On the Number of Species
When reading about biodiversity, it’s common to find a sentence like this:
“There are an estimated 352,000 species of flowering plants, or angiosperms1”.
It’s also common to find a sentence like this:
“There are currently 295,383 described angiosperm species” (paraphrased from Christenhusz and Bing, 20162).
The difference between these two numbers is large, but its cause is obvious—many species of angiosperms remain undiscovered or undescribed. The numbers of described species and estimated total species can be found for many groups of organisms, but you might ask: how does a scientist estimate the total number of species without knowing what all of the species are?
Before I address that, it’s important to note that the number of described species is also an estimate. Species classifications are constantly shifting as taxonomists review data about the divisions between genera, species, and varieties in their groups of interest, so maintaining up-to-date lists of species and accounting for old or synonymous species names is a difficult task. Furthermore, delineations between species are controversial and frequently effect disagreements between taxonomists who favor different species concepts. In 18593, Darwin noted that:
“No one definition (of species) has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species”.
Over 150 years later, this statement still holds true.
Species are a human construction that describe biodiversity, and the criteria that define species boundaries in one group may not work well in another. For example, many animal species are clearly defined by mating boundaries; mating between species will either not produce offspring or produce sterile offspring in the vast majority of cases. In contrast, many plants are capable of producing fertile offspring across species or even genus boundaries!
Aranda ‘Lucy Laycock’, a fertile, intergeneric orchid hybrid made by crossing Arachnis hookerana and Vanda tricolor. Photo by Scott Zona. CC BY-NC 2.0
Now, let’s return to our original question of how scientists estimate the total number of species in a given group. One method4 is to build a mathematical model that takes into account the rate of species descriptions over time, as well as other factors such as the number of taxonomists actively describing them. Initially, individual taxonomists (e.g., Carl Linnaeus) were able to quickly describe large numbers of species, but, eventually, diminishing returns have taken effect as the remaining undescribed species became rarer and rarer. These mathematical models describe the decreasing rate of new species descriptions, which is used to extrapolate the number of species needed to reach a point where new species descriptions stop altogether. That number is the difference between the number of described species and the estimated total number of species.
This method is ingenious, but it treats each species description the same, even though the methods used to delineate species have changed immensely over time. 200 years ago, a newly discovered species might have been classified purely by its physical similarity to other described species. Now, genetic and phylogenetic analyses would be used to determine the species’ genealogical relationships to other described species. If either method were applied consistently to all known species, it is unlikely that number of described species in any major biological group would be the same as it is today. This leads to a new question: can we trust species numbers (or estimates of species numbers) based on inconsistent methods of classification?
This is exactly why biodiversity research is so important. By closely examining the relationship between species, scientists at UF and elsewhere are revising old species classifications with new tools in order to build a more consistent and well-resolved description of life on Earth. These efforts, combined with continued exploration of the planet’s most diverse ecosystems, are critical not only for a more complete understanding of biodiversity, but also to gather information that will inform conservation policies and priorities.
Sources:
1. www.theplantlist.org
2. Christenhusz, M. J., & Byng, J. W. (2016). The number of known plants species in the world and its annual increase. Phytotaxa, 261(3), 201-217.
3. Darwin, C. (1859) On the origin of species by means of natural selection, or, the preservation of favoured races in the struggle for life. London: J. Murray.
4. Joppa, L. N., Roberts, D. L., & Pimm, S. L. (2010). How many species of flowering plants are there? Proceedings of the Royal Society of London B: Biological Sciences rspb20101004.
