Functional traits explain the Hutchinsonian niches of plant species

Aim The Hutchinsonian niche is a foundational concept in ecology and evolutionary biology that describes fundamental characteristics of any species: the global maximum population growth rate (r(max)); the niche optimum (the environment for which r(max) is reached); and the niche width (the environmental range for which intrinsic population growth rates are positive). We examine whether these characteristics are related to inter- and intraspecific variation in functional traits. Location Cape Floristic Region, South Africa. Time period Present day. Major taxa studied Twenty-six plant species (Proteaceae). Methods We measured leaf, plant-architectural and seed traits across species geographical ranges. We then examined how species-mean traits are related to demographically derived niche characteristics of r(max), in addition to niche optima and widths in five environmental dimensions, and how intraspecific trait variation is related to niche widths. Results Interspecific trait variation generally exceeded range-wide intraspecific trait variation. Species-mean trait values were associated with variation in r(max) (R-2 = 0.27) but were more strongly related to niche optima (mean R-2 = 0.56). These relationships generally matched trait-environment associations described in the literature. Both species-mean traits and intraspecific trait variability were strongly related to niche widths (R-2 = 0.66 and 0.59, respectively). Moreover, niche widths increased with intraspecific trait variability. Overall, the different niche characteristics were associated with few, largely non-overlapping sets of traits. Main conclusions Our study relating functional traits to Hutchinsonian niches demonstrates that key demographic properties of species relate to few traits with relatively strong effects. Our results further support the hypothesis that intraspecific trait variation increases species niche widths. Given that niche characteristics were related to distinct sets of traits, different aspects of environmental change might affect axes of trait variation independently. Trait-based studies of Hutchinsonian niches thus yield important insights into the mechanisms shaping functional biodiversity, which should reinforce the role of traits in functional biogeography.