Describing rarity: Pattern
There is no universally agreed-upon definition of rarity or what exactly
constitutes a rare species. While the term is most commonly used to
refer to species characterised by a restricted distribution, low
abundance, or both , some authors have incorporated different metrics
into their definitions of rarity, including habitat breadth, occupancy,
or persistence . As a starting point, we define rarity as being
uncommon. While this “uncommonness” can be defined at various
scales—e.g., within a particular biotic community, the boundaries of a
jurisdiction, or globally—we focus on species that are globally
uncommon. Thus, species that are rare only within a certain context,
such as a particular jurisdiction or habitat type, are not considered
here.
Despite the popularity and evident utility of the Rabinowitz framework,
it does have weaknesses. First, the three rarity dimensions (range size,
local abundance, and habitat specialisation) represent a combination of
pattern and process. Whereas range size and local abundance both
describe patterns in species’ occurrence, habitat specialisation is
arguably a process (environmental filtering) and it can influence both
patterns. Creating a process-based framework for rarity requires a clear
distinction between patterns of rarity and the processes that drive
them. Second, the Rabinowitz framework captures patterns of occurrence
either at very broad scales (i.e., range size) or very fine scales
(i.e., local abundance), but lacks a measure of regional-scale rarity.
As scale can substantially affect patterns of rarity, along with related
issues such as population decline , this omission may create an
important gap in assessing rarity.
We propose addressing both these issues by removing habitat
specialisation from the Rabinowitz framework, and replacing it with a
new dimension, namely occupancy. In some contexts, occupancy is a binary
variable that indicates whether a species occurs at a particular site
(as in occupancy surveys or models, e.g., . It has also been defined as
the proportion of cells in a gridded landscape currently occupied by the
species . Here, we define occupancy as the proportion of habitat patches
(or sites, grid cells, etc.) within a species’ range that is occupied by
the species. While it is somewhat unconventional to define occupancy
specifically in relation to a species’ range, by doing so we can capture
the density at which the species occurs within the range, which has
implications for the persistence of rare species and the type of
conservation measures they may require. We discuss these implications inConservation implications (below).
The distinction between occupancy and range size warrants some further
precision, as the two are often used interchangeably in the literature,
including some studies that have applied the Rabinowitz framework (e.g.,
. However, as shown in Figure 1, we defined occupancy in a way that is
distinct from range size: whereas range size refers to the global limits
of the area within which a species occurs (i.e., extent of occurrence),
occupancy denotes the density of occurrences within those limits. While
occupancy may provide a practical proxy for range size that is suitable
in the context of certain research questions, the two are not
conceptually equivalent. Furthermore, a species with a small range but
high occupancy may require different conservation and management
measures compared to a species with a large range but low occupancy
(Figure 1); this will be discussed in more detail in Conservation
implications . Given that studies of rarity (or rare species) often have
conservation objectives, the distinction between range size and
occupancy is important to maintain.
There are four major advantages to replacing habitat specificity with
occupancy: first, like range size and habitat specificity, occupancy
describes a pattern of occurrence. Second, our proposal yields a system
that acknowledges the importance of scale in rarity, as the proposed
dimensions of range size, occupancy, and local abundance correspond to
global, regional, and local scales, respectively (Figure 2), thus
filling the regional scale gap in the Rabinowitz framework. Third, as we
will argue in more detail in Conservation implications , using
occupancy rather than habitat specialisation will allow the framework to
more accurately identify species that are truly uncommon, which has
important implications when using the classification system for
conservation purposes. Finally, because habitat specialisation is among
the mechanisms that can influence occupancy , habitat specificity will
remain embedded in the framework, but as a causal driver rather than a
rarity dimension, as explained in the Explaining rarity: Processsection.