474 



inated by a single plant species as a rule. 

 Under such conditions there is a correla- 

 tive increase in the number of insects ad- 

 justed to this abundant, relatively uniform, 

 concentrated food supply. Many of our 

 agricultural pests, such as grasshoppers, 

 the squash bug {Anasa tristis), and the 

 chinch bug {Blissus leucopterus) , derive 

 their abundance from such unintentional 

 human cooperation. Man then attacks such 

 herbivores by spraying this stratum with 

 insecticides or otherwise applying economic 

 checks and controls to maintain a condition 

 )f biotic instability adjusted in his favor, 

 [n this he is aided by the natural stratal 

 predators. Included in this latter category 

 are numerous insects (phymatid and re- 

 duviid bugs, many carabid beetles, para- 

 sitoid wasps, and a large number of grass- 

 land spiders). 



Insectivorous lizards (for example, Taky- 

 dromus of Eastern Asia and Chamaesaura 

 of Africa) with elongated tails and/or 

 bodies, are structurally adjusted for mov- 

 ing rapidly through the grassland canopy 

 or over the surface of the herbaceous 

 stratum. In these grassland reptiles we have 

 a stratal convergence analogous to the 

 position occupied by the gyrinid whirli- 

 gig beetles and gerrid bugs, which skate 

 over the surface of fresh-water habitats. 

 The small gyrinid beetles also dive below 

 the surface somewhat as the lizards weave 

 back and forth between the grass stems. 

 In both hmnological and grassland com- 

 munities these organisms occupy equivalent 

 stratal and feeding niches. 



In this predation all strata are more or 

 less involved. Subterranean floor, and her- 

 baceous levels eflfect occasional or contin- 

 uous control on plant and animal increase. 

 The usually vegetarian meadow mouse will 

 eat stray insects; our most typical prairie 

 birds (homed lark, meadow lark, prairie 

 chicken, and their stratal equivalents) are 

 generally floor feeders. Such birds typically 

 have a seasonal shift in diet (Chap. 28). 

 For example, the prairie chicken feeds 

 upon insects (especially grasshoppers) 

 from April to October and chiefly on vege- 

 tation or its products between November 

 and March; the horned lark takes up to 

 20 per cent animal (chiefly insects) and 80 

 per cent plant food, while the meadow lark 

 takes 73 per cent animal (chiefly grasshop- 

 pers, beetles, and floor caterpillars), and 



THE COMMUNITY 



27 per cent plant food. This complex inter- 

 stratal predation forms but a part of the 

 complex food web of a community (Chap. 

 27). 



With stratification as a principle of or 

 ganization, it follows that there is a high 

 degree of stratal equivalence or replaceabil- 

 ity among the constituents of any stratum 

 within the limits of their position in the 

 gradient. The principle of stratal equiva- 

 lence directs attention to the fulfillment of 

 similar ecological requirements and, con- 

 sequently, to the segregation of community 

 constituents upon an ecological rather 

 than a taxonomic basis. Similarly, consider- 

 ation of stratal equivalence focusses atten- 

 tion upon the principle of community habi- 

 tus. The presence of strata, with all that 

 this implies, gives a characteristic aspect 

 to a given community type. 



Examination of the whole system of 

 grassland communities indicates that three 

 influences exert a profound effect upon 

 present day community maintenance and on 

 current shifts in community boundaries. 

 Two of these influences (precipitation and 

 temperature) exert their initial effect as 

 extracommunity forces, operating singly or 

 in unison, in extremes of local weather. The 

 third influence is biotic. Here should be 

 included the activities of man, involving 

 both economic and noneconomic aspects of 

 his civilization and having to do with all 

 manner of commercial and agricultural en- 

 terprises—for example, the effects of cer- 

 tain kinds of pollution upon the herba- 

 ceous cover of the grasslands, or their in- 

 tense cultivation for food or forage. In addi- 

 tion to man's influence, there is the in- 

 fluence of the vast populations of wild her- 

 bivores (Table 35), which certainly act as 

 an important biotic control within the grass- 

 land communities of which they are a nat- 

 ural part. 



Since the distribution and seasonal 

 growth of grasses are influenced by tem- 

 perature, local abnormalities of this factor 

 may alter the extent of a grassland com- 

 munity from year to year. Any disturbance 

 of the biotic balance is reflected in all 

 parts of the community. The subject has 

 been given increased attention, especially 

 the relative influence of air versus soil tem- 

 perature. In general it may be stated that 

 soil temperature has a much more impor- 

 tant role. In controlled experiments with 



