Aim Elevational Rapoport's rule, proposed in 1992 by Stevens, predicts that species ranges on mountains become larger in elevational extent with increasing elevation. Here we test this prediction using 160 datasets of range size measured by maximum elevational extents for bats, birds, frogs, non‐volant small mammals, reptiles, and salamanders from mountains around the globe. Location Mountains distributed globally and spanning 36.5° S to 48.2° N. Methods We compare three methods: (1) the Stevens method, which uses the average range size of all species within each elevational band (100‐m bands); (2) the midpoint method, which uses the average range size of species whose midpoints occur in each elevational band; and (3) a quartile method that examines the distribution of only the smallest ranges (less than one‐quarter of the mountain height) to see if their frequency distribution is negatively related to elevation. Results Support for the elevational Rapoport's rule was weak across all groups of montane vertebrates. For the Stevens method, the mean r2 value was 0.32, and strong support (positive relationship, r2 value > 0.50) was detected in 40% of the studies, ranging from 20% for salamanders to 57% for frogs. For the midpoint method, the mean r2 value was 0.06, and none of the datasets showed strong support. For the quartile method, the mean r2 value was 0.26, and strong support (negative relationship, r2 value > 0.40) was detected in 38% of the studies, ranging between 10.5% in salamanders and 58% in reptiles. Main conclusions Across vertebrates, and within the literature for plants and invertebrates, more empirical studies find a lack of trend than the predicted trend of increasing range size with increasing elevation. Thus, elevational Rapoport's rule is not a consistently predictive pattern for understanding montane patterns in range size.

The rush to assess species' responses to anthropogenic climate change (CC) has underestimated the importance of interannual population variability (PV). Researchers assume sampling rigor alone will lead to an accurate detection of response regardless of the underlying population fluctuations of the species under consideration. Using population simulations across a realistic, empirically based gradient in PV, we show that moderate to high PV can lead to opposite and biased conclusions about CC responses. Between pre- and post-CC sampling bouts of modeled populations as in resurvey studies, there is: (i) A 50% probability of erroneously detecting the opposite trend in population abundance change and nearly zero probability of detecting no change. (ii) Across multiple years of sampling, it is nearly impossible to accurately detect any directional shift in population sizes with even moderate PV. (iii) There is up to 50% probability of detecting a population extirpation when the species is present, but in very low natural abundances. (iv) Under scenarios of moderate to high PV across a species' range or at the range edges, there is a bias toward erroneous detection of range shifts or contractions. Essentially, the frequency and magnitude of population peaks and troughs greatly impact the accuracy of our CC response measurements. Species with moderate to high PV (many small vertebrates, invertebrates, and annual plants) may be inaccurate 'canaries in the coal mine' for CC without pertinent demographic analyses and additional repeat sampling. Variation in PV may explain some idiosyncrasies in CC responses detected so far and urgently needs more careful consideration in design and analysis of CC responses.

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