The Latitudinal Diversity Gradient

by

Jonathan Rolland, Benjamin G. Freeman

• LAST REVIEWED: 12 January 2023
• LAST MODIFIED: 12 January 2023
• DOI: 10.1093/obo/9780199941728-0144

Introduction

The latitudinal diversity gradient describes the fact that there are less species in the temperate regions than in the tropics. The latitudinal diversity gradient is observed in most groups of animals, plants, and microorganisms, and remains one of the oldest and most famous mysteries in ecology. The ubiquity of this pattern suggests the possibility of a single general explanation for the latitudinal diversity gradient, and scientists since von Humboldt and Darwin have formulated dozens of hypotheses to explain the causes of this gradient. This article reviews the literature describing the main evolutionary hypotheses related to the fundamental processes that can explain why some places (like the tropics) have more species than others: speciation, extinction, colonization (dispersal), and the time necessary for diversity to accumulate. The recent advances in global-scale datasets of species distributions, the fossil record, and molecular mega-phylogenies give some hope of determining the primary cause(s) of the latitudinal diversity gradient.

General Overviews

Hawkins 2001 identifies Alexander von Humboldt as one of the first authors describing the latitudinal diversity gradient in 1807. Otté and Bohn 1850 describes von Humboldt’s multiyear expedition to the biodiverse tropical Andes in South America, where von Humboldt noticed that tropical species (distributed between 23.4° North and 23.4° South) greatly outnumbered temperate species. This pattern, called the latitudinal diversity gradient, is a general pattern that applies to most taxa. Some examples are described in Wiens, et al. 2009 for amphibians; Ricklefs 2006 for birds; Condamine, et al. 2012 for insects; Davies, et al. 2008 for mammals; Jablonski, et al. 2006 for marine invertebrates; Rabosky, et al. 2018 for fishes; and Fuhrman, et al. 2008 for microorganisms. Hillebrand 2004 reviews latitudinal patterns of species richness, and formally shows that most taxonomic groups indeed harbor a latitudinal diversity gradient. However, there are exceptions. Some clades have no latitudinal gradient of diversity, or even an inverse gradient where species richness peaks in the temperate zones (for example in lagomorphs and for some bryophytes, fungi, and insect groups, such as shown in Kouki, et al. 1994). There are dozens of hypotheses that attempt to explain the pattern, but until recently, many remained untested. The main reviews of the latitudinal diversity gradient literature, such as Mittelbach, et al. 2007, propose three processes that can contribute to the greater accumulation of lineages in the tropics: higher speciation rate in the tropics, higher extinction rate in the temperate regions, or higher dispersal rate from high latitude to low latitude. Now the combination of increasingly powerful genomic datasets used to infer species’ evolutionary histories and global datasets describing species’ distributions at unprecedented scale has led to exciting tests of hypotheses to explain the latitudinal diversity gradient.

• Condamine, F. L., F. A. H. Sperling, N. Wahlberg, J. Y. Rasplus, and G. J. Kergoat. 2012. What causes latitudinal gradients in species diversity? Evolutionary processes and ecological constraints on swallowtail biodiversity. Ecology Letters 15.3:267–277.

  DOI: 10.1111/j.1461-0248.2011.01737.x

  Large-scale analysis of butterflies testing different potential drivers of the latitudinal diversity gradient. Shows a strong role of shifts in species distribution from high to low latitudes related to cooling events, and a possible role of diversification.

• Davies, T. J., S. A. Fritz, R. Grenyer, et al. 2008. Phylogenetic trees and the future of mammalian biodiversity. Proceedings of the National Academy of Sciences of the USA 105:11556–11563.

  DOI: 10.1073/pnas.0801917105

  Describes the gradient of species diversity in mammals and proposes causes for the extinction of mammals.

• Fuhrman, J. A., J. A. Steele, I. Hewson, et al. 2008. A latitudinal diversity gradient in planktonic marine bacteria. Proceedings of the National Academy of Sciences of the USA 105.22:7774–7778.

  DOI: 10.1073/pnas.0803070105

  Rare study of the latitudinal diversity of pelagic marine bacteria from 103 samples in coastal and pelagic water. Confirms a strong latitudinal gradient in the world of oceanic microorganisms.

• Hawkins, B. A. 2001. Ecology’s oldest pattern? Trends in Ecology & Evolution 16.8:470.

  DOI: 10.1016/S0169-5347(01)02197-8

  Discusses when the latitudinal diversity gradient was first discovered and proposes that Alexander von Humboldt was likely the first scientist to describe this pattern.

• Hillebrand, H. 2004. On the generality of the latitudinal diversity gradient. American Naturalist 163.2:192–211.

  DOI: 10.1086/381004

  Highly cited article studying the slope and the pervasiveness of the latitudinal diversity gradient in many clades.

• Jablonski, D., K. Roy, and J. W. Valentine. 2006. Out of the tropics: Evolutionary dynamics of the latitudinal diversity gradient. Science 314.5796:102–106.

  DOI: 10.1126/science.1130880

  Very important article testing the fundamental hypotheses about the construction of the latitudinal gradient with exceptional fossil data of bivalves. Suggests that the tropics are both a cradle and museum of biodiversity.

• Kouki, J., P. Niemalä, and Matti Viitasaari. 1994. Reversed latitudinal gradient in species richness of sawflies (Hymenoptera, Symphyta). Annales Zoologici Fennici 31.1: 83–88.

  Describes the example of sawflies, a clade that shows an inverse latitudinal diversity gradient, with higher diversity in the temperate regions.

• Mittelbach, G. G., D. W. Schemske, H. V. Cornell, et al. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecology Letters 10:315–331.

  DOI: 10.1111/j.1461-0248.2007.01020.x

  Classic review of the major hypotheses related to the construction of the latitudinal diversity gradient.

• Otté, E. C., and H. B. Bohn, trans. 1850. Views of nature: Or contemplations on the sublime phenomena of creation; with scientific illustrations. 3d ed. By Alexander von Humboldt. London: Henry G. Bohn.

  DOI: 10.5962/bhl.title.155802

  Translation of what could be a first description of the latitudinal diversity gradient according to Hawkins 2001.

• Rabosky, D. L., J. Chang, P. O. Title, et al. 2018. An inverse latitudinal gradient in speciation rate for marine fishes. Nature 559:392–395.

  DOI: 10.1038/s41586-018-0273-1

  Describes the latitudinal diversity gradient for most fish groups and proposes that diversification may not be correlated with diversity.

• Ricklefs, R E. 2006. Global variation in the diversification rate of passerine birds. Ecology 87.10:2468–2478.

  DOI: 10.1890/0012-9658(2006)87[2468:gvitdr]2.0.co;2

  Studies the variables, including latitude, that explain diversification rates in birds; it was one of the first studies to show faster diversification in tropical clades.

• Wiens, J. J., J. Sukumaran, R. A. Pyron, and R. M. Brown. 2009. Evolutionary and biogeographic origins of high tropical diversity in old world frogs (Ranidae). Evolution 63.5:1217–1231.

  DOI: 10.1111/j.1558-5646.2009.00610.x

  Studies the role of diversification and dispersal in a large group of Ranidae frogs.