Elevational Rapoport's rule is not pervasive on mountains
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.
Mountains distributed globally and spanning 36.5° S to 48.2° N.
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.
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.
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.