One Biodiversity Hotspot to Rule Them All: Southwestern Australia—an Extraordinary Evolutionary Centre for Plant Functional and Taxonomic Diversity

Abstract

The Southwest Australian Floristic Region (SWAFR) is a global biodiversity hotspot with exceptional plant species richness, endemism and rarity linked to ancient landscapes, extremely infertile soils, complex habitats and a relatively stable climatic history. It contains about 9000 plant taxa (~8000 species), the majority of which are endemic. Key functional traits for nutrition, fire and pollination were assigned to 77% of taxa and extrapolated for all genera using existing data sources and new observations. Plants with complex mineral nutrition traits are 3–14 times more abundant than global averages, including 18% of all known ectomycorrhizal plants, 40% of nonmycorrhizal plants with cluster roots, 18% of carnivorous plants and most Thysanotus species with unique mycorrhizas. Many SWAFR plants also have complex pollination relationships with specific insects (30%), birds (12%), or non-flying mammals (2%). Most also have single (90%) or multiple (48%) traits that are important for fire survival and recovery, such as soil seed banks (70%), canopy-stored seed (20%), resprouting (24%), fire-promoted germination (56%), enhanced flowering (33%), or fire avoidance (14%). Despite these adaptations, fire impacts can cause substantially altered plant diversity and dominance, loss of species, seed banks that take decades to recover, and increased presence of weeds. Most SWAFR plants have adaptive traits for nutrition, pollination and fire, with the most complex trait combinations in specific families and genera that also are the most taxonomically diverse in this biome. Trait variability within genera or even species reveals that strong selective pressures are still driving local adaptation. Species richness patterns of highly diverse trait-complex clades extend the SWAFR boundary into the interzone where ironstone ranges, shrublands and eucalypt woodlands include additional local hotspots for plant diversity and endemism. The SWAFR is globally unique due to the high proportion of plants with exceptionally complex functional traits that have evolved over long periods in response to adverse conditions, thereby making this region a key plant diversity hotspot for trait evolution. The SWAFR is the best location globally for studying long-term impacts of climate and soil conditions on plant functional and taxonomic diversity, and provides a preview of future conditions elsewhere. However, plants in this region now face even more severe fires, droughts, pollinator shortages and declining soil health. This greatly increases the need for well-resourced and science-based adaptive approaches for ecosystem management and rare flora conservation.

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