A mid-domain rule of environmental niche conservatism in tetrapods

Niche conservatism describes the tendency of species to retain ancestral ecological characteristics and climatic tolerances over time. Under this mechanism, stabilizing selection is expected to preserve climatic niches that maintain species persistence, while physiological constraints limit expansion into distinct conditions. Because speciation predominantly occurs in allopatry, niche conservatism predicts that species’ niches should remain centered around ancestral optima or expand symmetrically toward them within environmental space. To test this, we developed a niche field approach that overlays the niche ranges of co-occurring species within grid cells of environmental space for each tetrapod group independently. We constructed a gridded two-dimensional environmental space defined by temperature and precipitation and quantified niche field asymmetry as the Euclidean distance between focal environmental conditions and composition similarity-weighted niche conditions within each field. We then evaluated whether niche asymmetry varied with positional centrality within each clade’s environmental space, testing this relationship across major tetrapod groups and their lower taxonomic levels (orders and families). Across all tetrapods, niche field asymmetry displayed a consistent concentric pattern in environmental space, which was strongly consistent across taxonomic scales. Asymmetry increased with distance from the environmental space centroid of the clade. These results suggest that niche ranges expand deterministically toward environmental boundaries but remain constrained by physiological limits imposed by ancestral constraints. Our findings reveal a mid-domain-like rule emerging from niche conservatism, linking ecological constraints to large-scale patterns of niche organization.