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.