234 



ANALYSIS OF THE ENVIRONMENT 



The fact that the truncated hole-closing 

 head or abdomen is found to be indepen- 

 dently developed in many different types 

 of burrowing or crevice-inhabiting forms 

 clearly indicates an evolutionary response to 

 biotic pressure. It appears in annelids, in- 

 sects, arachnids, frogs, snakes, and mam- 

 mals. The development of an operculum in 

 snails, the rolling up of various beetles 

 and isopods, the closure of the combined 

 openings for the head and tail in the rolled- 

 up shield of the three-banded armadillo by 

 the head and tail shields and the various 

 modes of closure of the shell in turtles, form 

 a related class of protective devices. 

 Wheeler remarks further: "The phragmotic 

 insect, instead of secreting or construct- 

 ing a stopper, like the operculum or 

 epiphragm of snails and the earthen or 



special environments supplied by decaying 

 plant and animal masses are greatly modi- 

 fied in both physical and chemical pecu- 

 liarities by their organic components. 



The conspicuous biotic control of the cli- 

 matic environment as a whole shown by 

 social insects (p. 425) leads out of the 

 more casual or temporary modifications pro- 

 duced by aggregations of animals in more 

 simply organized groups, hke the sessile 

 marine forms. The modifications of currents 

 and wave action produced in the sea by 

 mats of algae and eel grass or by masses 

 of coral correspond to modifications of 

 microclimates on land. 



An illustrative example of biotic limiting 

 factors is afforded by the elf owl (Micr opal- 

 las whitneyi), which lives in arid parts of 

 California and Arizona. It nests exclusively 



Fig. 63. Phragmosis, illustrated by an ant and a spider. The ant (Colobopsis etiolatus) 

 is common in live oak galls in Texas. A, Soldier; B, head of soldier from in front. The spider 

 (right), Chorizops loricatus, of tropical America, shows the truncated end of the abdomen, 

 C, and a view from the rear, D. ( After Wheeler. ) 



silken barricades or doors erected at 

 the entrances of their burrows by many 

 ants, wasps, and trap-door spiders, actually 

 employs for the purpose a speciahzed por- 

 tion of its own body, thus affording proof 

 that no hard and fast fine can be drawn 

 between behavioristic activities on the one 

 hand and physiological and morphogenic 

 processes on the other," 



The microclimate may be influenced by 

 the animal components of the environmen- 

 tal matrix in various ways. The body 

 warmth of mammals must alter the air and 

 soil temperature in burrows or nests, which 

 become the habitat niche of a considerable 

 number of associated smaller forms. The 

 closely packed herd of musk oxen, in win- 

 ter, is said to be overhung by a sharply de- 

 fined fog blanket produced by the rising 

 exhalations of the animals. The important 



in holes made by two woodpeckers (Cen- 

 turus uropygialis and Colaptes chrysoides 

 mearnsi) in stems of giant cactus (Cereiis 

 giganteus). The range of the elf owl is 

 limited by the distribution of these two 

 biotic elements of its environment. The 

 woodpeckers, unhke the pigmy owl, are not 

 limited to this one cactus for nesting; they 

 excavate nest holes in other trees and 

 plants. The owl is Umited by absence of 

 cactus even though woodpeckers are pres- 

 ent, and by absence of woodpeckers even 

 though cactus is present, 



BIOTIC MODIFICATION OF THE 

 SUBSTRATE 



The concept of habitat niche as a specific 

 type of environment includes the broader 

 concept of habitat (as the total effective 

 environment within which an individual or 



