Contingencies
Beyond the processes described above, rarity can also be influenced by historical and geographical contingencies. Perhaps the most obvious of these are the effects of habitat quality, availability, and accessibility on abundance, occupancy, and distribution . These effects are illustrated by studies of niche position (i.e., the commonness or rarity of species’ habitat on the landscape), which has been found to be significantly correlated with occupancy and abundance across a range of taxonomic groups . Historical contingencies also affect species’ occurrence: phylogeny is correlated with range size , and glaciation can leave a lasting imprint on species’ range limits . Finally, neutral theory suggests that rarity can arise from random variations in species’ demography (ecological drift). While the complexity and diversity of contingencies precludes generalising their effects on rarity and integrating these effects into our framework, the role of contingencies cannot be ignored when considering the drivers of rarity for species of interest.