The evolution and coexistence of generalist and specialist herbivores under between-plant competition

Consumer-resource models have been used extensively to study the evolution and coexistence of generalist and specialist consumers. However, current consumer-resource models do not take into account competition between resources or only incorporate intraspecific competition phenomenologically with, for example, a logistic growth function. Here, we mechanistically incorporate competition in an existing two-resource model, by introducing nutrient-limited resource growth and setting the total amount of nutrients (free or contained in consumers and resources) to a fixed value. In addition to the three combinations of generalists and specialists found in previous models, we find four other evolutionary outcomes, depending on the strength of the consumer trade-off: coexistence of one specialist and a generalist and three types of evolutionary cycling. Furthermore, which outcomes are most likely depends strongly on the combination of intrinsic growth rate of resources and the total amount of nutrients in the system. Our results suggest that the realistic assumption of nutrient competition may shed new light on the evolution of the multitude of strategies in real systems.