Population consequences of constitutive and inducible plant resistance: herbivore spatial spread

Little attention has been paid to the impact that constitutive and inducible plant resistance traits will have on herbivore spatial dynamics. We investigate mathematical models in which herbivore demographic rates and movement rates respond to host plant quality, which in turn is determined by constitutive and inducible resistance. Models with and without induced resistance yield the same analytic expression for the asymptotic speed at which a herbivore population will spread through an initially uninduced plant population, suggesting that induced resistance will have no effect on the rate of invasion of herbivores that respond to plant resistance on small spatial scales. In contrast, constitutive resistance will influence the speed of an invasion. If herbivore movement is quite sensitive to plant quality, an increase in constitutive resistance can actually accelerate the rate of herbivore spread even while it reduces the herbivore's intrinsic rate of increase. In other scenarios, the rate of invasion attains a maximum at intermediate levels of constitutive resistance. These results argue that our view of plant resistance should be broadened to include herbivore movement if we are to understand fully the implications of differences in resistance for the dynamics of herbivore populations in natural and managed settings.