A 2021 study of 505 Neotropical epiphyte species (genera Peperomia and Elaphoglossum) documented two linked cross-gradient biogeographic patterns: a positive relationship across species between latitudinal and elevational range extents (termed 'Stevens' pattern'), and a negative within-species relationship between elevational and latitudinal occurrences, with most such species tracking a narrow band of mean annual temperature. Whether these patterns are a general macroecological regularity or contingent on regional topography and taxon, and what mechanisms (narrow thermal tolerances versus moisture regimes, biotic interactions, local adaptation, or cryptic species) generate them at biogeographic scale, remain unresolved. A 2024 follow-up field experiment by the same authors validated narrow thermal and moisture tolerances and extinction risk for 15 epiphyte species at one Central American mountain, but did not test the range-scale pattern, its other candidate mechanisms, or its generality across taxa and regions.

Background

Rapoport's rule is the macroecological observation, named by George C. Stevens in 1989, that species living at higher latitudes tend to have larger geographic (latitudinal) ranges than species at lower latitudes, plausibly because temperate species tolerate broader climatic variation; Stevens extended the idea in 1992 to elevation, predicting that species found higher on mountains occupy broader elevational ranges. Both the latitudinal and the elevational version have been tested many times with mixed results — for example, a 2012 global synthesis by McCain and Knight found the elevational rule to be far from pervasive on mountains — but the two gradients have almost always been analyzed separately. Epiphytes are non-parasitic plants that grow on other plants (here two species-rich genera of tropical understory and canopy plants), and they are useful for gradient studies because they are strongly elevationally zoned and climate-sensitive in tropical mountains. Hollenbeck and Sax (2021) compiled 35,382 occurrence records from GBIF (the Global Biodiversity Information Facility, an aggregator of species-occurrence data of uneven spatial sampling) for 505 Neotropical epiphyte species, computed each species' latitudinal extent (span of latitudes occupied) and elevational extent (span of elevations occupied), and examined how the two gradients interact. They found that about half the species show a significant negative relationship between their elevational and latitudinal occurrences — that is, individuals at higher elevations occur at lower latitudes within the species' range — and that most of these species occupy a non-random, relatively narrow band of mean annual temperatures across their whole latitudinal distribution ('temperature tracking'). Across species, latitudinal and elevational extents covary positively, a pattern the authors named 'Stevens' pattern', and wide-ranging species drive both this pattern and the elevational Rapoport pattern. Because the study is a case study of two Neotropical genera, its findings leave several questions open: the mechanistic causes of the negative elevation-by-latitude relationship (narrow thermal tolerances versus moisture regimes, biotic interactions, local adaptation, or cryptic species — distinct but morphologically similar species hidden within one name); the generality of Stevens' pattern across other taxonomic groups and non-Neotropical regions; and methodological biases from uneven GBIF sampling and coarse-resolution climate layers in topographically complex montane environments. The paper's main conclusions argue that many broad-ranging tropical species may be more temperature-sensitive than their broad ranges suggest and might be threatened by climate change, which raises the further question of whether global range metrics predict realized climatic tolerances and hence extinction risk under warming.

Problem Statement

Are the cross-gradient range-size patterns documented in Neotropical montane epiphytes — the positive covariation across species of latitudinal and elevational range extents ('Stevens' pattern') and the negative within-species relationship between elevational and latitudinal occurrences with non-random tracking of mean annual temperature — a general macroecological regularity, or are they contingent on regional topography and taxon-specific traits; and what mechanisms generate the negative elevation-by-latitude relationship at the scale of species' whole ranges? Concretely: (a) test Stevens' pattern and the negative elevation-by-latitude relationship in other taxonomic groups and in non-Neotropical mountain systems, with occurrence and climate data at resolutions and sampling-bias corrections adequate for topographically complex montane environments; (b) empirically disentangle the range-scale drivers of the negative elevation-by-latitude relationship — narrow thermal tolerances versus moisture regimes, biotic interactions, local adaptation, and cryptic species — using physiological measurements, reciprocal-transplant experiments (moving individuals between sites along environmental gradients) or common-garden experiments (growing individuals from different source populations under shared conditions), and finer-scale climatic data (local-scale evidence that temperature and aridity jointly limit survival already exists from one Central American mountain, but its generative link to the range-scale pattern remains untested); and (c) determine the degree to which latitudinal and elevational range extents predict realized climatic tolerances and hence extinction risk under warming, by pairing whole-range occurrence data with organismal tolerance and local demographic data. These components jointly serve one objective: establishing whether, why, and how latitudinal and elevational range gradients interact to structure species' distributions, so that global range metrics can serve as reliable predictors of climatic sensitivity for montane tropical biodiversity.

References

  1. Emily C. Hollenbeck and Dov F. Sax (2021), 'Novel biogeographic patterns across latitudinal and elevational gradients: A case study with tropical montane epiphytes lends insights to conservation', Journal of Biogeography 48:1076-1087, DOI 10.1111/jbi.14059, https://doi.org/10.1111/jbi.14059
  2. Emily C. Hollenbeck and Dov F. Sax (2024), 'Experimental evidence of climate change extinction risk in Neotropical montane epiphytes', Nature Communications 15:6045, DOI 10.1038/s41467-024-49181-5, https://doi.org/10.1038/s41467-024-49181-5
  3. Tarciso C. C. Leão, Peter B. Reich, and Eimear Nic Lughadha (2023), 'The geographic range size and vulnerability to extinction of angiosperm epiphytes in the Atlantic Forest of Brazil', Global Ecology and Biogeography 32:1607-1617, DOI 10.1111/geb.13718, https://doi.org/10.1111/geb.13718
  4. Gabriela C. de Mendonça, Yuri L. R. Leite, and Ana Carolina Loss (2025), 'The role of climate change in the past, present, and future of two species of montane neotropical mice, genus Juliomys', Mammalian Biology 105:787-798, DOI 10.1007/s42991-025-00518-7, https://doi.org/10.1007/s42991-025-00518-7
  5. George C. Stevens (1989), 'The Latitudinal Gradient in Geographical Range: How so Many Species Coexist in the Tropics', The American Naturalist 133:240-256, DOI 10.1086/284913, https://doi.org/10.1086/284913
  6. George C. Stevens (1992), 'The Elevational Gradient in Altitudinal Range: An Extension of Rapoport's Latitudinal Rule to Altitude', The American Naturalist 140:893-911, DOI 10.1086/285447, https://doi.org/10.1086/285447
  7. Christy M. McCain and Kevin Bracy Knight (2012), 'Elevational Rapoport's rule is not pervasive on mountains', Global Ecology and Biogeography 22:750-759, DOI 10.1111/geb.12014, https://doi.org/10.1111/geb.12014
  8. Kevin López-Reyes, Carlos Yáñez-Arenas, and Fabricio Villalobos (2024), 'Exploring the causes underlying the latitudinal variation in range sizes: Evidence for Rapoport's rule in spiny lizards (genus Sceloporus)', PLOS ONE 19:e0306832, DOI 10.1371/journal.pone.0306832, https://doi.org/10.1371/journal.pone.0306832