Rapoport's rule — the tendency for species' geographic ranges to widen with latitude — has classically been explained by the climatic variability hypothesis: species at higher latitudes experience larger seasonal temperature swings and therefore evolve broader thermal tolerances. Tomašových, Jablonski, Berke, Krug and Valentine (2015) showed that this argument ignores the geometry of latitudinal thermal gradients: because a given sea-surface temperature spans a much broader latitudinal band in the tropics than at higher latitudes, a model of temperature-limited range expansion along realistic gradients predicts an inverse relationship between range size and latitude — a reverse Rapoport's rule — a prediction matched by the ranges of 4,760 marine bivalve species. The open question suggested by that paper's discussion is whether this thermal-geometry explanation is general: does the reverse pattern hold across other marine taxa (the discussion names reef fishes and corals) and in terrestrial settings, where shallow tropical and steep high-latitude thermal gradients also characterize some environments but variation in annual precipitation can further modify range expansion? The model deliberately fixes thermal tolerances, keeps ranges continuous and non-disjunct, neglects depth distributions such as equatorial submergence, and excludes speciation and local interactions; how these simplifications, and non-thermal factors such as dispersal limitation at sea, bear on the mechanism's generality is not settled by the source. Later work has confirmed the inverse pattern in reef fishes, seagrasses and eastern Pacific cephalopods and has quantified phylogenetic signal in range size, but no study has quantified the non-thermal modifiers within the model, extended it to disjunct or depth-structured ranges, tested it in terrestrial taxa, or integrated thermal limitation with macroevolutionary dynamics, so the question remains open.

Background

Rapoport's rule is the macroecological generalization, proposed by Eduardo Rapoport in 1975 and developed by George Stevens in 1989, that species living at higher latitudes have broader geographic ranges (measured as latitudinal extent) than species living closer to the equator. Its standard explanation is the climatic variability hypothesis: species at high latitudes experience larger annual variation in temperature, are selected for broader thermal tolerances, and can consequently occupy wider latitudinal bands. The rule and this mechanism have been debated for decades, with support varying among regions, taxa and realms. Tomašových, Jablonski, Berke, Krug and Valentine (2015) pointed out that the classical argument implicitly treats the relationship between latitude and temperature as linear. In reality, latitudinal thermal gradients are strongly nonlinear: a given sea-surface temperature occupies a much broader latitudinal extent in the tropics than at higher latitudes, so thermally uniform habitat is spatially more extensive at low latitudes. They built a range-expansion model in which a species' geographic range is limited by its thermal tolerance: the minimum and maximum annual temperatures a species can withstand determine how far its range can extend, and the temperatures encompassed by the realized range (the macroecological thermal range) scale with the annual temperature minima and maxima at the species' point of origin. Because equal-temperature bands are wide in the tropics, temperature-limited ranges there can be latitudinally broad even though tropical species have narrow thermal tolerances; along the realistic nonlinear gradient the model predicts that range size decreases with latitude — the reverse (or inverse) of Rapoport's rule, sometimes called a reverse Rapoport pattern. Comparing the model against a database of 40,820 occurrences of 4,760 marine bivalve species from the western Pacific, eastern Pacific and western Atlantic, the authors found that the bivalves' latitudinal range sizes match this inverse prediction, and that the pattern is driven more by the nonlinearity of the gradients in annual temperature minima and maxima than by the latitudinal gradient in seasonal temperature range. The model, however, deliberately excludes everything except temperature: species' thermal tolerances are fixed, ranges expand continuously and never become disjunct (split into separate sub-ranges), depth distributions are ignored (a notable omission because of equatorial submergence — the tendency of tropical marine taxa to shift to greater depths near the equator, so that their surface-latitude ranges understate their true thermal distributions), and processes operating on evolutionary timescales — niche evolution (change in a lineage's environmental tolerances), speciation, and clade-level range expansion — are outside the model, as are non-thermal environmental factors such as precipitation on land and dispersal limitation (the restriction of range expansion by organisms' limited dispersal capacity) in the sea. Within macroecology these omissions matter because range-size gradients feed into explanations of the latitudinal diversity gradient and into forecasts of how species' ranges will shift under climate change.

Problem Statement

The problem suggested by the forward-looking remarks in the Discussion of Tomašových, Jablonski, Berke, Krug and Valentine (2015) is to determine whether the nonlinear thermal-gradient mechanism is a general explanation for broad-scale latitudinal gradients in geographic range size. The paper demonstrates the mechanism in marine bivalves along the three best-sampled ocean margins and observes that, at the global scale, shallow thermal gradients within the tropics and steep thermal gradients at higher latitudes also characterize some terrestrial environments, so that these effects "may be a general factor shaping latitudinal range-size gradients"; it suggests that range expansion along gradients that vary in the steepness of their temperature trends "may also explain reverse Rapoport's patterns in other marine groups, such as reef fishes and corals", while cautioning that "variation in annual precipitation can further modify range expansion in those settings". The central question is therefore empirical generality: does the inverse relationship between latitudinal range size and latitude (the reverse of Rapoport's rule) predicted by temperature-limited range expansion along nonlinear thermal gradients, and observed in marine bivalves, hold across other marine and terrestrial taxa, and does the thermal-gradient geometry alone suffice to reproduce observed range-size gradients in those settings, or must it be augmented by non-thermal factors such as precipitation? Both parts of this question are mandatory. A complete answer must determine, for at least one marine taxon (other than the bivalves, reef fishes, seagrasses and eastern Pacific cephalopods, for which the inverse pattern is already reported) and at least one terrestrial taxon, whether the latitudinal range-size gradient has the inverse sign predicted by temperature-limited expansion along the local nonlinear thermal gradient, and must also determine whether a temperature-only version of the mechanism reproduces the observed gradient in those taxa, identifying which non-thermal factor (such as precipitation) is required where temperature alone does not. The generality of the explanation is then measured by how far these outcomes extend as further taxa and differently shaped regional gradients are examined.

Closely related questions arise from the model's stated scope choices — which the source paper justifies rather than poses as open — and from factors the source treats as competing explanations; they are recorded here as context bearing on the generality question, not as jointly required components of an answer. How do the predictions change when the model's simplifying assumptions are relaxed: thermal tolerances that evolve rather than remain fixed, ranges that become disjunct rather than continuous (the models do "not allow discontinuous species ranges"), and depth-structured distributions (including equatorial submergence, which affects equatorward rather than poleward range limits) rather than purely surface-latitude ranges? What role does dispersal limitation in marine systems — argued against for bivalves in the source, where the marked clustering of poleward range limits associated with steep mid-latitude thermal gradients points to temperature limitation — play in other taxa and settings? And how does thermal limitation interact with evolutionary processes — niche evolution, speciation, and clade-level range expansion — over macroevolutionary timescales to shape latitudinal range-size gradients, given that the source focuses on range expansion "rather than speciation or local interactions, both of which might also influence range-size patterns"?

References

  1. Tomašových, A., Jablonski, D., Berke, S. K., Krug, A. Z., & Valentine, J. W. (2015). Nonlinear thermal gradients shape broad-scale patterns in geographic range size and can reverse Rapoport's rule. Global Ecology and Biogeography 24(2): 157-167. doi:10.1111/geb.12242, https://doi.org/10.1111/geb.12242 (source paper)
  2. Tomašových, A., Kennedy, J. D., Betzler, T. J., Bitler Kuehnle, N., Edie, S., Kim, S., Supriya, K., White, A. E., Rahbek, C., Huang, S., Price, T. D., & Jablonski, D. (2016). Unifying latitudinal gradients in range size and richness across marine and terrestrial systems. Proceedings of the Royal Society B: Biological Sciences 283(1830): 20153027. doi:10.1098/rspb.2015.3027, https://doi.org/10.1098/rspb.2015.3027
  3. Tomašových, A., & Jablonski, D. (2017). Decoupling of latitudinal gradients in species and genus geographic range size: a signature of clade range expansion. Global Ecology and Biogeography 26(3): 288-303. doi:10.1111/geb.12533, https://doi.org/10.1111/geb.12533
  4. Pie, M. R., Divieso, R., Caron, F. S., Siqueira, A. C., Barneche, D. R., & Luiz, O. J. (2021). The evolution of latitudinal ranges in reef-associated fishes: Heritability, limits and inverse Rapoport's rule. Journal of Biogeography 48(9): 2121-2132. doi:10.1111/jbi.14150, https://doi.org/10.1111/jbi.14150
  5. Tuya, F., Bosch, N. E., & Rodríguez, L. (2024). Larger distribution ranges of seagrasses towards the equator. Journal of Biogeography 51(3): 350-355. doi:10.1111/jbi.14756, https://doi.org/10.1111/jbi.14756 (online 2023)
  6. Ibáñez, C. M., Braid, H. E., Carrasco, S. A., López-Córdova, D. A., Torretti, G., & Camus, P. A. (2019). Zoogeographic patterns of pelagic oceanic cephalopods along the eastern Pacific Ocean. Journal of Biogeography 46(6): 1260-1273. doi:10.1111/jbi.13588, https://doi.org/10.1111/jbi.13588
  7. Saupe, E. E., Myers, C. E., Peterson, A. T., Soberón, J., Singarayer, J., Valdes, P., & Qiao, H. (2019). Non-random latitudinal gradients in range size and niche breadth predicted by spatial patterns of climate. Global Ecology and Biogeography 28(7): 928-942. doi:10.1111/geb.12904, https://doi.org/10.1111/geb.12904
  8. Gross, K., & Snyder-Beattie, A. (2016). A General, Synthetic Model for Predicting Biodiversity Gradients from Environmental Geometry. The American Naturalist 188(4): E85-E97. doi:10.1086/688171, https://doi.org/10.1086/688171
  9. Zacaï, A., Fara, E., Brayard, A., Laffont, R., Dommergues, J.-L., & Meister, C. (2017). Phylogenetic conservatism of species range size is the combined outcome of phylogeny and environmental stability. Journal of Biogeography 44(11): 2451-2462. doi:10.1111/jbi.13043, https://doi.org/10.1111/jbi.13043
  10. Zacaï, A., Brayard, A., Laffont, R., Dommergues, J.-L., Meister, C., & Fara, E. (2018). The Rapoport effect and the climatic variability hypothesis in Early Jurassic ammonites. Palaeontology 61(6): 963-980. doi:10.1111/pala.12389, https://doi.org/10.1111/pala.12389
  11. Reygondeau, G., & Egorova, Y. (2025). Rapoport's rule in the marine realm: wrong axis, right pattern. bioRxiv preprint posted 31 December 2025. doi:10.64898/2025.12.30.696717, https://doi.org/10.64898/2025.12.30.696717
  12. Radomski, T. (2025). Revisiting Rapoport's Rule: The History of an Idea and Alternative Explanations for Latitudinal Variation in Geographic Range Size. The Quarterly Review of Biology 100(2): 63-81. doi:10.1086/736000, https://doi.org/10.1086/736000
  13. López-Reyes, K., Yáñez-Arenas, C., & Villalobos, F. (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