Description
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 revealed that mammals affected the mortality of Ceanothus greggii seedlings to a greater extent than competing Adenostoma fasciculatum seedlings. Insecticide applications in conjunction with the enclosure experiments showed that insects caused a further significant increase in Ceanothus mortality, but did not affect Adenostoma. Close observations of 428 tagged seedlings combined with insect and mammal surveys indicated that the major mammalian herbivores (brush rabbits) preferred Ceanothus over Adenostoma seedlings; and that the major insect herbivores (psyllids) were specific to Ceanothus. These herbivore pressures were sufficient to reverse a competitive advantage of Ceanothus seedlings in the absence of herbivory. Plant physical and chemical parameters which may influence herbivore preference are the greater seedling water content of Ceanothus and the higher levels of non-tannic phenolics of Adenostoma. The relatively high levels of tannins in Ceanothus were ineffective in preventing herbivory by small mammals. Insect family diversity and relative density 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. The effects of sucking insects on shrub seedlings were studied by monitoring growth rates of root and shoot systems of potted seedlings sustaining different levels of infection by phloem feeders. A general model of the response of mediterranean shrub seedlings to sap-feeding herbivores was proposed.
