No evidence for the niche breadth - range size hypothesis in big plant genera

Although almost 40% of the world's plant species are likely rare, little is known about the determinants of range size, a major axis of rarity, particularly in the tropics. Narrow geographic ranges make species more susceptible to extinction, so understanding factors that correlate with small range sizes is key to prioritising conservation efforts. The niche breadth - range size hypothesis predicts range restricted species to have narrow environmental tolerances. Here, we tested the niche breadth - range size hypothesis in two big tropical plant genera, Solanum and Begonia.

We used taxonomically verified occurrence data to quantify the range size of 1,065 species native to Southern America, calculated their climatic niche breadth based on six bioclimatic variables, and used spatial null models to determine whether the observed niche breadth - range size relationship differed significantly from spatial autocorrelation.

The correlation between climatic niche breadth and range size did not differ significantly from null expectations in either genus. Range-restricted species across both genera occur at higher elevations and have narrower elevational ranges than widespread species, however. More than two-thirds of the species had their minimum niche breadth in temperature seasonality, with strong increases between range-restricted and widespread species, suggesting that niche breadth in terms of temperature seasonality plays a key role in shaping species distributions.

Our findings support the view that niche breadth and range size correlations can artificially arise just from spatial autocorrelation. Macroclimatic conditions capture only a part of the ecological niche, and additional niche properties such as adaptations to soil and fire and biotic interactions are likely to influence species' ranges in particularly in species diverse tropical groups. Our findings emphasise that tropical mountains such as the Andes and the Atlantic forests of Brazil are globally important centres of range-restricted plant species.