no. 3 dunkle i plant ecology, channel islands 265 



Climax Changes 



Climaxes are, in the long run, unstable, since changes are constantly 

 taking place in climate, in soil, and in the biotic environment. Weather- 

 ing, erosion, and sedimentation are unceasingly active and their rate 

 of activity is being affected by the mutations of the climate and the 

 biota, as well as by diastrophic agencies affecting the topography. Vegeta- 

 tion, through the accumulation of humus, and the disintegration of rock 

 materials, continually modifies the chemical and phj^sical properties of 

 the soil. These changes, operating singly or in various combinations, 

 inevitably affect the floristic composition of an area. That which may 

 be of minor importance in a continental area, may have a powerful 

 influence upon the vegetation within the narrow limits of an island. 



The gradual rise of the ocean level and increasing aridity have 

 affected the insular environment since Pleistocene time. The influence 

 of man has been active through only a few centuries, but this effect 

 has been catastrophic. The original vegetation has been affected over 

 wide areas by overgrazing, wind and water erosion, and fires. Yet 

 the most potent instrument of modification brought about by man has 

 been the introduction of plants from other regions of the world as 

 previously described. These exotics have thrived for two reasons: (1) 

 they have come, for the most part, from dry regions and are better 

 equipped to withstand the present arid or semi-humid conditions than 

 the indigenous plants which are relicts of a more humid time; (2) the 

 exotics have survived for centuries because of adaptations which have 

 enabled them to withstand all the destructive agencies of man, whereas 

 the native flora had experienced few previous contacts with either man 

 or herbivorous animals. 



Many areas on all of the islands have been so changed that it is 

 now extremely difficult to envision the nature of the original climax 

 or the possible successional stages of the present sub-sere. Thus, broad 

 terraces, wide interior uplands, and long gentle slopes are now dominated 

 by introduced grasses and forbs. These areas of fine-textured, deep soil 

 are those most favorable for grazing. That such parts, or even a major 

 portion of them, were originally grassland Is open to doubt. Coarse 

 grasses, such as Sttpa lepida, S. pulchra, and Melica imperfecta are 

 present among herbs and suffrutescent perennials. Grazing, erosion, 

 and fire must have destroyed many suffrutescents and most shrubs (cf. 

 ante p. 261). Many sea bluffs and canyon slopes near the sea are 

 now dominated by introduced Mesembryanthemum and Atriplex. In 

 all these areas the original nature of the vegetation may be estimated 



