Latitudinal biodiversity gradient

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Latitude gradient of species number for vertebrates (increasing red color means increasing species number)

A latitudinal biodiversity gradient or also latitudinal gradient of biodiversity is a pattern of change in biodiversity depending on the latitude . The best known and best studied is the latitude gradient of the number of species , also the latitude gradient of the number of species : The number of species decreases with increasing latitude - it is highest in tropical areas, and decreases significantly in the direction of both poles. This pattern of biodiversity can currently be observed globally for many groups of organisms and was probably also valid over long periods of earth's history. There are a number of hypotheses to explain this, on which no consensus has yet been reached. Researching the gradient is an important prerequisite for assessing and, if possible, limiting the further development of species extinction , for example due to habitat loss and climate change.

The increase in biodiversity towards the tropics can be found both in the sea and on land and on various spatial scales ( climatic zones , regions and local communities). The exact form of the relationship depends on many factors, including the taxa considered , areas or time periods. The few exceptions include some coastal and marine vertebrate groups such as seals and albatrosses. There have been times in the history of the earth when the gradient was flatter than it is now. At times there could even have been a “paleo-temperate peak” in which the number of species between the 30 ° and 60 ° latitudes could have been highest. This could have been true of the dinosaurs in the late Cretaceous period (100 mya to 66 mya). For the early Paleozoic (458 mya to 423 mya) the late Paleozoic (330 mya to 270 mya) and the last 30 million years there is good evidence of latitude gradients similar to today's.

Some hypotheses to explain this phenomenon assume that there has been no equilibrium in the number of species in the areas that were previously characterized by ice cover since the end of the last glacial period . A large part of the species would have developed under warm climatic conditions in the upper Mesozoic and early Paleogene , which is why there would be a head start in the evolutionary development of the species in the tropics. According to these hypotheses, the number of species in higher latitudes would approach that in lower latitudes over very long periods of time with a constant earth's climate through repopulation and diversification.

A second group of hypotheses assumes that such compensation processes do not take place (anymore), that there is an equilibrium and that the upper limit of species richness has been reached everywhere. Larger land and water areas in the tropics, which would be home to more species ( species-area relationship ), are named here as a possible cause of the gradient . Regions with the highest biodiversity, such as in Southeast Asia, are not always found in the largest areas. Therefore area hypotheses can only be a partial explanation.

Further hypotheses cite a higher diversification rate in the tropics as the cause. The net rate of diversification lies in the tropics; H. the number of newly added species minus the extinct species, higher than elsewhere. This could be due to a higher speciation rate , a lower rate of extinction, or both. The higher solar radiation in the tropics could play a role here, which could result in higher primary production and thus also more differentiated niches along the food chain . Climatic conditions could also favor diversification, so the more balanced climatic conditions in the tropics could make it easier for groups with newly developing characteristics to survive and colonize niches. At the same time, these species are not able to adapt to higher climatic variability. The genetic drift in the tropics could be higher, biotic interactions more intense.

Alexander von Humboldt was perhaps the first to report a decrease in the number of species with increasing width. He used temperature differences as an explanation. He wrote:

“But if the fullness of life is widespread everywhere, the organism is also incessantly trying to combine the elements unleashed by death into new forms, then this fullness of life and its renewal differs according to the differences in the lines of the sky. […] The closer, on the other hand, to the tropics: the greater the diversity of design, grace of form and color mix, eternal youth and the strength of organic life. "

- Alexander von Humboldt : Views of Nature

In 1878 Alfred Russel Wallace suggested the increasingly harsh climate as an explanation for the decreasing biodiversity towards the poles. In the tropical environment, which was balanced - both in the course of the year and over past ages - and unaffected by catastrophic cold periods, however, he saw above all the effect of the organisms on one another, which fills every free niche, as essential for the balanced biodiversity.

Since the mid-1950s there has been increasingly rigorous numerical evidence of relationships between species richness and latitude. Latitudinal gradients could also be demonstrated for higher taxa. Since the mid-2000s, latitude gradients have increasingly been viewed from a functional, phylogenetic , genetic and phenetic perspective.

literature

  • PD Mannion: Patterns in Palaeontology: The latitudinal biodiversity gradient . In: Palaeontology Online . tape 4 , 2014, p. 1-8 ( palaeontologyonline.com ).
  • Michael R. Willig and Steven J. Presley: Latitudinal Gradients of Biodiversity . In: Reference Module in Life Sciences . 2017, doi : 10.1016 / B978-0-12-809633-8.02174-9 (updated version of an article in the Encyclopedia of Biodiversity (Second Edition), 2013).

Web links

Individual evidence

  1. ↑ a b c d e P. D. Mannion: Patterns in Palaeontology: The latitudinal biodiversity gradient . In: Palaeontology Online . tape 4 , 2014, p. 1-8 ( palaeontologyonline.com ).
  2. ^ Lauren Gough and Richard Field: Latitudinal Diversity Gradients . In: Encyclopedia of Life Sciences . September 2007, doi : 10.1002 / 9780470015902.a0003233.pub2 .
  3. ↑ a b Michael R. Willig and Steven J. Presley: Latitudinal Gradients of Biodiversity . In: Reference Module in Life Sciences . 2017, doi : 10.1016 / B978-0-12-809633-8.02174-9 .
  4. ↑ a b c Jens Boenigk and Sabina Wodniok: Biodiversity and Earth History . Springer, 2015, ISBN 978-3-642-55389-9 , Chapter 3.2.1.5 Global gradients of biodiversity, p. 186-187 .
  5. ^ A b Gary G. Mittelbach: Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography . In: Ecology Letters . tape 10 , 2007, doi : 10.1111 / j.1461-0248.2007.01020.x .
  6. ↑ Alexander von Humboldt: Views of nature with scientific explanations . 1808, ideas for a physiognomy of plants ( limited preview in the Google book search).
  7. ^ Alfred Russel Wallace: Tropical nature, and other essays . Macmillan and co., London 1878, p. 65-66, 121-123 ( biodiversitylibrary.org ).