TABLE 9-4 A tab 



(Carpenter 1940). 



can, therefore, be carried with full or nearly full pro- 

 ductivity. Heavy grazing, however, should not be 

 permitted. In addition to reducing herbage produc- 

 tion, heavy grazing may destroy seed stalks prior to 

 the dropping of the seed or so weaken the plants 

 physiologically that seed is not even produced. The 

 growth of underground rhizomes and vegetative re- 

 production is retarded when photosynthetic activity is 

 reduced. The best pastures are those in which graz- 

 ing animals do not consume more than 70 to 80 per 

 cent of the total herbage productivity of the grasses 

 (Stoddart and Smith 1943). As a rule, not more 

 than about 60 per cent of the current forage volume 

 and 25 per cent of the flower stalks should be har- 

 vested by grazing animals. Overgrazing always 

 brings about a reduction in abundance of the more 

 palatable species and an increase in the less desirable 

 ones with the consequent deterioration of the range 

 and the productivity of the community (Weaver and 

 Tomanek 1951, Kucera 1956). The carrying capac- 

 ity of grassland or the largest number of animals 



that can be supported without deterioration of the 

 range varies with the type of grasses involved, the 

 climate, and the soil (Table 9-5). 



Although often overlooked, invertebrates consti- 

 tute one of the three important groups of grazing ani- 

 mals. Individually they may not consume appreciable 

 amounts, but in the aggregate they produce a very 

 significant effect. The total biomass of insects in a 

 New York pasture amounted to 3.2 g dry weight per 

 square meter. This is to be compared to 14.5 g for 

 the dry weight of cows per square meter that the pas- 

 ture was supporting. Feeding experiments showed 

 that in one pasture where grazing by the cattle was 

 moderate and the vegetation was ample, the insects 

 ate more of the grasses and clovers than the cows 

 did, but in another pasture which was being over- 

 grazed and in which the vegetation was short, the 

 cattle ate more than the insects did (Wolcott 1937). 

 Grasshoppers and Mormon crickets are sometimes 

 very destructive in the arid west. In one area in 

 Montana, a population of 25 grasshoppers per square 



126 Habitats, communities, succession 



