Biome specialisation in squirrels: phylogenetic and geographic patterns

Abstract

Aim: Habitat breadth shapes species' responses to environmental change and influences large-­ scale biodiversity patterns. According to Vrba's resource-­use hypothesis, biome specialists (inhabiting a single biome) exhibit higher speciation rates than generalists due to increased population isolation during habitat fragmentation, generally associated with past climate changes, particularly in biomes at extremes of the global climatic gradient. However, the phylogenetic and geographic distribution of biome specialists remains poorly understood. Here, we use squirrels to assess whether: (1) clades accumulate more specialist species than expected by chance, (2) this accumulation is associated with biomes at the extremes of the climatic gradient (tropical rainforest, subtropical desert, steppe and tundra), (3) habitat specialisation relates to geographic patterns of phylogenetic diversity and (4) species-­ level recent speciation rates. Location: Global. Taxon: Squirrels (Sciuridae, Rodentia). Methods: We compared observed biome specialisation indices (BSI) of species to MCMC-­generated null distributions. We then mapped global patterns of specialist and generalist richness, phylogenetic diversity and recent species-­ level speciation rates. Results: Squirrels include more biome specialists than expected under a null model that randomises biome occupancy, supporting the resource-­use hypothesis. Tropical rainforests and steppes harboured more specialists than expected, whereas deserts and tundra did not, the latter likely due to their low species richness. Regions with high specialist richness (e.g., Southeast Asia) exhibited lower levels of phylogenetic diversity, consistent with Miocene speciation. In contrast, high recent speciation occurred in Holarctic steppes and South America, likely reflecting Pleistocene glacial radiations and post-­ Panamanian colonisation, respectively. Main Conclusions: During periods of climatic change, the origin of new specialist lineages through rapid speciation led to specialist-­dominated and phylogenetically clustered faunas, particularly in fragmented biomes. However, recent speciation rates reflect emerging rather than historical diversification hotspots. The Sunda Shelf emerges as a key conservation priority due to its exceptional richness, low phylogenetic diversity and concentration of endemic specialists.