975). 

 The distinction corresponds broadly to an aboveground and below- 

 ground division (perhaps reflecting a paucity of information on below- 

 ground herbivory). The two trophic systems may converge to some ex- 

 tent, particularly at the top carnivore level, and a single animal popula- 

 tion may function in both trophic systems; this is a categorization of tro- 

 phic functions rather than animals. 



Several important conceptual differences distinguish the two trophic 

 systems. The herbivore-based system begins with the consumption of liv- 

 ing plant tissue, and thus impacts directly upon plant production, 

 growth, and reproduction. Thus herbivores (or, indirectly, carnivores 

 preying upon herbivores) modify the rate of input of chemical energy 



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