Evolutionary Biology

Species richness

Content Cover Image

Red-eyed Tree Frog (Agalychnis callidryas), photographed near Playa Jaco in Costa Rica (retouched version). (By Careyjamesbalboa (Carey James Balboa) [Public domain], via Wikimedia Commons)


Patterns of Species Richness


Some taxonomic groups of organisms have more species than other groups. For example, there are almost three times more species of beetles (Order Coleoptera) than there are in any other order of insects. In fact, there are so many species of beetles that there are more species of beetles than the total number of species currently living in the Phyla Echinodermata, Mollusca, Annelida, Platyhelminthes, Cnidaria, and Porifera combined!

Geographic patterns in species richness also exist. Latitudinal gradients in species richness are common in many taxonomic groups. Generally, species richness is higher in tropical regions than it is in temperate or polar regions. Not surprisingly, the two most species-rich habitat types, rainforests and coral reefs, are located in tropical regions. There are interesting patterns of species richness associated with island groups. Species richness is typically greater on larger islands and more species are found on islands that are nearer to the mainland (the source of colonization). Similar patterns can be observed in "habitat islands" such as forested mountaintops surrounded by arid land.

Local species richness can be influenced by ecological factors. For example, species richness is often higher in areas with higher productivity (the amount of carbon fixed by photosynthesis per unit area per time). Disturbances, such as fires, hurricanes, and floods, can also affect species richness. In many communities, species richness is greatest at intermediate frequency and/or intensity of disturbance (the intermediate disturbance hypothesis). This is because very frequent disturbance eliminates sensitive species, whereas very infrequent disturbance allows time for superior competitors to eliminate species that cannot compete.

New species arise through the process of speciation and species are lost by the process of extinction. Because extinction and speciation rates are not constant over time, species richness also varies considerably through evolutionary time as well. The number of species in particular taxonomic groups also can vary over time. Rapid rates of speciation can occur during adaptive radiations, resulting in an increase in the species richness of a taxon. Extinction rates also vary among taxa and over time. There have been at least five periods of mass extinction during the history of the Earth during which the species richness of the planet was reduced dramatically. Many scientists believe that Earth is currently experiencing the sixth mass extinction as a result of human transformation of the global environment.

caption Species richness as depicted by the biodiversity observed across the globe. A-I. Agricultural plant biodiversity; A. Apple; B. Zucchini; C. Egg plant; D. Tree; E. Potato plant; F. Lettuce; G. Rice grains; H. Wheat grains; and I. Potato tubers. J-L. Microbial biodiversity; J. Bacterial stab culture; K. Bacterial liquid culture; and L. Electron micrograph of powdery mildew fungi. M-R. Animal biodiversity; M. Malaysian tree frogs; N. Pigs; O. Cattle; P. Tiger; Q. Giraffe; R. Canada geese. S-T Food biodiversity; S. Vegetable biodiversity; T. Fish biodiversity. U-X. Fungal biodiversity; U. Bracket/shelf fungus (Basidiomycota) ; V. Enoki Mushrooms (Basidiomycota) and X. Mold on tomato (Zygomycoya). [Source: Saikat Basu, own work].


Further Reading

  • Wilson, E.O. 1992. The Diversity of Life. W.W. Norton & Company Inc. New York, NY. ISBN: 0393310477


McGinley, M. (2014). Species richness. Retrieved from http://www.eoearth.org/view/article/51cbeef17896bb431f69b531


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