The resource availability hypothesis revisited: a meta-analysis

P>1. Several theories have provided a framework for understanding variation in plant defence against herbivores. Among them, the plant apparency theory and the resource availability hypothesis (RAH) have aimed to explain the patterns of defence investment and the selective pressures that have led to the variety of defensive strategies across species. Here we provide a historical review of both theories, present evidence that shaped their development and contrast their predictions. 2. We present the results of a meta-analysis of the utility of the RAH 25 years after it was proposed and compare it to apparency theory. We performed a meta-analysis of 50 studies that have examined plant growth, defences and herbivory in relation to resource availability across latitude and ontogeny. Specifically, we tested four predictions that follow the RAH: (i) species adapted to resource-rich environments have intrinsically faster growth rates than species adapted to resource-poor environments; (ii) fast-growing species have shorter leaf lifetimes than slow-growing species; (iii) fast-growing species have lower amounts of constitutive defences than slow-growing species; and (iv) fast-growing species support higher herbivory rates than slow-growing species. 3. Our results confirm the predictions that species adapted to resource-poor environments grow inherently more slowly, invest more in constitutive defences and support lower herbivory than species from more productive habitats. Our data also showed that variation in growth rate among species better explains the differences in herbivory than variation in apparency, suggesting that the evolution of different defensive strategies across species is resource, rather than herbivore driven. We also found that the application of this theory appears robust across latitude and ontogeny, as the magnitude of the effect sizes for most of the predictions did not vary significantly between ecosystems or across ontogenic stages. 4. We conclude that the RAH has served as a valid framework for investigating the patterns of plant defences and that its applicability is quite general.