Biogeographic ranges do not support niche theory in radiating Canary Island plant clades

Aim: Ecological niche concepts, in combination with biogeographic history, underlie our understanding of biogeographic ranges. Two pillars of this understanding are competitive displacement and niche conservatism. The competitive displacement hypothesis holds that very similar (e.g. closely related) co-occurring species should diverge, forced apart by competition. In contrast, according to the niche conservatism hypothesis, closely related species should have similar niches. If these are fundamental structuring forces, they should be detectable when comparing the climatic niches of endemic species in radiating clades in oceanic archipelagos, where closely related species exist in both sympatry and allopatry and the species' entire ranges are known. We took advantage of this natural experimental system to test whether the climatic niche relationships predicted by the two hypotheses are found. Location: Canary Islands. Methods: For the plant clades Aeonium, Argyranthemum, Descurainia, Echium, Lotus and Sonchus, separately, we tested relationships between phylogenetic distance and climatic niche differentiation (in temperature, precipitation and their combination), using a high-resolution dataset. We also tested for niche conservatism using Blomberg's K and Pagel's λ. We compared climatic niche differentiation between pairs of species existing in sympatry with that for pairs of species in allopatry. For each comparison, we focused on the climatic niche space available to both species. Results: The relationships between phylogenetic distance and climatic niche differentiation were mostly non-significant; some weak but significant positive relationships were found, mainly for Aeonium and Sonchus. Where differences between sympatry and allopatry were found, niche differentiation tended to be greater in allopatry. Main conclusions: The expectations from niche conservatism were frequently not met; instead our results suggest considerable climatic niche lability. All significant differences in climatic niche differentiation were opposite to the predictions from competitive displacement. These forces may be less important in structuring biogeographic ranges than is commonly thought, at least on islands.