Abstract
AbstractAimEcological 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.LocationCanary Islands.MethodsFor 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.ResultsThe 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 conclusionsThe 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.
Highlights
Understanding the environmental factors that influence the local occurrence and persistence of species is one of the most fundamental tasks in ecology
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
Summary
Understanding the environmental factors that influence the local occurrence and persistence of species is one of the most fundamental tasks in ecology. Species distribution modelling can work well for interpolation, but is not designed to delineate the boundaries of a fundamental environmental niche (Blonder et al, 2014) Such models risk calculating unbounded niches if species occurrence is modelled as increasing (unchecked) with environmental variables whose global range is not captured by the dataset being used. Broennimann et al, 2007; Early & Sax, 2014), perhaps because of enemy release, suggesting that the relationship between fundamental and realized niches may be weak Neutral theories, such as that of Hubbell (2001), demonstrate that biogeographic patterns can be modelled remarkably well without ecological differentiation between species. Attempts to scale to biogeographic patterns from ecological mechanisms, determined using reductionist approaches at a very local level, have met with mixed success at best
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