11 



The Detritus-Based 

 Trophic System 



S. F. MacLean, Jr. 



INTRODUCTION 



The detritus-based trophic system is composed of animals that use 

 energy only after it has passed from living components through the pool 

 of dead organic matter. The system includes animals feeding directly 

 upon dead organic matter (detritivores), upon microbial tissue (microbi- 

 vores), or upon other animals (carnivores) (Figure 10-1). This chapter 

 considers the abundance, energetics, and ecological function of animals 

 in the detritus-based trophic system, and the contribution that they make 

 to the decomposition of organic matter and cycling of mineral nutrients 

 in the coastal tundra at Barrow. 



Even in years with high lemming populations, assimilation of energy 

 by herbivores amounts to only about 6% of net primary production 

 (Chapter 10). Another 13% of net primary production is returned to the 

 tundra as feces, while about 80% passes directly to the dead organic mat- 

 ter pool when unconsumed vegetation (including moss and vascular plant 

 roots) senesces and dies. Thus, each year, 93 to 99% of the annual pri- 

 mary production enters the pool of dead organic matter and becomes 

 available to microorganisms and invertebrate detritivores, which form 

 the first link in the detritus-based trophic system. 



The rate at which soil and litter organisms use this energy source is 

 limited by the quality of the organic matter, the length and temperature 

 of the period of biological activity, and other factors such as soil mois- 

 ture and aeration. These influence both the population density and the 

 rate of activity of individual organisms. The cumulative effect of these 

 rate-limiting factors is seen in the large accumulation of organic matter 

 in the soil which indicates that, by and large, the processes involved in 

 the decomposition of organic matter have been more limited by arctic 

 conditions than have the processes involved in the synthesis of organic 



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