activities of man. Clearing of the land for agricultural purposes, 

 roads, or housing has resulted in the destruction or alteration of 

 large areas of California. Man's accidental (or intentional) intro- 

 duction of various weedy plant species also has had an important 

 effect on native plant communities. His introduction of grazing 

 animals such as sheep, goats, and cattle, as well as his indirect 

 effect on increasing population size of native herbivores such as 

 deer and rabbits by destroying predators, have had an important 

 influence (generally a negative one) on California plant commu- 

 nities. 



There are some plant communities that have been all but 

 eliminated in CaUfornia as a consequence of man's activities. At 

 one time large tracts of the Central Valley were occupied by a 

 grassland composed of various genera of perennial native grasses. 

 With the advent of agriculture and particularly of grazing domes- 

 ticated animals, the native grasses disappeared and have largely 

 been replaced by introduced annual grasses as well as by a few 

 introduced non-native herbs (such as filaree, a name applied to 

 \2inous Erodium spp.). These introduced grasses are satisfactory 

 for forage purposes, but they are a rather poor substitute for the 

 natural and more enduring plant community that they have 

 replaced (Plate 9D). Perhaps one slightly favorable by-product 

 of man's interference with the natural ecology of large areas of 

 the state is that the display of spring wildflowers may have been 

 locally enhanced by the eradication of native perennial grasses. 

 There is evidence that the abundance of various colorful native 

 herbs is greater in some man-made grasslands than in natural 

 native grasslands. Probably in the latter it is difficult for seedUngs 

 of annual herbs to become established because of the extensive 

 root systems of perennial grasses, whereas in the introduced 

 grasslands, the grasses, like the herbs, are annual and may offer 

 less competition for the native herbs because all start growing 

 at approximately the same time. 



Other influences of man on vegetation are the pollutants 

 released by industry and automobiles that are resulting in the 

 death of Ponderosa Pine trees over extensive areas of southern 

 California. 



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