BIOLOGICAL FACTORS 



201 



population of herbivores and thus maintaining a balance in food 

 relationships. In spite of this, the feeding by herbivores may some- 

 times be excessive enough to cause serious disturbance or even 

 destruction of community structure. 



Under natural conditions, grazing was undoubtedly greatest 

 when buffalo ranged throughout our grasslands. Locally, as around 

 water holes, their feeding and trampling sometimes destroyed 



FlG. 94. Distinct browse line on stand of ironwood resulting from deer 

 feeding on low branches. Note the uninterrupted view under stand, and ab- 

 sence of shrubs and tree seedlings. Such damage commonly results when deer 

 population is high, and especially when winter supply of food is inadequate. 

 — U. S. Forest Service. 



most of the vegetation but otherwise probably did little damage 

 since they were constantly on the move and distributed themselves 

 where grazing was best. Moderate grazing by cattle does not 

 change the essential nature of a grassland community. A succession 

 of dry years in the time of the buffalo could have resulted in local 

 conditions similar to those in overgrazed pasture areas today. 



Deer and moose similarly have little effect on grassland or for- 

 est, where they browse, unless there is an overpopulation. Then, 

 especially as a result of winter browsing, the complete destruction 

 of tree reproduction might be possible. 



Prairie dogs may consume all the forage for some distance about 

 their villages. The total consumption of food bv such relatively 

 small animals is sufficient to reduce considerably the value of a 

 range for larger herbivores. The same may be said for jack rabbits, 

 but their feeding is less localized. 



