408 G. O. Batzli et al. 



winter nests. This would increase nutrient deposition in troughs. Many, 

 perhaps most, carcasses are taken by predators. Weasels deposit their 

 scats near the winter nests of lemmings, but avian predators regurgitate 

 pellets on higher ground, the rims of low-centered polygons and the cen- 

 ters of high-centered polygons. Deposition of pellets would therefore add 

 nutrients to rims, and this would counteract the trend of greater nutrient 

 removal from rims than from basins (Table 10-13). 



The total movement of phosphorus to troughs, assuming half of the 

 carcasses decayed in troughs, would be 0.2 kg ha"' (20 mg m"^) over a 

 three-year period, five times the average amount of soluble phosphorus 

 found there at present. While this simulation does not prove that con- 

 sumers move nutrients to polygon troughs, it does show that the nutrient- 

 transport hypothesis is feasible. 



We do not know how effective predators are as nutrient-transport 

 systems, but snowy owls spend long periods of time on favorite centers 

 of high-centered polygons (owl mounds). Extremely high levels of phos- 

 phorus accumulate in the soils of these mounds, and grasses such as Arc- 

 tagrostis, Calamagrostis and Poa dominate the vegetation there as no- 

 where else at Barrow. Jaegers deposit their pellets on lower mounds, such 

 as the rims of low-centered polygons, as well. The deposition of these 

 pellets affects the foraging patterns of shorebirds. MacLean (1974b) has 

 shown that female shorebirds must consume lemming bones when laying 

 eggs to obtain enough calcium for their eggshells. Shorebirds, which nor- 

 mally forage in lower areas, search on mounds for lemming bones. Thus, 

 nutrient transport by consumers can affect other consumers as well as 

 producers and decomposers. 



More observations, calculations and experiments are required to de- 

 termine the effectiveness of consumers as nutrient-transport systems in 

 tundra. If confirmed, these systems will be an important example of the 

 major impact consumers can have on ecosystem structure and dynamics, 

 much greater than that predicted by simple measurement of their 

 biomass. 



Consideration of changes in tundra vegetation and soils after re- 

 moval of lemming grazing provides additional insight into the effects of 

 grazers on coastal tundra. Batzli (1975b) sampled exclosures near Barrow 

 that had been in place for 15 years (Schultz 1964) and 25 years (Thomp- 

 son 1955c). Vegetation had changed little in low, wet sites that had stand- 

 ing water most of the summer. In mesic and dry sites, however, the net 

 production of graminoid stems and leaves was almost twice as high in 

 grazed areas as in the exclosures. Standing dead material and detritus 

 were greater within the exclosures, suggesting that nutrient cycling had 

 diminished in the absence of grazing. Reduced phosphorus in the soil 

 solution under exclosed areas corroborates the hypothesis of decreased 

 nutrient availability in the absence of grazing (Barel, pers. comm.). 



