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ECOLOGICAL FACTORS: 



GENERAL CLIMATE 



Present climatic classifications are based upon 

 fairly exact measurements of temperature and mois- 

 ture. These classifications consider average values, 

 seasonal distribution, and effectiveness. For example, 

 temperature monthly averages of mean, minima, and 

 maxima, plus indirect temperature estimates by re- 

 cording the average growing season are utilized. 

 Comparable data for precipitation are used. All these 

 criteria for climates are of prime importance to life. 

 Each criterion is reflected by particular limits of 

 tolerance and, for this reason, can be a limiting factor. 



Although climate is operating upon all aspects of 

 an environment, its main ecological act is to help 

 delimit the ranges of many species and of the mature 

 soil types. This is the case in both tropical and tem- 

 perate regions. However, in tropical regions there 

 may be no seasonal climatic differences and in tem- 

 perate regions there may be marked seasonal changes. 

 In the temperate zone most animal adaptations re- 

 lated to seasonal changes appear most closely associ- 

 ated with temperature changes, and not with both 

 temperature and moisture as in plants. However, in 

 spite of these close relationships, the climatic adapta- 

 tions of organisms probably are related to all physical 

 conditions producing climate. In fact, organisms are 

 influenced by all features of their environment and 

 usually react to the complete environment rather than 

 to any single factor. 



In spite of being closely correlated with organism 

 distribution, general climate is less important at the 

 local level. Locally, the individual factors of tempera- 

 ture, moisture, light, wind, soil, and other organisms 

 — usually in combination, but also singly — are most 

 important in determining the presence or absence of a 

 species. Here also, freak weather conditions, includ- 

 ing storms, can have severe consequences for plants 

 and animals. 



CLIMATIC RHYTHMS 



The rhythms or trends of climate are of six types: 

 equable, tropical wet-and-dry, Mediterranean, conti- 

 nental, polar, and desert. Normally, each rhythm 

 conforms to a major type of climate. 



Equable rhythms usually have little seasonal climatic 

 difference. Even the maximum possible seasonal 

 changes are insignificant, so the most typical equable 

 trends are near the equator in tropical climates where 



the predominant vegetation is rainforest. As one 

 might expect, existing climatic fluctuations cause, at 

 most, only minor responses among the organisms. 



Tropical wet-and-dry rhythms display seasonal changes 

 in temperature and precipitation. The temperature 

 cycle is associated with, and is mostly the conse- 

 quence of, the precipitation cycle. Temperature ex- 

 tremes occur during the dry season. Under such 

 rhythm, rainforests are first replaced by deciduous 

 plants (typical rainforests have evergreens); if climate 

 is less equable, an open type of vegetation, called 

 savanna, grows. The savanna plants are woody, gen- 

 erally deciduous, and sometimes quite specialized for 

 arid conditions; however, these adaptations are re- 

 sponses to progressively less equable trends. There- 

 fore, in these tropical rhythms the most equable en- 

 vironments contain a deciduous, or monsoon, rain- 

 forest and progressively less equable trends are 

 inhabited by a variety of savannas. 



Other rhythms are fairly distinctive, closely related 

 to climate, and more familiar to North Americans. 

 The Mediterranean rhythms have evergreen woody 

 plants, such as the chaparral of western North 

 America. The continental rhythms have three main 

 types of vegetation. In areas where rainfall is heavy 

 and is distributed with a summer maximum and 

 where the winter is cold but of moderate length and 

 conditions, the vegetation is a deciduous forest. Such 

 a forest covers much of the eastern United States. In 

 colder areas, such as the Great Lakes and north- 

 eastern United States, the vegetation is a needle-leaf 

 forest. Finally, in drier areas, steppe or grassland like 

 that of the Great Plains prevails. The polar rhythms 

 have a stunted woody and herbaceous vegetation 

 called tundra. The desert rhythms have a sparse cover- 

 age of specialized woody and herbaceous plants. 



Although each rhythm is most easily recognized by 

 its plants, there is also a definite association between 

 climatic trends and animals. Whether this animal 

 association actually reflects rhythm or vegetation, or 

 perhaps both, is difficult to ascertain. Perhaps the 

 various influences upon animals contain examples of 

 strict rhythm control, of vegetation control, of rhythm 

 -vegetation control, and probably even some inde- 

 pendence from these factors. 



PLANT LIFE FORMS 



Various schemes summarize the over-all responses 

 of plants, through adaptations, to ecological condi- 



