Species richness is simply the number of species present in a sample, community, or taxonomic group. Species richness is one component of the concept of species diversity, which also incorporates evenness, that is, the relative abundance of species. Species diversity is one component of the broader concept of biodiversity. About 1.75 million living species and 300,000 fossil species have been described by scientists. Estimates of the total species richness of the Earth range from three to 10 million, with some estimates as high as 50 million.
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.
- Wilson, E.O. 1992. The Diversity of Life. W.W. Norton & Company Inc. New York, NY. ISBN: 0393310477