Niche conservatism over deep timescales and diverse niche axes in land vertebrates

Aim: There has been considerable interest in niche conservatism, the idea that ecological variables are similar among related species. Much research has focused on climatic niches of recently diverged species, rather than deeper timescales or non-climatic niche axes. Furthermore, it has been suggested that conservatism disappears over deeper timescales, and is greater in alpha niche traits (like diet and microhabitat) than beta niche variables (like climate). Here, we test these latter two ideas by comparing patterns of phylogenetic conservatism among 10 niche variables across major clades of land vertebrates. Location: Global. Time Period: Present to 350 million years ago. Major Taxa Studied: Tetrapods, including amphibians, mammals, lepidosaurs (including lizards and snakes), turtles, crocodilians and birds. Methods: The 10 niche variables included four alpha niche components (diet, diel activity, habitat, body temperature) and six beta niche components (related to climatic temperature and precipitation). We analysed these variables on time-calibrated phylogenies with similar taxon sampling (similar to 1700 species), using phylogenetic signal (lambda) to estimate conservatism, along with the D statistic and estimates of evolutionary rates. Results: Phylogenetic signal was generally strong across all variables, with lambda generally >0.80 (with 1.0 representing maximum signal). Nevertheless, mean phylogenetic signal was lower in beta niche traits than alpha niche traits (based on lambda and especially the D statistic), and alpha niche traits showed significantly slower rates of evolution. Main Conclusions: We address two long-held views in the literature on niche conservatism, rejecting one but supporting the other. We show that phylogenetic signal does not disappear over deep timescales for many important niche variables, even over 350 million years. We also generally support greater conservatism in alpha niche traits than beta niche traits over hundreds of millions of years, a pattern that was previously suggested (but not explicitly tested) based on closely related species.