108 



COACTION: THE INTERRELATIONS OF ORGANISMS 



in character, but similar in producing a new chain of coactions, is the 

 introduction, intentional or otherwise, of exotic plants or animals. In 

 the case of cultivated plants and weeds, this coaction has played the 

 paramount role in every agricultural region, as a sequel to the coac- 

 tions concerned in clearing of land of all kinds. It is likewise seen 

 in the introduction of domestic animals, or of such semi-feral ones 

 as the rat, English sparrow, and numerous insect pests, as well as in 

 that of disease-producing organisms, both plant and animal, of which 

 wheat rust and the bacillus of cholera are examples. 



1800 1810 1820 1830 1840 1850 1860 1870 1880 1890 1900 1910 



Fig. 20. — Showing the effect of settlement on the mammals of central Illinois. 

 The decline of the Wildcat, Lynx rujus (Schreb), and wolf, Canis nubiliis Say, 

 early reduced by trappers and first settlers, was accompanied by an increase in 

 deer, Odocoileus virginianus (Bod.). The gray fox continued in full numbers 

 until about 1850-1860. The decline of this species and of the deer was probably 

 due as much to destruction of the forest habitat as to hunting. (After Wood, 



1910.) 



IModifications of the community by native animals are less wide- 

 spread and on a smaller scale as a rule, but are essentially identical 

 in nature. The grazing coaction of antelope and bison probably dif- 

 fered only in degree and perhaps in slightly different preferences 

 from that of cattle on the open range today. The aggregation of cer- 

 tain rodents in "towns" produces more serious effects, though these 

 are too local and restricted in area to modify the climax materially. 

 Most striking of all such coactions are those caused by grasshoppers 

 in migration, which sometimes leave hardly a vestige of field crops. 

 Successive defoliations by caterpillars have been known to cause the 

 death of an aspen subclimax over many square miles of mountainside 



