Can the richness-climate relationship be explained by systematic variations in how individual species' ranges relate to climate?

AimAt large spatial extents, the species richness of high-level taxa is generally strongly positively correlated with temperature and precipitation, and consistently so across space and time. Here, we test whether this richness-climate relationship is driven by systematic associations between climate and characteristics of the geographical ranges of individual species. Specifically, we test the hypotheses that spatial variations in richness are driven by variations in species mean range size, climatic niche-breadth, climatic range filling, frequency distribution of climatic niche position and/or frequency distribution of extant climatic conditions. LocationThe Americas. MethodsWe tested hypothetical effects of climatically constrained ranges on species richness using the breeding ranges of 3277 birds and 1659 mammals. We tallied species richness in 10(4)-km(2) quadrats in the Americas as well as summary statistics describing the geographical ranges and climatic niches of the species occurring in each quadrat. We then used regression models to relate species richness to those characteristics. ResultsWe found that species mean range size, climatic niche-breadth and range filling were generally, but inconsistently, negatively related to species richness. As predicted, species richness per quadrat increased with the number of species having their climatic niches centred in the climatic conditions of the quadrats and with the geographical extent of those conditions, although these relationships were relatively weak. Main conclusionThe richness-climate relationship appears to be largely decoupled from systematic variations in the characteristics of species climatic niches. Species generally have larger geographical ranges, wider climatic niches and higher range filling in species-poor areas, each of which, all else being equal, should generate a richness-climate relationship the inverse of what we generally observe in nature. More species have their ranges centred on warm, wet and common climatic conditions. However, temperature and precipitation variables themselves explain more of the variance in species richness than the measured characteristics of species climatic niches.