290 Origin of Biomes and Succession 



A similar example explained by Bess and Haramato ( 1958 ) is 

 afforded by the tropical American plant Eiipatoriiim adenophorum. 

 commonly known as pamakani. When introduced into Hawaii in 

 1860 it thrived and spread over the grassland areas, forming im- 

 penetrable thickets ten feet high which killed the grass. The change 

 in the dominant converted the grassland into a dense shrubland 

 (Fig. 126, top). A Mexican tephritid fly Procecidochares utilis, 

 which makes large galls on the Eupatorium stems and branches, 

 was introduced into the Hawaiian area in 1945. In a few years the 

 fly practically eliminated the Eupatoriinn from the more open and 

 drier areas, which reverted to grassland. In other areas, notably 

 some of the wetter slopes, the Eupatorium was less affected by 

 the fly. In this example the net result of the two successive in- 

 troductions was the initial threatened extinction of one biome by 

 the substitution of a dominant, then a checking effect by a plant 

 predator which has resulted in a new shrub community being 

 added along the wetter edges of the somewhat reduced original 

 grassland (Fig. 126, bottom). 



The examples cited above concern species artificially introduced 

 into the biomes, but nonetheless they give a good indication of the 

 effects of changes in community dominants and in host-parasite 

 (prey-predator) relationships. There is every reason to believe that 

 comparable changes in community and biome structure have oc- 

 curred many times and in many places due to the natural dispersal 

 of organisms from one part of the world to another. 



The possible origin of the angiosperm temperate deciduous forests 

 of the Holarctic region presents an interesting set of speculations. 

 In Cretaceous pollens of Minnesota, Pierce (1957, 1958) found 

 evidence suggesting that the Cretaceous forests of that area were 

 chiefly pine and other conifers, with a smaller, presumably sub- 

 dominant element of angiosperm trees. Many belonged to extant 

 genera such as Magnolia, Phtamis, Hamamelis, Quercus, and Tilia. 

 This situation is highly suggestive of two possibilities. One is that 

 the forest was much like that of the pine woods of southeastern 

 United States, in which the preponderance of pine may be main- 

 tained to a large extent by natural fire (sec p. 272). If so, the Min- 

 nesota observations may be one of the earliest indications so far 

 observed of the occurrence of natural fires. 



A second possibility is that the aggregation of species described 

 by Pierce was the true climax of that community and that since 

 then the angiosperm trees have supplanted the pines as the final 

 dominants. 



