Background/Question/Methods
Though plants' facing early damage is a common scenario, few studies focused on the fitness consequences of herbivory damage at early ontogenetic stages. In the present plant-herbivore system, previous studies found that selection acted favoring plants (Datura stramonium) damage by its herbivore (Epitrix sp.) during early ontogenetic stages, and concluded that herbivory damage increases plant fitness. The aim of this work was to disentangle the mechanism behind the proposed positive effect of damage on plant fitness. During the summer of 2005, a total of 800 plants (400 per treatment) were grown in an experimental plot (Teotihuacán, México). The abundance (natural or low) of the herbivorous insect was manipulated applaying an insecticide during the first 30 days of growth to half of the experimental plants. For each plant we measured: initial size of plantings, foliar damage, growth rate, flowering time, and total fruit production.
Results/Conclusions
The application of the insecticide produced a reduction of 52% in the mean level of early damage which in turns reduced the mean of the flowering time in four days, and increased growth rate (6%) and fruit production (47%). The covariance between early damage and plant fitness was not significant, suggesting that the response of plants was not related to a mechanism devoted to reduce (resistance) or increase the amount of damage (overcompensate). Results from path analyses within each tratment indicated that herbivores preferred large juvenile plants, which flowered earlier and had higher fruit production. Because early flowering increases plant fitness and herbivory accelerated time to flowering, a positive indirect effect of herbivory on plant fitness was observed. The significant interaction treatment x growth x flowering time indicated that the amount of damage altered the selective value of the correlation between growth and flowering time with plant fitness. At higher herbivory damage selection favored an increment in growth and rapid swith to reproduction incurring a cost reducing plant size. If plant size before reproduction is an important ecological trait under competitive condition, the selective pressure exerted by early damage can interact with selection imposed by competition. The results indicated that the plants response to early damage by Epitrix sp. was related to a change in the time to flowering rather than to a mechanism of overcompensation.