Community Organization of Erigeron Glaucus Folivores: Effects of Competition, Predation, and Host Plant

Experiments were conducted to determine the relative importance of interspecific competition, predation, and host plant quality on the success of the common herbivores of Erigeron glaucus, the seaside daisy. Leaves of E. glaucus were attacked by spittlebugs (Philaenus spumarius), plume moth caterpillars (Platyptilia williamsii), and thrips (Apterothrips secticornis). Each species of herbivore was affected strongly by only one of the biotic factors examined. For spittlebugs, damage caused by caterpillars increased rates of desiccation and mortality of nymphs. This competitive effect probably caused the negative association between late—instar spittlebug nymphs and caterpillar damage wherever both species were encountered along the California coast. Plume moth caterpillars were most strongly affected by predator exclusion, at least during the late spring. Chicken—wire cages that excluded birds resulted in greater numbers of caterpillars on single rosettes and on whole clonal mats. Passerculus sandwichensis, Savannah Sparrows, were likely responsible for this effect which was only found during the season when adult birds were feeding insects to nestlings. On San Miguel Island, Savannah Sparrows were absent, densities of caterpillars were greater than on the mainland, and caging had no effect on caterpillar survival; these observations were all consistent with the hypothesis that Savannah Sparrows were important predators on the mainland. The factor that was most important for each herbivore species did not interact with other biotic factors. The relative ranking of the factors for each species was similar among the 3 yr of the experiments, with only one exception. Observations suggested that experiments conducted at Bodega Bay were representative of this community at other locations in California where all of these species were present. A scheme is proposed as a first step in predicting the importance of various biotic factors in herbivore community organization. If effects of predation, parasitism, or physical disturbance are very strong, host plant related competition and host quality are likely to be unimportant. If predation and disturbance are relatively less important, factors that reduce the choices available to an individual herbivore make host quality and interspecific competition more likely to play a significant role.