**1. Introduction**

There is growing evidence that rare species play key ecological roles in the functioning and structuring of communities [1–3]. Additionally, rare species tend to be more affected by habitat disturbance compared to common species [4–6], and should thus comprise important targets of conservation programs [6]. A better understanding of rarity patterns is therefore necessary to help with guiding conservation efforts [6].

In the context of populations and communities, ecologists tend to define rare species as those showing low abundance and/or a small distribution area (see reviews in [7,8]). In conservation biology, the meaning of rare is usually associated with extinction risk [6], given that low abundance and small distribution areas are linked to heightened extinction risk of species [9]. Although the term rarity has been broadly used, a more precise concept of rarity was proposed by Rabinowitz [10] as a combination of three components: Small geographical range, high habitat specificity, and low local abundance. The so-called "seven forms of rarity" of Rabinowitz [10] is often suggested to have some degree of subjectivity in the classification of species (e.g., [11–13]). However, although several other strategies for the classification of rarity have been proposed (see review [6]), the "seven forms of rarity" approach is

still a simple, e ffective, and widely used framework to characterize rare species from ecological and conservation points of view (e.g., [14–22]).

Rarity patterns resulting from the "seven forms of rarity" approach tend to be strongly correlated with extinction risk assessed by traditional methods like the International Union for Conservation of Nature (IUCN) categories and criteria [14,22]. In fact, this correlation is expected because most methods aimed to detect extinction risk also use two of the criteria used to define rarity, geographical range and population size, among their main criteria [23]. However, using just one aspect of rarity, such as small geographical range, could lead to an underestimation of extinction risk of a species. On the other hand, the interplay between processes at di fferent scales that make species common or rare can create associations between di fferent components of rarity [24,25]. The investigation of such associations is thus necessary for a better understanding of rarity and also has practical consequences like the use of one aspect as a proxy for the other aspects when data are incomplete. Furthermore, detecting intrinsic factors that could predict rarity and its di fferent aspects from species trait data would also help in conservation e fforts by accelerating assessments of rarity and subsequent prioritization exercises. Rarity patterns in tropical woody plants, for example, have a strong phylogenetic component, indicating that related species tend to be similar regarding rarity [26]. In birds, abundance is negatively related to body size, and range sizes are better predicted by habitat specificity, migratory status, and clutch size of di fferent species [27]. Thus, identifying correlations among the di fferent aspects of rarity and possible intrinsic factors that can predict rarity would help us to increase the e fficacy of conservation strategies.

Vipers (family Viperidae) comprise about 350 snake species distributed in most continents. New World (NW) vipers are all in the subfamily Crotalinae (pitvipers), comprising about 12 genera and 150 species (~ 43% of all vipers [28]) distributed throughout the Americas [29]. NW pitvipers diverged from their Old World relatives around 28 million years ago and arrived in the New World from a single invasion into North America [30–32]. Once their ancestor reached the NW, pitvipers rapidly occupied virtually all terrestrial habitats available [29,30,32,33] and are today conspicuous predators in most NW ecosystems. There is a large body of literature on the biology of NW pitvipers, especially focusing on their medical importance [34,35], complex venom apparatus [36–38], and feeding habits [29,39,40]. Additionally, compared to other snake lineages and reptiles in general, vipers are significantly more threatened, mainly due to habitat loss and persecution by humans [41–43]. Given their high extinction risk but substantial availability of biological information, NW pitvipers comprise an interesting snake lineage to explore rarity patterns, the intrinsic factors a ffecting these patterns, and the possible associations between rarity and extinction risk.

Herein, we compiled a dataset of NW pitvipers to categorize species according to Rabinowitz [10]. Additionally, we explored how rarity is distributed across space and among lineages. If rare species are located in a more restricted area and/or lineage, this could guide conservation e fforts. We also investigated whether intrinsic factors predict rarity patterns. We expected that body size and latitude could predict rarity and thus could be used as proxies in circumstances where data on distribution, habitat breadth or population abundance are not available. We chose these variables because body size is frequently related to life history traits and range size, and abundance was found to be related to latitude in some clades (see additional details in *Predicting rarity* below). Finally, we compared the rarity patterns found with assessments of extinction risk and other prioritization methods in order to detect rare species which may have so far been overlooked in conservation e fforts. As Rabinowitz's classification uses three di fferent criteria (geographical ranges, local abundances, and habitat breadths) to define rare species, we expect that some rare species would not be listed as threatened and/or prioritized for conservation, and thus, this would demonstrate that assessments of rarity could provide an additional tool to guide conservation e fforts for these key ecological species.

## **2. Materials and Methods**
