The Herbivore-Based Trophic System 377 



Lemming reproduction remains low during the summer and winter 

 after a decline, and the standing dead material begins to accumulate. The 

 increase in standing dead material and litter improves insulation over the 

 soil and reduces the depth of thaw over the next two or three summers. 

 As the depth of thaw decreases, plant roots are confined to soil with 

 higher nutrient concentrations, and forage quality improves. Lemming 

 reproduction then increases, and the population grows until a new peak 

 is reached, usually 3 to 4 years after the last. 



Our evaluation of the major premises of the nutrient-recovery hy- 

 pothesis follows. Heavy grazing by lemmings can drastically reduce the 

 standing crops of live and dead aboveground biomass (Dennis 1977) and 

 increase feces and urine output (steps 1-3, Figure 10-13). The total con- 

 sumption of graminoids by lemmings during a high year amounts to over 

 40 g Tn'\ nearly 50% of the annual aboveground production and 20% of 

 the total net production. Consumption is less than 1 g m"^ yr"' when pop- 

 ulations are low (Figure 10-14). 



About 70% of the dry weight consumed is returned to the surface as 

 feces and urine. Except for nitrogen, potassium and sulfur, minerals are 

 primarily returned in feces (Wilkinson and Lowrey 1973). While the urin- 

 ary minerals are readily available to plants, those in feces may not be. 

 Most fecal phosphorus probably occurs as calcium diphosphate, a form 

 that is soluble in a weakly acidic (pH 5) solution (Barrow 1975). The rate 

 of nutrient loss from feces will depend upon where they are located. For 

 instance, feces in ponded troughs or basins of low-centered polygons 

 should lose their phosphorus more rapidly than feces on rims. Since most 

 lemming feces are deposited in places where standing water occurs, at 

 least during snowmelt, and since the tundra soil solution is acidic (pH 4.5 

 to 5.5), fecal phosphorus should be readily available to plants. Prelimin- 

 ary leaching experiments, using a solution that mimicked the soil solu- 

 tion, showed that over 80% of phosphorus was removed from feces in 24 

 hours (Chapin et al. 1978). 



Standing dead plant material can amount to 40 g m"^ at snowmelt 

 (Chapter 3), four to five times the dry weight of live material. Most of 

 this represents the previous summer's production less those nutrients that 

 have been removed by translocation and leaching. By felling standing 

 dead over the winter, lemmings do add organic material to the tundra 

 surface where it will decompose more rapidly, but its nutrient content is 

 less than half of that from feces and urine deposited during the high win- 

 ter. Disruption of mosses and lichens, which also takes place when the 

 lemming population is at its peak, may also increase the rate of decom- 

 position, but the amount is unknown. 



The total influx of nutrients produced by lemming activity may be 

 considerable. The average amount of soluble phosphorus in the top 5 cm 

 of soil is 0.5 to 4 mg m'\ whereas the amount deposited in lemming feces 



