How and why species are rare: A mechanistic reappraisal of the Rabinowitz rarity framework

The three-dimensional rarity typology proposed by Rabinowitz in 1981, based on range size, habitat specificity, and local abundance, is perhaps the most widely used framework for describing rarity in ecological and conservation research. While this framework is descriptive and does not explain the causes of rarity, recent advances in ecology may be leveraged to add explanatory power. We propose a modification of Rabinowitz’s typology to better distinguish between the dimensions of rarity and the processes that drive them and explore the conservation implications of our modified framework. We suggest replacing habitat specificity, which is arguably a cause of rarity, with occupancy (the proportion of occupied sites within a species’ range), yielding a modified classification based on range size, occupancy, and local abundance. Abundant, widespread habitat specialists are no longer considered rare; however, we argue that this modification more accurately identifies truly rare species, as habitat specialists may be common if their habitat is abundant. Finally, we draw on the functional literature to identify the key processes and associated traits that drive each rarity axis. In this respect, we identify four processes (environmental filtering, movement, demography, and interactions), and hypothesise that range size and occupancy are primarily driven by environmental filtering and movement, whereas local abundance is more strongly influenced by demography and interactions. Our work aims at providing a basis for developing hypotheses about the causes of rarity in particular taxa and identifying suitable conservation measures targeting different types of rare species.