Université de Sherbrooke Sherbrooke, Quebec, Canada
The three-dimensional rarity typology (“The Seven Forms of Rarity”) proposed by Rabinowitz in 1981 is perhaps the most widely used framework for exploring rarity in ecology and conservation research. While this framework, which classifies species into rarity types based on their range size, local abundance, and habitat specificity, is useful for describing patterns of rarity, it does not explain their underlying causes of rarity. In fact, Rabinowitz referred to a causal typology of rarity as a “distant goal”. However, the past four decades have seen substantial progress in ecology, and recent advances may be leveraged to add explanatory power to this populare framework. We sought to strengthen the conceptual underpinnings of studies of rarity by updating the Rabinowitz framework, and, in the process, link patterns of rarity to their underlying causes. We first modified the classical Rabinowitz framework to better distinguish between forms of rarity and the processes that drive them, and explored the conservation implications of the modified framework. We then used insights from theoretical and empirical work in macroecology and functional ecology to identify the fundamental ecological processes that result in different forms of rarity.
We reconceptualised the three dimensions of rarity by replacing habitat specificity, which is arguably a cause of rarity, with occupancy (the proportion of available sites occupied by a species), yielding a modified framework based on species’ range size, occupancy, and local abundance. This modification acknowledges the importance of spatial scale in rarity, as the three “new” dimensions correspond to global, regional, and local scales, respectively. Under the modified framework, abundant, widely distributed habitat specialists are no longer considered rare. Our proposed modification more accurately identifies species of potential conservation concern—often a focus of studies on rare species—as habitat specialists may be common if their habitat is abundant. Finally, we identified four major ecological processes (environmental filtering, movement, reproduction, and interactions) that explain variations in our three proposed rarity dimensions. We hypothesise that range size and occupancy are primarily driven by environmental filtering and movement, whereas local abundance is more strongly influenced by reproduction and interactions. Our work will strengthen the conceptual foundation of studies of rarity, help to identify traits associated with different forms of rarity, and provide a basis for developing hypotheses about the causes of rarity in particular taxa.