Testing hypotheses on the ecological patterns of rarity using a novel model of study: snake communities worldwide

Abstract. The theoretical and empirical causes and consequences of rarity are of central importance for both ecological theory and conservation. It is not surprising that studies of the biology of rarity have grown tremendously during the past two decades, with particular emphasis on patterns observed in insects, birds, mammals, and plants. I analyse the patterns of the biology of rarity by using a novel model system: snake communities worldwide. I also test some of the main hypotheses that have been proposed to explain and predict rarity in species. I use two operational definitions for rarity in snakes: Rare species (RAR) are those that accounted for 1% to 2% of the total number of individuals captured within a given community; Very rare species (VER) account for ≤1% of individuals captured. I analyse each community by sample size, species richness, continent, climatic region, habitat and ecological characteristics of the RAR and VER species. Positive correlations between total species number and the fraction of RAR and VER species and between sample size and rare species in general were found. As shownin previous insect studies, there is a clear trend for the percentage of RAR and VER snake species to increase in species-rich, tropical African and South American communities. This study also shows that rare species are particularly common in the tropics, although habitat type did not influence the frequency of RAR and VER species. This analysis also confirms the commonly accepted ecological hypothesis that body size and rarity are clearly and widely correlated in natural animal communities. However, in snake communities there is often an association between large and small species among the rare species, and a tendency for ophiophagous species to be rare. In addition, there was no support for the hypothesis that rare species should be typically phylogenetically primitive. The hypothesis that species with narrower realized ecological niches are more likely to be rare or very rare is supported by the evidence presented here on snake communities. In general, this study shows that snakes may make ”model organisms” for studies on the biology of rarity.