Abstract
Species’ geographic ranges could primarily be physiological tolerances drawn in space. Alternatively, geographic ranges could be only broadly constrained by physiological climatic tolerances: there could generally be much more proximate constraints on species’ ranges (dispersal limitation, biotic interactions, etc.) such that species often occupy a small and unpredictable subset of tolerable climates. In the literature, species’ climatic tolerances are typically estimated from the set of conditions observed within their geographic range. Using this method, studies have concluded that broader climatic niches permit larger ranges. Similarly, other studies have investigated the biological causes of incomplete range filling. But, when climatic constraints are measured directly from species’ ranges, are correlations between species’ range size and climate necessarily consistent with a causal link? We evaluated the extent to which variation in range size among 3277 bird and 1659 mammal species occurring in the Americas is statistically related to characteristics of species’ realized climatic niches. We then compared how these relationships differed from the ones expected in the absence of a causal link. We used a null model that randomizes the predictor variables (climate), while retaining their broad spatial autocorrelation structure, thereby removing any causal relationship between range size and climate. We found that, although range size is strongly positively related to climatic niche breadth, range filling and, to a lesser extent, niche position in nature, the observed relationships are not always stronger than expected from spatial autocorrelation alone. Thus, we conclude that equally strong relationships between range size and climate would result from any processes causing ranges to be highly spatially autocorrelated.
Highlights
Species’ geographic ranges have long been thought to be strongly limited by their tolerances of climate [1]
Consistent with earlier literature, we find that the range size of bird and mammals species in the Americas is positively correlated with their climatic niche breadth, their range filling and their niche position
We found that strong correlations between range size and species’ climatic niche properties at macro-scales could arise from spatial autocorrelation alone
Summary
Species’ geographic ranges have long been thought to be strongly limited by their tolerances of climate [1]. Temperature and water availability, in particular, are generally thought to be important axes of the fundamental niche [2], which encompasses all environmental conditions in which a species can survive [3]. Species that are able to tolerate broader environmental conditions are hypothesized to be more widely distributed: niche breadth positively affects range size [11, 12] through an increase in species’ potential range size. Biotic interactions (e.g., greater parasitism in warm climates) and dispersal limitation (e.g., colonization lags following glaciations) are hypothesized to exclude species from occurring in all suitable climatic conditions, such that species whose biology allows them to fill a greater proportion of their fundamental niche should have larger ranges [13,14,15,16]. Niche position, i.e. where species’ niche occurs along a given climatic gradient, has been hypothesized to affect range size through various mechanisms [17,18,19]
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