Nitrogen and carbon stable isotope analysis sheds light on trophic competition between two syntopic land iguana species from Galápagos

Coexistence between closely related species can lead to intense competition for resources. Stable isotope analysis (SIA) is a reliable tool to estimate the extent of species competition. We employed SIA to evaluate niche partitioning among two syntopic species of Galapagos land iguanas: Conolophus subcristatus and C. marthae. Samples were collected on Wolf Volcano, Isabela Island, where C. marthae is endemic and syntopic with C. subcristatus. We determined delta C-13 and delta N-15 ratios and described the isotopic niche of each species using corrected standard ellipse area (SEA(c)). We tested for differentiation between the isotopic niches, while controlling for sex, body size, spatial location of samples and mean annual primary productivity at capture points, using bivariate linear models. Despite the extensive overlap of the isotopic niches, we found species and sex to be a significant, interacting predictor of a sample's location in the delta C-13, delta N-15 space, indicating the existence of niche partitioning mechanisms acting between species and sexes. We also found that body size and productivity at the capture points, compounded with yet undetermined spatial effects, explain ca. 75% of the differences observed between species and sexes, providing evidence for differential microhabitat and food-items usage. Our study provides essential baselines for evaluating conservation actions for C. marthae, such as the potential translocation to a sanctuary area free of competition from C. subcristatus.