Why Are Species' Traits Weak Predictors of Range Shifts?

We examine the evidence linking species' traits to contemporary range shifts and find they are poor predictors of range shifts that have occurred over decades to a century. We then discuss reasons for the poor performance of traits for describing interspecific variation in range shifts from two perspectives: (a) factors associated with species' traits that degrade range-shift signals stemming from the measures used for species' traits, traits that are typically not analyzed, and the influence of phylogeny on range-shift potential and (b) issues in quantifying range shifts and relating them to species' traits due to imperfect detection of species, differences in the responses of altitudinal and latitudinal ranges, and emphasis on testing linear relationships between traits and range shifts instead of nonlinear responses. Improving trait-based approaches requires a recognition that traits within individuals interact in unexpected ways and that different combinations of traits may be functionally equivalent.