POPULATION CONSEQUENCES OF CONSTITUTIVE AND INDUCIBLE PLANT-RESISTANCE - HERBIVORE SPATIAL SPREAD

Little attention has been paid to the impact that constitutive and ind ucible plant resistance traits will have on herbivore spatial dynamics . We investigate mathematical models in which herbivore demographic ra tes 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 a n initially uninduced plant population, suggesting that induced resist ance will have no effect on the rate of invasion of herbivores that re spond to plant resistance on small spatial scales. In contrast, consti tutive resistance will influence the speed of an invasion. If herbivor e movement is quite sensitive to plant quality, an increase in constit utive resistance can actually accelerate the rate of herbivore spread even while it reduces the herbivore's intrinsic rate of increase. In o ther 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 resis tance for the dynamics of herbivore populations in natural and managed settings.