The multiple faces of indirect defences and their agents of natural selection

P>1. Plants commonly attract predatory and parasitoid organisms, and may thereby increase resistance against antagonistic herbivores. Mechanisms for indirect resistance include the provision of resources (e.g. food and shelter) for predators as well as the provision of information (e.g. herbivore-induce volatiles) on the location of hosts and prey that can facilitate the foraging behaviour of natural enemies of herbivores. 2. These ecologically divergent mechanisms for attracting bodyguards should also differ fundamentally in their evolution, particularly in how herbivores and their predators select on plant resistance traits. However, these different strategies are typically lumped together in theoretical discussions of indirect defences. 3. Here we highlight the fundamental differences between resource-mediated and information-mediated indirect resistance and discuss the relative evolutionary impacts of the interacting organisms as agents of natural selection in shaping indirect defence traits of plants. 4. We review clear evidence for a defensive function of resource-mediated indirect resistance, and recognize a significant lack of evidence for an adaptive function of information-mediated traits. A comparison of the underlying factors driving coevolution in each category of indirect defences, suggests that information-mediated indirect resistance is less likely to be subject to coevolution between plants and the third trophic level.