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.