Competition for light, plant-species replacement and herbivore abundance along productivity gradients

This chapter argues that patterns of herbivore abundance along productivity gradients depend on the kind of resource that plant species are competing for. Recently published plant-herbivore theory that considers competition for nutrients as its point of departure predicts a general increase in herbivore abundance with increasing productivity, as well as a change in plant species composition from good nutrient competitors at low productivity to good grazing tolerators at high productivity. We develop a model that combines plant-herbivore theory with a mechanistic model of competition for light, and show that competition for light may lead to other patterns than competition for nutrients. For example, suppose that tall plants of low palatability shade small plant species preferred by the herbivore. In this case, the herbivore may decrease in abundance or even disappear with increasing productivity. Furthermore, multiple stable states may occur. In one state tall plants of low forage quality outshade small plant species, whereas in the other state small plant species of high forage quality support a large herbivore population. Field data from salt marshes grazed by intermediate-sized vertebrate herbivores like rabbits, hares and geese support these predictions.