A global model of discretized rarity and its restrictions

Understanding the causes and consequences of species rarity is a fundamental goal in ecology and a critical challenge under climate change. Rarity is often defined using competing frameworks incorporating one or two dimensions of rarity and abundance. Multidimensional rarity plays a key role in shaping biodiversity; however, multidimensional frameworks are often data intensive, conceptually dense, and difficult to apply. We introduce a global model of discretized rarity (GDR) that incorporates geographic, functional, and phylogenetic dimensions at regional and local scales to create 63 restrictions that define rarity based on research question, management goal, spatial scale, and data availability. We test these restrictions' ability to explain variation in flowering phenology and distribution changes in British flora over 32 years. The global model performs well in explaining these biological processes; however, some restrictions outperform the richer GDR model. One such restriction, GRLFRLPL consistently emerges as the most informative definition of rarity, though its high dimensionality and data requirements limit practical use. Other novel restrictions also perform well, and we highlight how they can be readily integrated into conservation and research, opening new avenues for linking rarity to community and ecosystem processes within a unified conceptual framework.