Evidence of environmental niche conservatism among pairs of allopatric avian lineages in two Neotropical dry forests, the Caatinga and the Chaco

Abstract The Caatinga and the Chaco are the two largest expanses of seasonally dry forests in the Neotropics and host multiple allopatric avian lineages with similar biogeographic patterns. Among these, there are 10 avian lineages, which include (1) isolated populations in 1 monotypic species, (2) pairs of subspecies in 5 polytypic species, and (3) pairs of allopatric species in 5 species complexes. The main goal of this study was to evaluate whether there are common patterns of realized climatic niche differentiation or conservatism in these allopatric populations. Despite their current taxonomic status, under scenarios of environmental niche divergence, these populations may be undergoing ecological shifts driven by natural selection, potentially progressing toward differentiation. Conversely, under climatic niche conservatism, allopatric populations may retain similar ecological preferences despite geographic isolation. Intermediary cases are also possible when nonadaptive divergence or convergent evolution are acting. Here, we obtained occurrence data and abiotic variables (e.g., climatic and topographic) for the selected lineages and quantified realized niche differences using environmental principal component analysis (PCA-env) and the N-dimensional hypervolume framework to investigate differences in their climatic tolerances and to assess their level of niche conservatism and environmental similarity. We found low-to-mid environmental niche overlap among Caatinga and Chaco avian populations, pointing out different climatic tolerances between allopatric populations. Although we found dissimilar climatic niche hypervolumes between pairs, the niche equivalency and similarity tests indicate that two-thirds of the bird lineage pairs analyzed show results that support the hypothesis of environmental niche conservatism, whereas 3 pairs showed nonidentical environmental niches under the equivalency test, suggesting possible local adaptation to the different climatic conditions they are exposed to. These results indicate that, although there may be some signs of local adaptation among some Caatinga-Chaco lineages, most pairs indicate a tendency to climatic niche conservatism, rather than cases of ecological differentiation.