plasticity; some kinds are readily modified in regard to charac- 

 ters such as size, number, and structure of leaves, or number of 

 seeds; other kinds are much more rigidly fixed. The principles 

 may be outlined as follows: 



( 1 ) Relations of species to the physical environment: 



(a) Every species has certain essential requirements, 



(b) Every species possesses ecological amplitude, i.e., a 



characteristic potentiality for growth within a limited 

 range of environmental conditions, 



(c) Every species has a characteristic capacity for utilizing 



the available resources of the environment in which 

 it occurs. 



(2) Relations among individuals of the same or of different 

 species. Species differ in the following ways: 



(a) In competitive capacity, 



(b) In capacity of association, 



( c ) In reproductive processes, 



(d) In resistance to grazing, mowing, or other treatment, 



(e) In susceptibility to parasites, 



(/) In mutualistic and commensal relationships. 



The ecological success of a species depends upon its capacity 

 to cope with the physical environment and with associated species 

 in the relationships stated in the above principles. 



RELATIONS OF SPECIES TO THE PHYSICAL ENVIRONMENT 



The physical environment is a complex of factors which may 

 be classified into three rather arbitrary groups: (1) climatic 

 factors, which include light, heat or cold, precipitation, humidity, 

 wind, gases, and evaporating power of the air; (2) soil factors, 

 which include texture, structure, depth, and ingredients such as 

 water, gases, mineral constituents, acidity, alkalinity, and salinity; 



(3) topographic factors, which include the degree, extent, and 

 direction of slope, relief, and altitude, ground water, and snow 

 accumulation or removal (Figure 1-1). Fire, although often 

 caused by man, may also be included under climatic factors, but 



Species andl Popsmlatiomms 



