The weakness of evidence supporting tropical niche conservatism as a main driver of current richness–temperature gradients

Abstract

Aim Geographical variations in species richness are highly correlated with current temperature. The tropical niche conservatism hypothesis proposes that this relationship is driven by the evolutionarily conserved ancestral tolerances of species to the warm environments in which most clades originated. The hypothesis predicts that the slope of the richness–temperature gradient is positively related to the temperature of the period during which the clade originated. Here, we test this prediction for bird and mammal families in the A mericas and by revisiting a global analysis of 343 groups of organisms ( R omdal et al ., 2013, G lobal E cology and B iogeography , 22 , 344–350). Location The A mericas and world‐wide. Methods We computed the slope of the species richness–current temperature relationship within each bird and mammal family in the A mericas. We used palaeoclimate reconstructions to estimate temperature at the time of the family's origination, taken from the fossil record. We then tested how much of the among‐family variance in the richness–temperature slope could be explained by the temperature at the time of family origination. We repeated this test for the 343 groups of organisms world‐wide. Results Contrary to the prediction of tropical niche conservatism, the temperature at which bird and mammal families originated does not explain variations in the richness–temperature relationships. Similarly, we show that, although the predicted relationship was statistically detected in a broad range of groups of organisms in a previous study, it in fact statistically explains only 1.2–2.9% of the variance among groups in the slope of richness gradients. Main conclusions We found evidence inconsistent with the hypothesis that tropical niche conservatism is the main mechanism underlying the ubiquitous richness–climate relationship.

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