Special Cases of Common Boundaries 173 



mal boundary for their distribution. Under unusual circumstances 

 the horizontal temperature gradient may change abruptly. At the 

 northern edge of the Gulf Stream, for example, the temperature may 

 drop as much as 5°C within a few miles— an extreme thermal change 

 for the oceanic environment. We are not surprised that many species 

 inhabiting Sargasso water have northern boundaries and species in- 

 habiting continental slope water have southern boundaries at the Gulf 

 Stream. Similar sudden changes of climatic conditions occur on 

 land in relation to unusual topographical changes, such as an abrupt 

 change in altitude at the edge of a plateau. In these situations a 

 considerable number of plants and animals share common boundaries 

 based primarily upon the temperature factor. 



Frost Line. The frost line on land constitutes another special 

 thermal situation but one in which there is no sudden change in tem- 

 perature geographically. The fact that the tissues of animals and 

 plants in active condition freeze not far below 0°C results in the 

 occurrence of a boundry of general importance at the frost line. 

 All plants and all animals that are not frost hardy and that do not 

 have modes of escape are limited in their poleward and altitudinal 

 distribution by the harmful results of freezing. At the same time 

 any plants or animals dependent upon these frost-sensitive species 

 will drop out. The palm trees are an example of plants that are 

 unable to stand a continued frost. In North America the natural 

 range of the true palms does not extend north of the central part 

 of Florida, the Gulf coast, and the southern part of California. 

 Smaller plants and animals associated with the palms are necessarily 

 limited in their distribution to essentially the same boundaries. 



Tree Line. Other abrupt changes in the fauna and flora commonly 

 occur where one major type of vegetation gives place to another, since 

 the dominant vegetation influences the presence of many subordinate 

 species of plants and animals. An outstanding instance of common 

 boundaries due to the dependence of many species on the type of 

 vegetation is seen at tree line. Here again there is no sudden 

 change in the distribution of temperature. When thermal and other 

 conditions have been modified to such a point that trees no longer can 

 grow, an important change in the whole ecological situation takes 

 place. In any forested region trees have a profound controlling in- 

 fluence on the subordinate plants and on the associated animal life. 

 Thus the environmental situation that limits the trees indirectly sets 

 boundaries to the distribution of many other species as well. Tree 

 lines dependent in part at least on temperature occur across the con- 

 tinents at the poleward margins of the forests and also on mountain 



