Exploring the Abiotic and Biotic drivers of uneven Species Richness in Salamanders and Newts.

Species richness varies widely among taxonomic groups, and understanding the drivers of these disparities remains a key challenge. Using the most recent time-calibrated phylogeny of Caudata (763 species), we investigated how abiotic and biotic factors shape diversification in this amphibian clade, known for its ecological and life-history diversity. We first assessed the respective roles of clade age and diversification rates in explaining species richness using two branch-specific models. We then tested the influence of past temperature, continental fragmentation, and life cycle on diversification using environment- and state-dependent models. Our results show that diversification rates, rather than clade age, primarily explain richness differences. High speciation rates are found in young lineages and in early branches of the three most diverse families, suggesting early radiations followed by slowdowns. Likelihood-based analyses further suggest a positive association between temperature fluctuations and diversification. Weak temperature effects in Plethodontidae, and absence of effect in Salamandridae and Hynobiidae point to lineage-specific dynamics. However, Bayesian inference-based models failed to recover reliable diversification dynamics, likely reflecting the inherent difficulty of estimating time-varying speciation and extinction rates from this phylogeny. Consequently, the precise temporal trajectory of diversification and the causal interpretation of environmental associations remain uncertain. State-dependent models show no significant effect of life cycle. Instead, other traits likely drive diversification patterns. Together, our findings highlight the central role of diversification rate heterogeneity in shaping species richness disparity, and the challenges of disentangling the macroevolutionary processes driving salamander evolutionary history.