210 



ANALYSIS OF THE ENVIRONMENT 



on temperature and humidity corrected for 

 rainfall, and for biotic associates. Bonitation 

 may depend on temperature-rainfall condi- 

 tions; the intensity of the attack of wilt 

 disease on plants of the cucumber family 

 gives an illustration (Fig. 51). Heavy rain- 

 fall kills many kinds of insects, chinch bugs 



occupied by animals may be different from 

 the general meteorological conditions; this 

 is especially obvious for irrigated lands. 



Fire is an environmental factor to which 

 forest and grassland animals have been ex- 

 posed spasmodically from time immemorial. 

 The fire hazard depends on the combina- 



RELATIVE HUMIDITY IN PER CENT 



Fig. 50. Temperature-humidity relations of the Mediterranean fruit fly. (Based mainly on 



Bodenheimer. ) 



among them. Temperature-rainfall graphs 

 may show whether a given season is favor- 

 able or unfavorable for the development of 

 large population of such insects. Under the 

 weather conditions diagrammed in Figure 

 52, A, crop damage by chinch bugs in Illi- 

 nois was severe. By contrast, Figure 52, B 

 shows average conditions for years in which 

 crop damage was light. The difference in 

 rainfall during the warm growing season of 

 May, June, and July is the important factor. 

 We cannot overemphasize the fact that 

 estimates of bonitation based climatic fac- 

 tors must be seasoned with sound common 

 sense. A large amount of rain in a month 

 with favorable growing temperature is not 

 necessarily fatal to rain-sensitive insects. 

 Much gentle rain may not be harmful even 

 though an impressive total amount falls 

 during critical periods, and heavy localized 

 showers do not necessarily prevent disas- 

 trous outbreaks of chinch bugs. The tem- 

 perature-moisture relations jn the habitat 



tion of a number of variables. Given com- 

 bustible material and an igniting spark such 

 as might be furnished by a stroke of light- 

 ning, the danger of fire depends on wind 

 velocity, temperature, and fuel humidity, 

 both directly and in terms of the time since 

 the last precipitation. These elements can 

 be combined rather simply into an index of 

 fire danger that is used in practical fire con- 

 trol work in forestry.' 



Enough selected detail has been given to 

 document the generalization that the phys- 

 ical factors of the environment can work in 

 effective combination in pairs or other 

 groups of factors as well as singly. It is now 

 useful to consider some of the relations of 

 many elements of the environment consid- 

 ered together and of the environment as a 

 whole. 



" Anonymous, 1939a, Fire Control Hand- 

 book, Region Two, Forest Service, U. S. De- 

 partment of Agriculture, 



