Herbivores and Early Postfire Succession in Southern California Chaparral

The effects of insect and mammal herbivores on the regrowth of shrub seedlings and crown sprouts were studied during the early postfire period in southern California chaparral. Exclosure experiments and tagged seedling observations revealed that mammals affected the mortality of Ceanothus greggii (ceanothus) seedlings to a greater extent than competing Adenostoma fasciculatum (chamise) seedlings. Insects significantly reduced the growth of ceanothus, but did not affect chamise. The major mammalian herbivores (brush rabbits, Sylvilagus bachmani) preferred ceanothus over chamise seedlings, and the major insect herbivores (psyllids) were specific to ceanothus. These herbivore pressures were sufficient to eliminate a competitive advantage of ceanothus seedlings in the absence of herbivory. Plant physical and chemical parameters that may have influenced herbivore preference were the greater seedling water content of ceanothus and the higher levels of nontannic phenolics of chamise. The relatively high levels of tannis in ceanothus were ineffective in preventing herbivory by small mammals. Phytophagous insect population levels were monitored for two seasons and the proportion of total growth removed by chewing insects and mammals was measured for two seedling and three crown sprouting species.