The Refugial Squeeze: A Globally Relevant Mechanism of Species Decline Revealed by a Threatened Amphibian.

A key outcome of the Global Biodiversity Crisis is the contraction and range shift of species distribution into refugial habitats that afford protection against threatening processes. Range contractions driven by spatially overlapping threats may fragment populations to the point where functional habitat is lost. This study identifies a range contraction of the Vulnerable green and golden bell frog (Ranoidea aurea) in south-east Australia using a multi-method approach. We fitted alpha hulls, species distribution models (MaxEnt, Random Forest, Boosted Regression Trees and an AUC-weighted ensemble model) and conducted a hypervolume analysis to quantify change in the species' range and niche between full (historical) and contemporary periods. Our findings suggest R. aurea has undergone niche contraction and displacement with a directional shift into coastal saline-influenced refugia. Our results demonstrated a 76% reduction in extent of occurrence, a suitable habitat reduction of ~96%, and an environmental niche contraction of 57%. The remaining habitat was strongly associated with coastal proximity with shifts towards more saline environments. The concentration of R. aurea in coastal regions suggests intensified dependence on coastal refugia, persisting in salt-exposed habitat that may suppress chytrid fungus. Yet, the threat abatement afforded by low-lying, saline coastal environments is wedging R. aurea into chytrid refugia that is at risk from sea level rise. Refugial squeeze is a globally relevant phenomenon likely affecting a suite of coastal species that highlights the value of integrating geometric and probabilistic niche approaches with species distribution models to resolve temporal contraction in species distributions. Protection of refugial squeeze zones from sea level rise will ultimately determine the future range and population viability of coastal species.