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.