The practical implications of defining rarity axes
While some ecologists are interested in rarity from a theoretical
perspective, many studies on rarity and rare species are explicitly
conservation oriented. While rare species are not always of conservation
concern, and many naturally rare species may be stable over long periods
of time , rare species as a group are more likely to be at risk of
decline and extinction . In addition, historically stable rare species
may experience rapid declines because of anthropogenic pressures, as
illustrated by the vaquita . As such, rare species are often the focus
of conservation efforts such as monitoring and protected area planning .
However, how rarity is defined determines which species are, or are not,
targeted by such efforts, and what types of conservation measures are
used. As such, any framework for structuring rarity should consider the
practical implications of including or excluding particular dimensions
of rarity.
Our proposed replacement of habitat specificity with occupancy implies
that habitat specialists will not necessarily be classified as rare.
However, habitat specialists may be quite common if their preferred
habitat is widely available ; such abundant, widespread habitat
specialists stretch the definition of rarity, and are unlikely to be of
conservation concern. For example, brown peatmoss (Sphagnum
fuscum ), a bryophyte, is a circumboreal habitat specialist that
occupies relatively dry microhabitats in peatlands . However, this
habitat type is widely available, and brown peatmoss is common and
abundant throughout north temperate and low Arctic regions . Thus, while
brown peatmoss would be considered rare under the original Rabinowitz
classification, it would be considered common under our modified
framework. Conversely, for specialists associated with uncommon habitat
types, the effects of their high niche position will likely manifest as
low occupancy or restricted distribution. The koala (Phascolarctos
cinereus ), which is range-restricted owing to its strong reliance onEucalyptus forests, is an example of this phenomenon . A
contrasting example is Porsild’s bryum (Haplodontium
macrocarpum ), a moss species that occurs only in shaded, continually
moist localities on calcareous substrates . While widely distributed
throughout northern temperate and polar regions, populations are few and
disjunct owing to the limited availability of suitable habitat . In
contrast to brown peatmoss, both the koala and Porsild’s bryum would be
considered rare under our classification owing to the effects of the
availability of their highly specific requirements on their range size
and occupancy, respectively.