EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



723 



Any one of the large community systems 

 is made up of many biocoenotic parts with 

 varying degrees of independence and inter- 

 dependence. Each part of the whole eco- 

 system exliibits a degree of independence, 

 and relatively high degrees of independ- 

 ence characterize the major communities of 

 the globe (p. 436). 



In addition to the physical boundaries of 

 biocoenoses and communities that are often 

 fairly obvious, subtle biotic barriers occur 

 at boundary hnes or regions (Cain, 1944, 

 p. 16). The evidence for the existence of 

 biotic barriers— biotic Hmitations to disper- 

 sal and survival— may serve to give us a 

 glimpse of some of the properties of bio- 

 coenoses and communities conceived as 

 large and highly complex interspecies units. 

 Boundaries assist in defining entities and 

 may later be used in the further analysis 

 and synthesis of the systems they limit. 



If natural selection gradually results in 

 balanced competition, exploitation, tolera- 

 tion, and mutuahsm leading to the adaptive 

 integration of the biocoenose or community, 

 one might expect to find that organisms 

 from other associations would not always 

 fit into such a balanced and coordinated 

 system, even though the physical environ- 

 ment were favorable. Favorable niches in 

 long-established systems would be satu- 

 rated with forms adapted to the biotic as 

 well as to the physical conditions (Robert- 

 son and Pearse, 1945). 



The concept of biotic barriers may be 

 tested by introducing animals and plants 

 from foreign associations and observing the 

 results. In most instances such tests have 

 not been performed consciously. With the 

 advent of modern transportation, many or- 

 ganisms are inadvertently introduced into 

 ancient balanced communities. These un- 

 witting experiments may be studied with 

 profit. 



The introduced organism sometimes 

 seems to be preadapted to the new environ- 

 ment, both physical and biotic. Such a 

 species may overrun the new habitat to the 

 detriment of the whole community. An ex- 

 ample is the introduction of the European 

 rabbit into Austraha (p. 643), where this 

 placental mammal found httle competition 

 trom the native marsupials, and an abun- 

 dance of food in a climate not dissimilar to 

 its original habitat. 



An instance without such drastic efiFects 



upon the natural community is found in 

 the introduction of the pheasant {Phasia- 

 nus colchicus torquatus) into North Amer- 

 ica, where it is kept within bounds by the 

 impact of the habitat and the sportsman. 

 Errington (1946) states that introduced 

 pheasants and Hungarian partridges {Per- 

 dix perdix), co-occupying the same tract 

 of land with bobwhite quail (Colinus vir- 

 ginianus) in Wisconsin, Uved at the ex- 

 pense of the quail, while native grouse did 

 not affect quail populations, possibly be- 

 cause of less ecological overlap. 



The introduction of the common honey- 

 bee {Apis mellifica) to the New World by 

 the early European colonists is another ex- 

 ample of an animal that adjusts to the com- 

 munity without previous evolutionary adap- 

 tation to the particular species assemblage. 

 Once brought in, honeybees would doubt- 

 less be abundant in the New World even 

 without domestication by man. 



Ancient invasions of preadapted animals 

 are indicated by correlated taxonomic, zo- 

 ogeographic, and paleontologic patterns. At 

 the time of the late Pliocene or early Pleis- 

 tocene land connection between South and 

 Central America about two million years 

 ago, physical and climatic highways for dis- 

 persal were established (p. 662). Many 

 mammals, including pumas, jaguars, small 

 cats, deer, peccaries, tapirs, and squirrels, 

 invaded South America from the north and 

 sometimes evolved endemic genera, while 

 others originating in South America, includ- 

 ing the armadillos and porcupines, invaded 

 Central and North America. Ground sloths 

 apparently reached North America earlier, 

 possibly via island connections, while pro- 

 cyonid carnivores and monkeys invaded 

 South America by the same means (Mayr, 

 1946). 



Biotic barriers did not prevent the dis- 

 persal of these animals, but other species 

 with equal physical opportunities did not 

 move into the new available regions. Para- 

 sites of these dispersing mammals often 

 moved with their hosts (Jellison, 1942). 



Although some preadapted organisms 

 succeed in entering new regions, it is note- 

 worthy that the majority of introduced 

 species that maintain themselves succeed 

 only in the highly modified environment of 

 man or in the impoverished biota of is- 

 lands and are largely excluded by the more 

 complex natural environment of continental 



