Abstract
Plants have two principal defense mechanisms to decrease fitness losses to herbivory: tolerance, the ability to compensate fitness after damage, and resistance, the ability to avoid damage. Variation in intensity of herbivory among populations should result in variation in plant defense levels if tolerance and resistance are associated with costs. Yet little is known about how levels of tolerance are related to resistance and attack intensity in the field, and about the costs of tolerance. In this study, we used information about tolerance and resistance against larval herbivory by the butterfly Anthocharis cardamines under controlled conditions together with information about damage in the field for a large set of populations of the perennial plant Cardamine pratensis. Plant tolerance was estimated in a common garden experiment where plants were subjected to a combination of larval herbivory and clipping. We found no evidence of that the proportion of damage that was caused by larval feeding vs. clipping influenced plant responses. Damage treatments had a negative effect on the three measured fitness components and also resulted in an earlier flowering in the year after the attack. Tolerance was related to attack intensity in the population of origin, i.e. plants from populations with higher attack intensity were more likely to flower in the year following damage. However, we found no evidence of a relationship between tolerance and resistance. These results indicate that herbivory drives the evolution for increased tolerance, and that changes in tolerance are not linked to changes in resistance. We suggest that the simultaneous study of tolerance, attack intensity in the field and resistance constitutes a powerful tool to understand how plant strategies to avoid negative effects of herbivore damage evolve.
Highlights
Plant fitness may be reduced by herbivores directly, through the consumption of reproductive parts, or indirectly through consumption of vegetative parts impairing resource acquisition [1,2,3]
Our results show that damage both reduced the probability of flowering and altered the flowering phenology the year after treatment
Populations with higher attack intensities by A. cardamines in the field were more tolerant to larval herbivory under controlled conditions than populations experiencing low attack intensity [13]
Summary
Plant fitness may be reduced by herbivores directly, through the consumption of reproductive parts, or indirectly through consumption of vegetative parts impairing resource acquisition [1,2,3]. In the absence of herbivory, investment in high tolerance might reduce the available resources to other functions and plant fitness [7]. Variation in the intensity and type of herbivory among populations should create a pattern of varying tolerance mechanisms and degree of investment in tolerance as a defense among plant populations [11]. Relatively few studies have investigated differences in tolerance among populations within species, and examined how such differences are related to variations in herbivory intensity. Of these studies, some have found differences in tolerance between populations [12,13] whilst others have not [14,15,16]
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