172 Temperature 



Results of Temperature Limitation 



One far-reaching consequence of the action of temperature in the 

 various ways that have been reviewed is to impose a north-south 

 gradation on the distribution of many animals and plants both in 

 water and on land. The ranges of species that are controlled pri- 

 marily by temperature, rather than by moisture or some other ecologi- 

 cal factor, have boundaries tending to run generally east and west. 

 Latitudinal zonation is considerably modified, however, by tempera- 

 ture anomalies due to currents in the ocean, and to differences in alti- 

 tude, proximity to oceanic areas, and other influences on land. Across 

 the United States, for example, less severe cold is experienced along 

 the Atlantic and Pacific coasts than in the center of the continent, and 

 cold weather occurs farther south in the mountain areas. The ranges 

 of many species correspondingly extend farther north along the coasts 

 but are restricted to more southerly regions in the Rockies and in 

 the Appalachians. On the other hand, in northwestern Europe 

 species for which the northern boundary is determined by summer 

 warmth range farther north toward the interior of the continent be- 

 cause the temperature in summer is higher there than on the coast. 



There are no temperature boundaries to which the ranges of all 

 plants and animals conform. The complex influence of thermal con- 

 ditions must be investigated for each species separately before gen- 

 eralities applying to groups of animals and plants can be established. 

 As we go north or south species drop out irregularly and their ranges 

 overlap. The pines of the north, for example, extend into the realm 

 of the palms of the south. The live oaks, typical of southern United 

 States, overlap in their distribution the white oaks, whose center of 

 distribution lies farther north. The quail extends its range northward 

 into New Hampshire, but the grouse, a similar type of bird common 

 in Canada, spreads southward as far as Virginia. 



The same sort of overlapping occurs with aquatic organisms. Of 

 40 species of marine invertebrates common along the eastern coast of 

 the United States, 16 have their northern boundary at Cape Cod, 14 

 have their southern boundary at the same point, but 10 species occur 

 on both sides of the Cape. In the study of geographical distribution 

 as influenced by temperature, we see again the necessity for the ana- 

 lytical approach— for dealing primarily with the mechanism of control 

 of individual species. 



Special Cases of Common Boundaries. Only in special instances 

 do a large number of plant and animal species share a common ther- 



