Who Is Paying the Extinction Debt? Phylogenetic and Functional Structuring on Greek Islands Is Shaped by Sea-Level Rise Since the Last Glacial Maximum.

Since the Last Glacial Maximum (LGM) ~21.5 kya, global sea-level rise has reshaped coastal areas by contracting subaerial regions, severing mainland connections and driving the progressive fragmentation of pre-existing islands. These changes led to supersaturation of island communities, triggering community relaxation through local extinctions. However, it remains unclear which ecological processes are responsible for patterns of species extinction. Here, we integrate a paleo-coastline model with well-characterized squamate community data from 163 Mediterranean islands to assess how past geographical changes influenced community structure. We evaluate phylogenetic and functional community structure and test links to paleogeographic variables. Our findings suggest phylogenetic overdispersion (co-occurring species being more distantly related than expected by chance) dominates and is higher on older islands, implying ongoing community relaxation and that extinctions are driven more by interspecific competition than environmental filtering. Island time-since-isolation emerges as the strongest predictor of phylogenetic structuring, indicating that longer isolation drives phylogenetic overdispersion via selective extinction of close relatives. These results underline the significance of relaxation dynamics in shaping insular communities.