While latitudinal and elevational gradients of range size have been thoroughly studied, the ways these gradients might interact with each other to shape species distributions in complex montane environments are not well understood. We examined how elevational and latitudinal gradients interact to structure individual species’ distributions and larger geographic range size patterns.
Neotropics.
Two epiphytic plant genera:
We compiled 35,382 GBIF records for 505 species, calculated species’ latitudinal and elevational range extents, and examined gradients in richness and range size. For individual species, we analysed the relationship between elevational and latitudinal occurrences.
Approximately 50% of species demonstrate a significant, negative relationship between their elevational and latitudinal occurrences; most of these species occupy a non‐random, relatively narrow range of mean annual temperatures across their latitudinal distribution. Across species there is a positive relationship between latitudinal and elevational extent, a phenomenon we hereafter refer to as ‘Stevens’ pattern’. Average latitudinal extent of species’ ranges increased at higher latitudes, in support of Rapoport's rule. Average elevational extent increased with elevation in the global dataset (consistent with Rapoport's elevational rule), but most subsets of the data demonstrated a peak in average extent size at mid‐elevations.
The prevalence of species with negative elevation‐by‐latitude relationships, along with their non‐random tracking of temperature, suggests that many tropical species with broad geographic distributions are more temperature sensitive than their broad ranges might otherwise suggest. Consequently, even tropical species that occur across a wide range of latitudes and elevations might be threatened by climate change. These wide‐ranging species drive the occurrence of two biogeographic patterns: Rapoport's elevational rule and Stevens’ pattern. Finally, while Rapoport's rule and its elevational corollary were supported in part, the unexpected occurrence of many species restricted to high elevations near the equator suggests a possible focus for conservation effort.
