Intraspecific competition and subterranean herbivory: individual and interactive effects on bush lupine

High mortality of plants growing in dense monospecific stands (i.e. self‐thinning) usually results from intense intraspecific competition. However, inconspicuous below‐ground insect herbivory might be a potent but overlooked source of mortality within dense stands of plants, particularly if crowding limits a plant's ability to compensate for herbivore damage. Here I ask how high conspecific density influences a plant's ability to cope with heavy below‐ground insect herbivory.
 I manipulated conspecific density and exposure to an abundant root‐borer, the ghost moth ( Hepialus californicus ), and examined the impacts on the fecundity, growth, and survival of bush lupine ( Lupinus arboreus ), a fast‐growing shrub that grows in dense monospecific stands in coastal grasslands. Both herbivory and intraspecific competition affected seed production, size, and mortality of bush lupine over the two years of the experiment. Plants consistently produced fewer seeds when growing at high versus low density and ghost moth herbivory also significantly reduced seed production. The negative effects of herbivory on plant fecundity were similar, regardless of plant density. In contrast, plant survival was affected by both competition, herbivory, and the interaction of these factors. In high density plots, plant survival was uniformly low (averaging 0.45–0.50); plants exposed to herbivores died from heavy herbivory, and plants protected from herbivores died due to intense intraspecific competition that compensated for losses due to herbivory. In low density plots, ghost moth herbivory similarly reduced lupine survival, from an average survival probability of 0.94 in plots protected from these herbivores to 0.55 in plots exposed to herbivory. Thus, results show that regardless of plant density, below‐ground herbivory can be a potent source of mortality.