Community richness is the main factor determining the structure of ant-plant mutualistic networks along environmental water gradients

Many plant species rely upon ants to protect against herbivores. In arid environments, these plants often form stronger bonds with dominant ant species that, in turn, provide a better anti-herbivory defense than low aggressive subordinate ants. Dominant ants typically claim the plants producing more nutritious nectar, commonly leaving less valuable plants to be guarded by lower-quality bodyguards. As water scarcity increases, the value of the extrafloral nectar also increases, which can increase the control of the most valuable plants by dominant ants and of the displacement of outcompeted ants to less valuable plants, reducing niche overlap among ant species and, consequently, the generalization of ant-plant interactions at the community level. To evaluate this hypothesis, we crossed data from 63 empirical ant-plant networks with the mean precipitation rate of the sites and period in which the interactions were sampled. As the environment dries, ant-plant networks decreased in species richness but maintained other network properties. Surprisingly, the decline in the number of ant and plant species engaged in the mutualism along the precipitation gradient increased the interaction generalization in drier habitats. But this increased generalization is possible due to the increase in probability of interaction between all plant and ant species due to the lower richness. Hence, water availability primarily influences ant-plant interactions through its impact on ant and plant communities. This has significant eco-evolutionary consequences, as possibly increasing the persistence of this networks in drier environments, demonstrating a new pathway through which environmental factors can impact ecological interactions.