Unexpected density‐dependent effects of herbivory in a wild population of the annual Collinsia torreyi

Summary 1  Herbivory is known to affect plant fitness, but has rarely been studied under realistic conditions, i.e. by manipulation of real herbivores in a wild plant population. Furthermore, its interaction with competition, another factor known to affect plant physiology and reproduction, has usually been neglected. 2  Interactions among plant density, insect herbivore damage and plant fitness components (survival and lifetime fecundity) were investigated by both observation and experiment in a population of a native montane annual, Collinsia torreyi , and its primary insect herbivore, the butterfly Euphydryas editha . Larvae were manipulated to inflict predetermined levels of experimental damage. 3  Increases in either plant density or herbivory generally had negative effects on fitness. Loss of only 5% of seedling leaf area significantly reduced adult lifetime fecundity. 4  Herbivory reduced fitness at low, but not at high, plant density. This form of interaction between effects of competition and herbivory is theoretically unexpected and opposite in direction to interactions found in other published studies. Low resource availability may be an important factor. 5  Where such an interaction occurs, it would cause the selective effects of herbivores to be density‐dependent, i.e. when plants are crowded, the absence of selection would oppose any evolutionary increase in costly defences.