Origm of Biomes and Succession 289 



been more restricted to the area occupied by the hve-oak wood- 

 land. On this basis, it is possible that more recently the chaparral 

 community has become adapted to xeric conditions beyond the 

 limit to which the live-oak woodland has become adapted. If such 

 is the case, then where the woodland does occur, the chaparral is 

 a true subclimax community, but in areas slightly too xeric for 

 the woodland, the chaparral would be the climax community. This 

 concept gives a rational explanation for the previous observation 

 that many species may be members of a subclimax community in 

 one area and of a climax community in another. The concept leads 

 further to the idea that a community may be a subclimax stage 

 under certain conditions and the climax stage under others. 



2. Substitution of Dominants 



During the history of life, dominants in biomes such as the tropical 

 rain forest and marine organic reefs have changed without chang- 

 ing the general physical nature of the community. In other in- 

 stances, dominants supplanting the old have changed the physical 

 nature of the community drastically. The part thus changed has 

 in essence become a new biome. 



The effect on the community of substituting even a single domi- 

 nant species is shown strikingly by the introduction of the Eu- 

 ropean perennial herb, St. John's wort, Hypericum perforatum, into 

 the arid grazing areas of Australia and California ( Holloway, 1957 ) . 

 In California the native dominants in range land communities are 

 arid-tolerant annual grasses. The Hypericum out-competed these 

 grasses for water and virtually supplanted them wherever the Hype- 

 ricum spread. Thus a grass community was converted into one 

 dominated by a luxurious shrubby herb growing to a height of up 

 to five feet and casting a dense shade. 



The possible effect of adding additional species to the same 

 community is shown by several insect species introduced as control 

 agents for this Hypericum. These insects were imported first into 

 Australia, then into California, in an effort to reduce the weed. A 

 leaf -feeding beetle Chrysolina gemellata proved to be especially ef- 

 fective in reducing the extent of the Hypericum. The beetle did 

 not exterminate its host but reduced it to a negligible part of the 

 vegetation or to small ephemeral patches. Thus this one species of 

 plant predator has eff^ected another substitution of a dominant, 

 suppressing the Hypericum to the point that the area has reverted 

 to being primarily an annual grassland. 



