The Dominance of Introduced Plant Species in the Diets of Migratory Galapagos Tortoises Increases with Elevation on a Human‐Occupied Island

ABSTRACT The distribution of resources and food selection are fundamental to the ecology, life history, physiology, population dynamics, and con-servation of animals. Introduced plants are changing foraging dynamics of herbivores in many ecosystems often with unknown conse-quences. Galapagos tortoises, like many herbivores, undertake migrations along elevation gradients driven by variability in vegetationproductivity which take them into upland areas dominated by introduced plants. We sought to characterize diet composition of two spe-cies of Galapagos tortoises, focussing on how the role of introduced forage species changes over space and the implications for tortoiseconservation. We quantified the distribution of tortoises with elevation using GPS telemetry. Along the elevation gradient, we quantifiedthe abundance of introduced and native plant species, estimated diet composition by recording foods consumed by tortoises, andassessed tortoise physical condition from body weights and blood parameter values. Tortoises ranged between 0 and 429 m in elevationover which they consumed at least 64 plant species from 26 families, 44 percent of which were introduced species. Cover of introducedspecies and the proportion of introduced species in tortoise diets increased with elevation. Introduced species were positively selectedfor by tortoises at all elevations. Tortoise physical condition was either consistent or increased with elevation at the least biologically pro-ductive season on Galapagos. Santa Cruz tortoises are generalist herbivores that have adapted their feeding behavior to consume manyintroduced plant species that has likely made a positive contribution to tortoise nutrition. Some transformed habitats that contain anabundance of introduced forage species are compatible with tortoise conservation.Abstract in Spanish is available in the online version of this article.