Grazing optimization, nutrient cycling, and spatial heterogeneity of plant-herbivore interactions: Should a palatable plant evolve?

Can the evolution of plant defense lead to an optimal primary production? I n a general theoretical model, Loreau (1995) and de Mazancourt et al. (1998 , 1999) have shown that herbivory could increase primary production up to a moderate rate of grazing intensity through recycling of a limiting nutrien t, provided several conditions are fulfilled. In the present paper, we assu me: (1) grazing intensity is controlled by plants through their level of pa latability; and (2) plant fitness is determined by its productivity. We exp lore the conditions under which such an optimal production may be reached t hrough natural selection. We model two competing plant types that differ on ly in palatability and are distributed in a patchy landscape determined by the plant-herbivore interaction. Patch size is determined by herbivore beha vior: herbivores recycle nutrient homogeneously within patches, but recycle nutrient proportionally to consumption between patches. The model shows th at a strategy of intermediate palatability can be adaptive in response to a small herbivore that lives on and recycles nutrient around one or a few in dividual plants. For moderately small herbivores, plant palatability may ev olve towards one of two local convergent strategies, depending on the initi al conditions. For medium- to large-sized herbivores, the nonpalatable stra tegy is always selected. We discuss the functional and evolutionary implica tions of these results, and suggest that the traditional dichotomy describi ng antagonistic and mutualistic interactions may be misleading.