EFFECTS OF MAMMAL AND INSECT HERBIVORY ON POPULATION-DYNAMICS OF A NATIVE CALIFORNIAN THISTLE, CIRSIUM-OCCIDENTALE

We explored consequences of spatial and temporal heterogeneity in herb ivory on the survival, growth, and reproduction of the Californian nat ive dune thistle, Cirsium occidentale, in coastal and inland sites, fo r 2 years. We assessed the relative impacts of insect and mammalian he rbivores and compared the relative importance of herbivory in coastal and inland habitats and among locations with different microclimates a cross a coastal dune. Effects of insect and mammalian herbivores were tested with a combination of insecticidal spray and cage exclusion tre atments in a factorial experiment at the coastal site. Mammalian herbi vores strongly affected the population dynamics of C. occidentale in b oth years, and their effects were augmented by fungal infection (1991) , herbivory by stem-borers (1990) and, to a lesser extent, by insect s eed predators in both years. Mammals caused most plant deaths, but the mammal species responsible differed among sites. Rabbit herbivory alt ered the vegetative growth of coastal thistles and significantly modif ied other key aspects of Cirsium demography, including growth rate and timing of reproduction. Small, uncaged plants grazed by rabbits took at least 1 year longer to mature than did caged plants. Larvae of Pyra usta subsequalis were the only insects that killed established plants. In 1990 and 1991, the numbers of insects damaging seed heads before d ispersal were low, but were sufficient to cause receptacle and seed da mage. The number of mature, undamaged seeds (and percent successful se ed production) was reduced significantly only for heads infested by fu ngi near the ocean in 1991: the fungus occurred in 37% of heads and ca used a 77% reduction in mature seeds.