Low Nutritive Quality as a Defense Against Optimally Foraging Herbivores

Abstract

Using a simple optimization model, we analyze whether low nutritive quality in terms of low nutrient concentration can be a profitable anti-herbivore strategy for individual plants. Contrary to most previous studies, ours has considered vertebrate herbivores feeding on discrete food items such as trees. The food items are regarded as patches and exploited according to the marginal-value theorem. Low-nutrient strategies (possibly associated with correspondingly high levels of defense compounds) are expected if the browsing pressure is high (i.e., high encounter rate with individual plants and/or long handling time set by large interplant distances) in inherently fast-growing plants and in plants with long exposure time to potentially deleterious herbivory. Moreover, the critical size for escape from deleterious herbivory influences the optimal solution. We predict that plants in resource-poor habitats should tend to maximize their nutritive quality and therefore rely on carbon-based defenses, further increasing net growth. However, the nutritive quality of plants in resource-rich habitats should be below the maximum allowed by the habitat (and, therefore, they should also be able to produce nitrogen-based defense compounds). The optimal solution is not necessarily also the evolutionarily stable solution. If the browsing pressure is relatively high, most optimal solutions are also evolutionarily stable; but if the browsing pressure is significantly relaxed, most optimal solutions are not. In such situations, the average nutritive quality of the stand converges to a small range of relatively high stable values. The results are discussed in relation to a recent resource-availability hypothesis. We also argue that the theory of optimal patch use might be a powerful tool for analyzing the exploitation of individual plants by herbivores.