726 



ECOLOGY AND EVOLUTION 



island habitats, thus indicating ancient dis- 

 tribution. Thus, for long periods of time, 

 what appears to be the biotic barrier of 

 the continental communities has prevented 

 the establishment of this otherwise tropi- 

 copolitan group of insects that probably 

 has been continually dispersed in floating 

 logs. 



Phalangers are the only marsupials that 

 have successfully invaded the Celebes. 

 Mayr (1944) thinks it hkely that phalan- 

 gers have repeatedly landed on Borneo, 

 but have been unable to withstand the pre- 

 dation by the abundant placental mammals. 

 Borneo was recently part of the Indomalay- 

 an continent, while the Celebes have prob- 

 ably been insular through Tertiary times. 



We do not know the exact nature of the 

 biotic barriers that exclude these various 

 animals from habitats foreign to them. The 

 introduced or invading organism first must 

 have the biotic essentials, such as food, to 

 enable it to exist (p. 634). There is some 

 indication that phytophagy or predation 

 may exclude some organisms. Various de- 

 grees of competition may exclude others. 



Phytophagy is probably the reason why 

 many vegetable and ornamental plants fail 

 to survive when introduced by man into 

 natural communities. In the vicinity of a 

 biological station in the British Guiana 

 rain forest, leaf-cutting ants (Atta) de- 

 stroyed such plants almost as soon as 

 leaves were produced. 



Predation seems to exclude certain in- 

 troduced animals from native habitats. The 

 Norway rat (Rattus norvegicus) lives far 

 away from buildings in Wisconsin during 

 the spring, summer, and fall, but the win- 

 ter population under corn shocks is nearly 

 annihilated, particularly by the great 

 homed owl, Bubo virginiamis (Errington, 

 1946). 



Biotic restriction to certain natural habi- 

 tats is surmised by Barney and Anson 

 (1920), who suggested that the pigmy 

 sunfish, Elassoma zonatiim, which is popu- 

 lous in thick submerged vegetation, would 

 succumb to predator fishes outside this 

 habitat. 



An example that possibly illustrates an 

 ancient barrier of predators may be drawn 

 from termite distribution. In the subfamily 

 Nasutitermitinae (Fig. 263), the most 

 primitive genera (Syntermes, Cornitermes, 

 Armitermes, and so forth) are confined to 



the neotropical region, while specialized 

 derived genera {Nasutitermes and Subuli- 

 termes) are tropicopolitan. The primitive 

 genera all have mandibulate soldiers ex- 

 hibiting gradations of defensive adaptation 

 progressing in the direction of the nasute 

 soldier, which has regressively lost its 

 functional mandibles and protects the 

 colony from such general predators as ants 

 by the forceful ejection of a chemically 

 irritating viscid fluid from a frontal projec- 

 tion of the head (p. 426; Fig. 149). From 

 the present distribution of Nasutitermes and 

 Subulitermes and their relatives, including 

 many indigenous derived genera in such re- 

 gions as Australia, Indomalaya, Africa, and 

 South America, it may be presumed that 

 dispersal of these nasute groups occurred 

 during Cretaceous times or earlier. The 

 primitive genera of the subfamily were 

 doubtless in existence at the time when 

 Nasutitermes and Subulitermes invaded the 

 Old World from South America. There is 

 no indication that a geographic, climatic, or 

 food barrier existed. The flying powers of 

 the primitive genera and the nasute genera 

 are about equally weak. The obvious adap- 

 tive diflFerence between the genera in the 

 phylogenetic sequence is in the defensive 

 ability of the soldiers. The most probable 

 hypothesis is that a predatory barrier pre- 

 vented the dispersal of the less defended 

 genera, while the more specialized and de- 

 fended genera were able to overcome this 

 barrier. 



Competition \vith ecologically equivalent 

 species is possibly the major biotic barrier 

 in the majority of cases. Ecological investi- 

 gations are insuflBcient at present, however, 

 to allow more than a guess concerning the 

 exact nature and quantitative eflFects of the 

 excluding factors. Competition vvdth re- 

 motely related species seems to operate in 

 some instances, while in others the com- 

 petition is between closelv related species 

 It has already been pointed out (p. 656) 

 that ecological equivalence is likely to be 

 greater the more closely related the species 

 are phylogenetically. 



We may imagine that native birds, bet- 

 ter adapted through long selection to their 

 natural habitat, succeed in excluding the 

 English sparrow through direct competi- 

 tion. A flicker (Colaptes auratus) has been 

 seen destroying nests of sparrows with eggs 

 and young occupying former flicker holes 



