Introduction
Rare species have been the focus of considerable attention in ecology
and conservation biology . Rare species are more prone to extinction
than common species , and are often among the targets of conservation
efforts . Rarity is also of interest from a theoretical perspective
as it touches on fundamental
questions in ecology, particularly those related to the drivers that
influence distribution and abundance; though in the case of rare species
we are more precisely interested in what constrains their
distribution and abundance. However, as with other aspects of ecology,
the study of rarity is complicated by a sea of contingencies, and
identifying generalities has proven difficult.
Numerous ecologists have sought to bring order to the seemingly anarchic
phenomenon of rarity. Early work tended to focus on single factors: for
example, proposed that rare species were newly evolved taxa which have
not yet occupied their full niche, whereas hypothesised that they are
relictual species that have decreased in abundance or distribution.
emphasised low heterozygosity as a root cause of rarity. While overly
simplistic, these early studies began the work of identifying traits
associated with rarity, an undertaking that continues today in the form
of comparative studies of the traits of rare and common species (e.g., .
More recent work has taken a less monolithic perspective on rarity and
has attempted to parse the concept into a variety of types or causes.
The most well-known of these efforts is the framework developed by ,
which classifies species based on three dimensions of rarity: local
abundance, habitat specialisation, and range size. Local abundance
refers to a species’ typical population size at the local scale; habitat
specialisation refers to the range of habitat types in which a species
is found, and is roughly analogous to the concept of niche breadth; and
range size refers to the geographic extent within which a species
occurs. The three dimensions are dichotomised and then combined to form
eight possible groups, seven of which represent different types, or
forms, of rarity (Table 1). Rabinowitz’ seven forms of rarity have been
widely used to describe rarity in a variety of assemblages and regions
identify conservation priorities , and assess extinction risk .
Other ecologists have taken a mechanistic approach to understanding
rarity, and causal theories have been proposed to explain rarity and
endemism in plants. based their system on two axes, taxon age and range
size, which are dichotomised and combined to form four different rarity
types. They then propose a different hierarchy of explanatory causes for
each of the four resulting groups. emphasized the need for a synthetic
approach to understanding endemism, and proposed a system that
incorporates the effects of historical, genetic, and ecological
processes.
Though frequently cited, the theories of and have rarely been applied to
real species or assemblages (though see ). While this is in striking
contrast to the popularity of the Rabinowitz framework, the difference
may be attributable to the availability of the type of information they
require to classify species. Rabinowitz’ system requires comparatively
basic ecological information on range, abundance, and habitat
requirements, which can be obtained with relative ease from expert
knowledge and/or survey data. Conversely, the information required to
apply the theories of Stebbins or Fiedler & Ahouse, such as taxon age
or genetic information, may be less readily available, particularly for
very rare species.
As a result, we are left with a widely applied classification system
that is well-suited to describing rarity, but not to explaining its
causes, and two theories that explain rarity, but which are scarcely
used in practice. While the system developed by Rabinowitz is useful as
a phenomenological scheme, it does not explain the root causes of rarity
(nor was it intended to). asserted that a typology of the causes of
rarity “is a distant goal”; however, the four decades since the
development of the seven forms of rarity have seen considerable progress
in ecology, and linking Rabinowitz’ scheme to ecological theory may
serve to update the framework and expand its utility from description
towards explanation. The fields of functional ecology and macroecology
may be of particular value to this quest: the former seeks to identify
the processes that influence species’ patterns of occurrence based on
measurable traits, and has frequently been used to infer mechanistic
causes of rarity, whereas the latter provides insight into the
fundamental processes that drive broad-scale ecological patterns.
Stebbins’ emphasis on synthetic explanations is likely salient, as
patterns of abundance and distribution in species are driven by a
variety of factors operating across a range of spatial and temporal
scales . However, to provide practical value for conservation, such an
explanation must also be accessible and usable despite the knowledge
gaps that often exist for rare species. Here, we seek to increase the
robustness of the conceptual underpinnings of studies of rarity by
developing a process-based framework of rarity. To achieve this
objective, we revisit the Rabinowitz framework and link it to insights
from theoretical and functional ecology. We propose a modification to
the Rabinowitz framework that will more clearly distinguish between
dimensions of rarity and their underlying causes, use theory and
empirical work to explore the common causes of these dimensions, and
discuss the implications of our proposed framework for conservation.