724 



ECOLOGY AND EVOLUTION 



communities (Allan, 1936). Species in the 

 United States, such as the house mouse, 

 house rat, German roach, Mediterranean 

 fruit fly, European comborer, and Enghsh 

 sparrow, succeed mainly under the protec- 

 tion of human agriculture or architecture. 



Just what prevents these organisms 

 from invading those communities not modi- 

 fied by man is not known in detail, but it 

 is fairly obvious that there is a biotic 

 barrier. It is true that most success- 

 fully introduced animals succeed under cli- 

 matic conditions similar to those of their 

 native habitat, and the number of pests 

 introduced into the United States from 

 temperate Europe and Asia far outweigh 

 the number originating in the American 

 tropics with an easy access by land. 



However, climate is hardly the explana- 

 tion for their limitation to the crops and 

 dwellings of man. A few species adapted 

 to warm climates may extend their normal 

 climatic range by hving in heated dwell- 

 ings—for example, the Argentine ant, 

 Iridomyrmex hwnilis, in University of 

 Chicago buildings, and the common east- 

 ern termite, Reticulitermes flavipes, in 

 buildings in Superior, Wisconsin. In many 

 other organisms, the factor preventing the 

 extension of range seems to involve the 

 biotic environment. 



First let us examine a few examples of 

 introduced animals that succeed only in a 

 man-modified habitat. The English sparrow 

 (Passer domesticus) is a fairly critical case. 

 This species was first introduced into New 

 York City in 1850, and more individuals 

 were subsequently again released in the 

 same area. The sparrows rapidly spread 

 over most of the United States. The ntrni- 

 bers of individuals, however, are in direct 

 proportion to the degree of environmental 

 modification wrought by man. The great- 

 est abundance is reached in the cities 

 where few native birds are to be foimd, 

 the numbers decrease on the edge of tovvoi 

 and in the country districts where native 

 birds are more abundant, and the sparrow 

 is about as rare in virgin woods or prairies 

 as native birds are rare in the centers ot 

 large metropohtan areas (see Bumpus, 

 1898). 



Other examples in which introduced ani- 

 mals are bmited to human habitats are 

 found among insect pests causing serious 

 economic damage. The Mediterranean fruit 



fly {Ceratitis capitata) was discovered in 

 Florida in 1929 attacking citrus fniits and 

 avacado pears. Although it spread over 

 about a third of the state, it was com- 

 pletely exterminated by state and federal 

 agencies in 1930 through the control of 

 the citrus and avacado crops at a cost of 

 seven million dollars. In the laboratory it 

 was found that this fly could be raised on 

 a great many different kinds of wild fruits, 

 but it was never found in wild host plants 

 away from cultivated orchards in Florida. 

 In Hawaii the Mediterranean fruit fly com- 

 pletes its hfe cycle in the introduced wild 

 guava. 



The yellow fever vector, Aedes aegypti, 

 introduced into Brazil, spread only through 

 human habitats in cities and rural regions, 

 but did not invade natural habitats similar 

 to those originally harboring these mosqui- 

 toes in Africa. 



The mosquito. Anopheles gambiae, 

 native to the tropical belt of Africa, was 

 introduced into eastern Brazil in 1930 and 

 rapidly spread over 12,000 square miles 

 north and west, but always in the vicinity 

 of human habitations. Malaria of a virulent 

 form accompanied the introduction of this 

 species. In the first half of 1938, 100,000 

 cases and 14,000 deaths occurred. Non- 

 human communities were not invaded, thus 

 making complete eradication possible by 

 1940 through the joint efforts of the Rocke 

 feller Foundation and the Brazilian govern- 

 ment (Soper and Wilson, 1942). 



In 1942, Anopheles gambiae invaded 

 upper Egypt, reaching within 200 miles of 

 Cairo. Again, the mosquito successfully 

 bred only in the vicinity of human habita- 

 tions. Again, it carried virulent malaria 

 that caused the death of 130,000 persons. 

 By early 1945 the species was completely 

 eradicated from the area of its introduction 

 by appropriate control measures. 



The termite, Cryptotermes dudleyi, was 

 introduced from the Orient into Panama as 

 early as 1890 and is still a common termite 

 in Panamanian houses, but it has not in- 

 vaded the natural communities inhabited by 

 closely related native species (Emerson, 

 1947). In one instance, this termite oc- 

 cupied furniture in a house within 40 feet 

 of dense native rain forest; a thorough 

 search for the species in the natural habitat 

 failed to disclose its presence, although 

 ecologically equivalent termites were abun- 



