CHAPTER V 



BARRIERS TO DISTRIBUTION AND MEANS OF 



DISPERSAL 



The extremely rapid reproduction of living organisms causes them 

 to spread in every direction, constantly tending to enlarge the area 

 which they inhabit, so that no place in any way capable of supporting 

 life remains unoccupied. 



When a catastrophe such as a flood or a volcanic eruption destroys 

 life at some place, it is soon replaced. On August 26, 1883, the small 

 volcanic island of Krakatoa, 41 km. east of Java, was the scene of a 

 tremendous volcanic explosion, which destroyed it in part and covered 

 the remainder so thickly with ash and pumice that no plant or animal 

 was left. After only three years the soil was thickly filled with blue- 

 green algae, thereby being prepared for the advent of higher plants; 

 11 species of ferns and 15 flowering plants were found. A visit in 1897 

 showed further progress, with 12 ferns and 50 flowering plants estab- 

 lished. In 1906 a new examination yielded 114 species of plants, and 

 the composition of this flora was notably different from the earlier 

 ones. Animals had promptly followed the plants. Even in 1889 there 

 was a whole list of arthropods: spiders, flies, bugs, beetles, butterflies, 

 and moths; and even a species of lizard [Varanus salvator) was 

 present. A visit in 1908, 25 years after the eruption, yielded a collec- 

 tion of 263 species, of which 240 were arthropods; 4 species of land 

 snails were found; 2 species of reptiles and 16 birds composed the 

 vertebrate element. Investigations in 1920-1921 yielded 573 species of 

 animals, among which were one snake (Python reticulatus) , 26 breed- 

 ing birds, and 3 mammals (2 bats and Rattus rattus). Comparison 

 with neighboring islands shows that the fauna has about 60 per cent 

 of the expected number of species. 1 Rhizopods, rotifers, and tardi- 

 grades have been reported recently from Krakatoa that had not pre- 

 viously been recorded from that region. 2 The nearest island not 

 destroyed by the eruption, Sibesia, is 18.5 km. away, so that the new 

 inhabitants must have been brought from at least that distance by 

 means of wind and waves and other agencies of "fortuitous dispersal." 



The possible distribution of plants and animals, however, is not 

 unlimited. Dispersal does not take place with equal success in all 



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