Recent and Rapid Assembly of an Island Species–Area Relationship Threatened by Human Disturbance

The island species–area relationship (ISAR) describes how larger islands support more species. ISARs of isolated oceanic archipelagos, assembled over millions of years, typically show positive relationships, steep slopes, and species richness equilibrium. However, it remains unclear how quickly such characteristics emerge. We compiled a dataset for fish communities of 79 postglacial peri‐Alpine lakes and report an ISAR, formed de novo in less than 15,000 years, that partially mirrors older systems, but has an asymptotic shape. Immigration and speciation, the main ISAR drivers, are primarily associated with area and depth, respectively. Immigration increases with area, while speciation is promoted by greater depth, likely due to species depletion in the source pool and ecological constraints on speciation. This young ISAR has been reshaped by anthropogenic activities, with species introductions erasing its asymptotic shape. We demonstrate that ISARs can develop rapidly after insular habitat formation, offering insights into patterns of biodiversity assembly.