210 P.J. Webber et al. 



Natural Perturbations 



The control over ecosystem function exerted by various factors can 

 best be studied when the system is perturbed. Enlargement and drainage 

 of lakes may be viewed as natural perturbations occurring in the coastal 

 tundra as part of the thaw lake cycle. Ecosystem function changes dras- 

 tically in response to these alterations between aquatic and terrestrial en- 

 vironment, and thousands of years are required for completion of the 

 thaw-lake cycle and return of the ecosystem to its original state. 



Tundra is often viewed as easily disturbed or changed (Bliss et al. 

 1970) but it is quite stable and resilient to major environmental changes 

 (Bliss et al. 1970, Webber and Ives 1978). It appears to be adapted to 

 large, natural, often sudden environmental fluctuations. For example, 

 the pulse of water runoff during snowmelt is dramatic but does not have 

 immediate effects upon most other factors such as an efflux of nitrogen 

 or phosphorus, even though 40% of the annual phosphorus return from 

 litter to the soluble pool occurs during this 10-day period (Chapin et al. 

 1978). Apparently, the exchange capacity of the mosses and the nutrient 

 demands of plants and microorganisms at this time are sufficient to ex- 

 tract most of the dissolved nitrogen and phosphorus from runoff. Dur- 

 ing the growing season, microbial populations in the soil may build up 

 and crash, releasing a large proportion of the annual nutrient flux in 

 brief periods. However, the rapid nutrient uptake by plants and the high 

 exchange capacity of peat are sufficient to remove nutrients as they are 

 made available. 



The recurrent peaks in lemming abundance at 3- to 4-year intervals 

 constitute another natural perturbation of the coastal tundra at Barrow 

 (Chapter 10). Heavy winter grazing removes dead leaves from the can- 

 opy, and computer simulations predict that the associated increase in 

 light intensity will stimulate photosynthesis and increase net production 

 the following summer (Miller et al. 1976). Moreover, Htter decomposes 

 more rapidly once it is felled and in contact with the wet ground surface, 

 releasing nutrients and further stimulating production. 



Heavy grazing during summer, removing 50% or more of leaf bio- 

 mass, depletes plant carbon and nutrient reserves as new leaves are pro- 

 duced and reduces the length of time that new leaves can photosynthesize 

 (Mattheis et al. 1976, Chapin 1975); computer simulations predict a re- 

 duced annual production. The tundra vegetation recovers from intense 

 grazing perturbations within 3 to 4 years, as discussed in Chapter 10 and 

 by Schultz (1964, 1969). Briefly, grazing increases nutrient availability 

 through leaching of urine and feces and more rapid decomposition of 

 felled leaves and litter. The stimulation of primary production by in- 

 creased nutrient availability returns the vegetation to its original state. 



