124 Species and Species Change 



POPULATION DISCONTINUITY 



A range map, such as that in Fig. 48, seldom represents an area 

 over which the species is uniformly distributed but almost invariably 

 includes a series of discontinuous local populations. This discon- 

 tinuity is due to a combination of three factors: (1) each species 

 can exist only in certain ecological conditions; (2) an area of any 

 sizable geographic extent contains ecological differences sufficient 

 that any one species could not exist over every square foot of it; 

 and (3) a species is at times annihilated from local pockets within 

 its normal range. If the area of the range is extremely small, such 

 as the small island abode of the Kankakee mallow or a beetle known 

 from only a single cave, the species may indeed occur uniformly 

 over the entire range. If, however, the range is much larger than 

 these local examples, it will include differences in elevation, water 

 areas which cut up the land surface, local differences in soil types, 

 and differences in climatic factors from one edge of the range to 

 another. Any one species within this whole area is adapted to live 

 in only certain ecological conditions; where those conditions exist, 

 there the species may be. In the American grasslands, for example, 

 the grama and buffalo grasses occur only on the better drained 

 areas which are not too sandy. In the sloughs another set of plant 

 species occur and on the sand ridges a third set is found. Some 

 species of marine plankton probably approach most closely a uni- 

 form distribution of individuals over the entire geographic range, 

 but these species also encounter ocean currents or areas having 

 inimical ecological conditions which result in lacunae within the 

 species range. As a result each species occupies only certain hab- 

 itats; the exact areas in which these specific habitats occur will be 

 interrupted or separated by habitats of other types. 



RANGE OSCILLATIONS 



The range of a species is dynamic; its edges continually change to 

 a greater or lesser extent in amoeboid fashion. Peripheral denies 

 may become connected with or disconnected from each other and 

 more central populations. As Andrewartha and Birch (1954) ex- 

 plained in detail, the dynamic nature of a range is caused by two 

 factors: the dispersal of the species and fluctuations in ecological 

 factors limiting the geographic distribution of the species. 



Fluctuation is as much a characteristic of each ecological factor 

 as genetic change is characteristic of a species. Some fluctuations 

 such as daily, seasonal, or annual rhythms of temperature or rain- 



