The Herbivore-Based Trophic System 383 



Summary 



Even though Httie can be said about the role of intrinsic factors in 

 lemming population dynamics, it seems clear that extrinsic factors can 

 exert a strong and overriding influence. In order for lemming popula- 

 tions to reproduce and grow during winter, good quality forage must be 

 available, the snowpack must be suitable and mammalian predators must 



o 



(0 



Surface 

 temperature 



Snowpack 



Soil 

 moisture 



Nest sites 



Vegetation 

 (quantity and quality) 



Forage 

 consumption rate 

 T" 



Decomposition 



Soil nutrients 



Predation 

 rate 



,. w 





O 



c 



Hormonal 

 state 



Reproduc- 

 tive rate 



Nutritional 

 state 



Genetic 

 state 



a EnxTiTH ] 



Growth 

 rate 



Survival 

 rate 



Ei^ 



Dispersal 

 rate 



Density 

 (Biomass) 



I 



Aggression 



FIGURE 10-15. Diagram of relationships among factors influencing 

 lemming population density. 



