Functional Rarity: The Ecology of Outliers

Abstract

Rarity has been a central topic for conservation and evolutionary biologists aiming to determine the species characteristics that cause extinction risk. More recently, beyond the rarity of species, the rarity of functions or functional traits, called functional rarity, has gained momentum in helping to understand the impact of biodiversity decline on ecosystem functioning. However, a conceptual framework for defining and quantifying functional rarity is still lacking. We introduce 12 different forms of functional rarity along gradients of species scarcity and trait distinctiveness. We then highlight the potential key role of functional rarity in the long-term and large-scale maintenance of ecosystem processes, as well as the necessary linkage between functional and evolutionary rarity.

Figure I (Box 1)

Functional rarity types in local communities are assessed in both abundance and trait space by combining the classical view of taxonomic rarity and the modern view of trait rarity. Using a 10-individual community of 4 species, we highlight different facets of functional rarity integrated into a single framework. The 4 species correspond to archetypal situations at the extremes of the abundance scarcity and functional distinctiveness gradients, species A being the ecological outlier (highest functional rarity value) in the community, while species C is the ecological norm (lowest functional rarity value).

Figure 1

Hypothetical consequences of biodiversity loss on local ecosystem functioning, for the four scenarios of functional rarity (i.e. when species of each group are extirpated first when biodiversity declines). The letters correspond to the categories on the distinctiveness-scarcity biplot at local scale, as described in the Fig. I of Box 1.

Figure 2

Relationship between evolutionary and functional uniqueness of mammals at both global and European scales calculated with two different sets of traits. All mammals of the words that contained both traits and phylogenetic information were included (4616 species). Functional uniqueness was calculated without accounting for abundance. The global mammal functional distance matrices (Gower distance for multiple traits and Euclidean distance for log transformed body-mass) together with the phylogenetic distances were extracted from [81]. The list of mammal species for Europe was extracted from [82]. Colours represent the 10 and the 5 most frequent orders at global and European scales, respectively. The remaining orders (e.g. monotrema) are grouped into the Others category.