Plant genetic and species diversity can impact herbivory and herbivore populations. However, the functional consequences of plant trait variation on herbivores is poorly understood. The two mechanisms by which trait variation might impact herbivores are nonlinear averaging, in which variability effects differ from the mean because of nonlinear impacts of plant traits on herbivore performance; and time-sensitive effects, in which emergent impacts of trait variation on herbivores are due to constraints on the physiology and behavior of individual herbivores. We conducted a series of experiments to manipulate the mean, variance, and properties of the timing of the plant defensive chemical (xanthotoxin) in the diet of a generalist herbivore (Trichoplusia ni).
Results/Conclusions
Defense variability consistently decreased herbivore performance. The impacts of defense variability on herbivore performance were caused by time-sensitive effects and were poorly anticipated by nonlinear averaging. As a consequence, the exact timing (frequency, phase, and skew) with which an herbivore experienced variation in plant defense strongly affected the consequences of that variability on individual herbivores and population growth rates. The time-sensitive consequences of trait variability carried on to a higher trophic level, where trait variability affected the performance of T. ni’s specialist parasitoid (Copidosoma floridanum). Defense variability decreased herbivore performance, but it did so in a way that is scale-dependent and difficult to anticipate based the effects of mean defense. This highlights the need for work on the impacts of trait variation on species interactions at multiple scales and to find generalities among systems.