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Abstract Although savanna woody encroachment has become a global phenomenon, relatively little is known about its effects on multiple dimensions and levels of savanna biodiversity.Using a combination of field surveys, a species‐level phylogeny, and functional metrics drawn from a morphological dataset, we evaluated how the progressive increase in tree cover in a fire‐suppressed savanna landscape affects the taxonomic, functional, and phylogenetic diversity of neotropical ant communities, at both the alpha and beta levels. Ants were sampled along an extensive tree cover gradient, ranging from open savannas to forests established in former savanna areas.Variation in tree cover had a significant influence on all facets of diversity at the beta level, whereas at the alpha level tree cover variation affected the taxonomic and functional but not the phylogenetic diversity of the ant communities.In general, ant community responses to variation in tree cover were largely non‐linear as differences in taxonomic alpha diversity and in the taxonomic, functional, and phylogenetic composition of the sampled communities were often much stronger at the savanna/forest transition than at any other part of the gradient. This indicates that savanna ant communities switch rapidly to an alternative state once the savanna turns into forest.Ant communities in the newly formed forest areas lacked many of the species typical of the savanna habitats, suggesting that the maintenance of a fire suppression policy is likely to result in a decrease in ant diversity and in the homogenisation of the ant fauna at the landscape scale. 

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.