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.