Note of caution
Despite the advantages outlined above, replacing habitat specificity
with occupancy may pose certain conceptual and technical challenges.
First, occupancy is correlated with both range size and abundance ,
which may be perceived as problematic. Second, it may be difficult to
quantify occupancy in practice, given that estimates can be influenced
by the scale of observation and imperfect detection . However, similar
problems exist for the other Rabinowitz dimensions; for example, habitat
specificity and range size are also correlated (Slatyer et al., 2013),
but this relationship has not hindered the utility of the framework.
With respect to the difficulty of quantifying occupancy, it is generally
challenging to quantify any of the rarity dimensions in practice, and a
good deal of creativity has been used in applying the framework. Each of
the three rarity dimensions have been defined in remarkably variable
ways in the literature. For example, some studies have defined species
restricted to a particular ecological or administrative region of
interest as having a small range size, and more broadly distributed
species as having a large range (e.g., . Similarly, while most studies
assess habitat specificity based on the number of ecoregions or habitat
types within which a species occurs , others base their assessments on
additional, related factors, such as habitat scarcity or species’
reliance on a focal habitat type . The flexibility and creativity
demonstrated by these examples can be used to assess occupancy in the
literature is usually based on available information and study
objectives. The strict definition of occupancy we proposed here may be
impractical to quantify in many situations; however, the concept can be
approximated, for example, by re-defining occupancy as the proportion of
study sites within a dataset in which a species was observed, or by
gridding the study region and estimating occupancy based on the
proportion of occupied cells. In well-surveyed regions such as the
United Kingdom or Switzerland , dot maps may also provide an excellent
basis for assessing occupancy.
Finally, while our framework is intended to identify the key mechanistic
drivers of rarity, it is impossible to capture all the idiosyncrasies of
individual species while also retaining a meaningful degree of
generalisability. As such, our proposed framework is by necessity a
simplification of reality. For example, for species that rely on
biogenic habitats (e.g., epiphytes), interactions may be a key driver of
range size as the presence of these species is dependent on the presence
of the host or substrate species, a factor that is not accounted for in
our framework. As such, the ecology and biology of focal taxa or groups
must be considered when applying the framework.