COMBINATIONS OF ENVIRONMENTAL FACTORS 



207 



pull of gravity. They may also be given as 

 an active response to one or more of the 

 wide variety of physical gradients just hsted 

 or to diese plus various biotic influences. 

 Vertical migrations also result from passive 

 conveyance by convection currents in water 

 or air and from equally passive relations 

 to changes in density or viscosity that are 

 closely associated with daily temperature 

 rhythms. 



TEMPERATURE AND MOISTURE 



Temperature and moisture acting to- 

 gether make a much-studied environmental 

 team. Each affects the potency of the other 

 under many conditions, and both together 

 are influenced by other phases of the en- 

 vironmental complex, notably by Hght and 

 by mineral nutrients. The relations of mois- 

 ture and temperature to plants have been 

 summarized by a hydrothermal index 

 (Livingston, 1916) based on the following 

 formula: 



T — T " 



■Imt — *■ 1^ 



I „u is the moisture-temperature (hydro- 

 thermal) index; Ij represents an index of 

 temperature efficiency (Klages, 1942); Ip 

 and Ig give indices based on precipitation 

 and on evaporating power of the air respec- 

 tively. As in most other work in which the 

 evaporating power of the air is considered, 

 this index is weak in that vapor tension rela- 

 tions are not yet known (p. 189). It also 

 has a number of other defects, one of the 

 more important of which is the failure to 

 consider the effects of soil moisture stored 

 during the nongrowing season and used 

 later. Even with these handicaps, maps 

 showing hydrothermal zones of the United 

 States differentiate fairly well between 

 southern regions of wann-weather crops and 

 northern ones with cool-weather plants, and 

 also serve to separate in a general way the 

 eastern moist from western more arid 

 regions. The correlation coefficient for 112 

 stations between the hydrothermal index 

 and the length of the growing season is rea- 

 sonably conclusive, with r = 0.63 ± 0.04. 

 The relation of this index to animal life has 

 not yet been critically evaluated. Maps 

 based on data calculated from the hydro- 

 thermal formula show some correlations 

 with broad generalities of animal distribu- 

 tion in the United States. 



Moisture and temperature are two impor- 

 tant elements in comparative climatology 

 (Taylor, 1918). These are graphically 

 shown in temperature-rainfall or tempera- 

 ture-humidity charts in which mean month- 

 ly temperature and mean monthly mois- 

 ture values are indicated by a single dot 

 properly entered on a grid that indicates 

 temperature on the vertical and moisture on 

 the horizontal axis. The twelve dots for a 

 year may then be connected to form a 

 polygon that may be irregular in form. 

 Temperature is recorded in Centigrade or 

 Fahrenheit degrees for dry-bulb or wet-bulb 

 readings. The moisture record is usually 

 based on precipitation or relative humidity. 

 Judging which of these is to be used should 

 depend on knowledge of which is more im- 

 portant for the ecological relations under 

 consideration. Soil animals in general are 

 more likely to be sensitive to precipitation, 

 and fresh-water animals may be decidedly 

 under the influence of temperature and 

 rainfall, especially in streams, ponds, and 

 the smaller lakes. On the other hand, many 

 animals that five above ground are more 

 affected by humidity, especially insects 

 with thin exoskeletons (p. 187). It is not 

 always easy to know the relative value of 

 humidity and rainfall in the ecology of in- 

 sects; both are often important, as in the 

 ecological fife history of the codfing moth 

 and the chinch bug (Shelford, 1927, 

 1932). 



Selected examples of temperature-mois- 

 ture graphs are shown in Figures 48 to 52. 

 Wet-bulb temperatures are plotted against 

 relative humidities in Figure 48, because 

 these correlate more closely with subjective 

 human estimates of discomfort. Dry-bulb 

 temperatures for Jhansi (India) average 

 95° F. in May in place of 75° for the wet 

 bulb; the whole graph for Jhansi would be 

 moved up the temperature axis if dry-bulb 

 temperatures were used. Different types of 

 cUmates are illustrated in this figure by 

 using data from selected regions. They are 

 to be compared with each other and with 

 the shaded region that is based on the cli- 

 mates of twelve large cities of the world in 

 which the population is predominately 

 white. No graphs are given for combinations 

 of low temperatures and low relative humid- 

 ities; such chmates are rare. Tropical cli- 

 mates usually show a small range in mean 

 monthly temperature and a large variation 

 in relative humidity; their graphs tend to 



