Explaining rarity: Process
A key challenge in ecology is to identify generalities that transcend
the idiosyncrasies of specific focal assemblages or regions. Through the
use of traits (i.e., morphological, physiological, phenological,
behavioural, biochemical, or other characteristics of species that
influence their fitness), functional ecology offers a
taxonomically-neutral approach to inferring information about ecological
processes from species’ functional traits . Similarly, traits may offer
a path towards identifying the key processes that drive different rarity
dimensions. However, despite the large body of literature that has
sought links between traits and rarity (Table 2), identifying
generalities has proven difficult for a variety of reasons. First,
rarity is defined differently across studies, e.g., species may be
considered rare based on range size , conservation status , or frequency
of occurrence within a study region . The traits associated with such
different types of rarity can be expected to vary substantially. A
second challenge is the large diversity of taxa and traits covered in
studies focussing on trait–rarity relationships: the traits related to
rarity may vary substantially among taxonomic groups owing to
differences in physiology and life history. In addition, species exhibit
numerous traits, not all of which will necessarily impact or even
correlate with their degree of commonness or rarity. Furthermore, traits
may interact within a single individual or species, and the effects of
traits that promote rarity may be offset by other traits that promote
commonness. Finally, studies on traits naturally focus on those traits
for which we have adequate data, and it is unclear to what degree the
available data correspond to the key drivers of rarity .
Owing to the difficulties in finding consensus among studies of the
traits associated with rarity, we propose a top-down approach linking
rarity to traits. Instead of inferring trait–rarity relationships by
synthesising the available evidence, we instead propose a suite of trait
dimensions that can be mechanistically linked to the three proposed
rarity dimensions (i.e., range size, occupancy, and local abundance).