80 REACTION: THE INFLUENCE OF COMMUNITY ON HABITAT 



community of ants often producing a much greater effect than a single 

 large mammal. 



In the brief outline that follows, the divisions are based upon 

 activity and effect, the subdivisions being made in general correspond- 

 ence with similarity in life form, size, or behavior. The multiform 

 effects exerted by man are reserved for treatment at the end of the 

 section on land reactions. 



Digging and Burrowing. Digging may produce hollows, holes, or 

 galleries, or a more or less complicated system of tunnels and cham- 

 bers to form a burrow or den. These may serve for nests, shelter and 

 housing, for storage, sanitation, or for various other purposes. The 

 dirt freed may be merely thrown out for a short distance, it may be 

 utilized for diking, or may be carried to some distance to be deposited ; 

 some of it may be compacted into plugs for sealing abandoned bur- 

 rows and entrances as is done by some pocket gophers. Its transport 

 may lead to the formation of paths with the compacting of particles, 

 and its piling, systematic or otherwise, amounts in effect to primitive 

 cultivation, while both processes produce coaction effects in destroying 

 plant cover and the latter by stimulating it also. In general, the con- 

 sequence brought about by a single individual or family is slight 

 (see Fig. 15, page 74), with the exception of such large mounds as 

 those of kangaroo rats, and hence digging coactions are most signifi- 

 cant where aggregation becomes pronounced, as in the so-called towns. 



By far the most important burrowers are the rodents belonging to 

 the fossorial life habit or mune. While some members of the carni- 

 vores, such as badgers, skunks, and even some wild dogs, do dig holes, 

 they are usually scattered and the importance correspondingly less. 

 In the absence of quantitative studies of burrowing reactions, it is 

 impossible to do more than compare different genera on the basis of 

 size and activity, degree of aggregation, and general effect. Perhaps 

 the most widely important in North America are the pocket gophers, 

 followed more or less closely by marmots and prairie dogs (Fig. 16), 

 ground squirrels, kangaroo rats (Vorhies and Taylor, 1922), rats and 

 mice generally being of much less consequence. Some of these may 

 burrow to a depth of fifteen to twenty feet, translocating a prodigious 

 amount of earth for a relatively small animal, while others, penetrat- 

 ing but a foot or two into the soil, may construct mounds several yards 

 across and two or more feet high, or may group small mounds so 

 closely as to cover more than half the surface. In constructing a 

 system of tunnels, the kangaroo rat is probably the most skillful ; the 

 viscacha of South America develops a unique set of surface trenches 

 a foot or more in depth for conveying loose dirt (Hudson, 1892:294), 



