Abstract
Global fire regimes are changing, raising concerns about the ability of fire-prone ecosystems to maintain biodiversity. We tested whether the pyrodiversity-biodiversity hypothesis (i.e., variation in postfire characteristics promotes biodiversity) or alternative hypotheses better explain patterns of biodiversity in a true island system. Using fixed-area sampling plots in a chronosequence of 42 boreal lake islands spanning gradients in island area (1-350.4 ha), isolation (0.1-7.9 km from mainland), and fire history (1-231+ year since fire), we tested whether alpha and beta diversity of beetles, plants, and birds increased with spatial (within-island variation in burn severity) and temporal (variation in time since fire among islands) pyrodiversity, respectively. Species richness of plants and birds increased with spatial pyrodiversity indicating that habitat heterogeneity from localized variation in burn severity supported more species in some groups. Beta diversity of all taxa increased with temporal pyrodiversity, highlighting the importance of conserving age-class variation within the boreal patch mosaic. In contrast, the habitat amount hypothesis and island biogeography theory were weak predictors of species richness across all taxa, and island area and isolation did not consistently affect beta diversity among the islands. Our findings emphasize the importance of maintaining pyrodiversity in boreal landscapes to combat biodiversity loss in the age of "megafires" and suggest leveraging the fire refugia effects of large lakes within the region to conserve vital components of temporal pyrodiversity such as old-growth forests.