ADAPTATIONS AND DISTRIBUTION 



tion of a specific type; ami it is es[iecially usetul when correlated with many 

 data of a positive kind on a given species. It has been noted, tor exani[-)le, that 

 Scaphiopus couchil and 5. hurtcrii are found breeding ahnost exclusively in 

 the shallower pools where both shallow and deep pools seem ec]ually avail- 

 able as hundreds of the animals are breeding. Species of Bufo manifest the 

 same tendency, but Scaphiopus bombijrons and S. hammondu show the op- 

 posite. Selection as to depth of water, therefore, tends to serve as an isolating 

 mechanism among groups of species of spadefoots (Scaphiopus) but not 

 among species of Bufo in regions where these selections are possible to the 

 animals. In playas of the Texas and Oklahoma panhandles, however, species 

 rarely found together may use various portions of a single pool, the spade- 

 foots breeding in the deeper water, the garden toads in the shallows at the 

 edges. Pseudacris triseriata, normally breeding in twig or grass strewn pools, 

 also uses playas on the high plains, since these are the only places available 

 in many regions. The adaptability of this species to these situations (and 

 others) is probably one of the main reasons for its wide distribution, for no 

 species of animals can long occupy an area in which its individuals cannot 

 successfully breed. 



Some positive observations also must be used with discretion. For ex- 

 ample, if one finds tadpoles in a pool or juveniles about it, this affords no 

 assurance in and of itelf that the pool was used by the species for breeding. 

 This is evident from the following considerations: (1) juveniles may have 

 migrated here to escape desiccation as their natal pool evaporated. Rana ber- 

 landieri, R. catesbeiana, and Acris crepitans are known to have done so in 

 several instances. (2) In streams (or in some pools) a current, not evident 

 later, may have deposited tadpoles in the location where observed. One case 

 involving S. bombijrons is known with certainty, and several involving R. 

 berlandieri , R. catesbeiana, Bufo t. americanus, B. ii'oodhottsii fowlen, and 

 B. iu. woodhousii were probably of this type. 



Some of the factors apparently influencing successful prediction of the 

 use of water as a breeding site are depth, temperature, turbidity, shade (or 

 lack of it), water movements, extent, permanence, type of bottom, presence 

 and relative abundance of potential food (algae and other aquatic plants), 

 associated invertebrate fauna, and nature of the terrain about the site. These 

 factors are, of course, rarely independently acting agencies; all are or may 

 be interrelated. The presence of some invertebrates (e.g., fairy shrimps) sug- 

 gests temporary water; of others (e.g., clams), permanent water. Extent and 

 depth together similarly indicate temporary or permanent pools, although 

 the type of bottom often influences this. Turbidity influences the types of 

 algae present, as well as the amount produced per unit time; but this also 

 is modified by temperature as well as by the chemical nature of the pool 

 which in turn is influenced by the terrain and the geological nature of the 

 exposed rocks and soil. 



In these complex situations it is impossible as yet to trace precisely the 

 interactions of all factors, but those of major importance can usually be de- 

 limited roughly. However, a minor influence in one place may become a 



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