ism when exposed to a gradient of one or more environ- 

 mental conditions, cf. Optimum, Minimum, Pessimum, 

 Tolerance. 



Preferential Species 



Species in Class 3 of Braun-Blanquet's classification of 

 Fidelity (q. v.); species occurring more or less abundantly, 

 but predominantly or with greater vitality, in a certain 

 community. 



Premature Grazing 



The grazing of vegetation before the most important 

 forage species have grown sufficiently, or before the soil has 

 become dry and firm enough, to prevent cumulative injury 

 to the range, cf. Range readiness. 



Prescribed Burning 



The use of fire under control to improve growth condi- 

 tions in vegetation. 



Presence 



The degree of occurrence of a species in Stands of a 

 Community-type, cf. Constancy. 



Pressure, Biotic 



See Biotic pressure. 



Prevernal 



Refers to early spring, cf . Aspcction. 



Prey 



An animal that is attacked and killed by another, Pre- 

 dator (q. v.), e.g., a ground squirrel killed by the coyote as a 

 predator. 



Primary Succession 



Succession (q. v.) beginning on a bare area such as a lava 

 flow, not previously occupied by plants or animals, cf. 

 Secondary succession. 



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