The effect of geographic range and climate on extinction risk in the deep-time amphibian fossil record

Abstract

Geographic range is of crucial importance concerning the extinction risk of species. However, our understanding of how the influence of this trait on extinction risk has varied through Earth history and across different climate regimes is still rather unexplored. This is especially true for taxa that are currently of strong interest in conservation biology, displaying a strong mismatch between paleontological studies and modern efforts to quantify the extinction risk of threatened species. We herein calculated a continuous measure for the connection of geographic range and extinction risk in the deep-time amphibian fossil record. Furthermore, we evaluated the impact of climate on this connection, using two climate proxies derived from oxygen isotope data (global relative temperature change and variations in the latitudinal temperature gradient). We show that geographic ranges tended to be larger during times of comparatively small latitudinal temperature gradients. Moreover, the strength of geographic range influencing extinction risk fluctuated temporally, but remained positive at all times. The variation in range size impact on extinction risk showed a strong connection with the latitudinal temperature gradient. Our findings indicate that geographic range persists as a factor influencing species’ extinction risk through all times. However, geographic range seemed less important during times of higher environmental variability. Reasons might be the restriction of species ranges due to environmental constraints, causing ranges to become more similar and therefore lose relative importance in buffering for extinction risk, while other factors gain in importance. Simultaneously, similar ranges might be more prone to alteration by e.g. conservation biases (related to sedimentary deposition and fossilization) in relation to their real range size, resulting in a smaller signal-to-noise ratio, potentially affecting the correlation strength. We show that trait-extinction risk dynamics can vary in their intensity, and that specifically the observed impact of geographic range on extinction risk can vary with climatic changes.