How many species are there on Earth?

Why should we care?

To ask whether there is importance in quantifying the number of species on Earth, is much like asking whether the Earth is spherical; the answer is a definitive and an undisputed yes. The extensive influence of evolution on the planet’s inhabitants has left the ecosystem somewhat unique across time and space. Our oxygen rich atmosphere, to name but one of these complex ecosystem traits, was first pioneered by early prokaryotic organisms. This surely highlights the significance of understanding the diverse array of species, and their impact on the planet, especially in today’s scientific world. With the climate rapidly changing and biodiversity decreasing, scientists have a duty to preserve what forms of life are left, and this can only truly begin when it is known what organisms these are. It is not only this, which scientists must consider. A large proportion of modern medicine derives from biological compounds found in plants, and there are approximately 2000 new species of plant discovered every year (Christenhusz M.J.M. & Byng J.W. 2016). At least 12,000 compounds have been isolated and are now in use, but this is a mere 10% of the estimated total (Tapsell L.C.; Hemphill I. & Cobiac L. et al, 2006). From a utilitarian stand point, counting and categorising species should be considered on a more significant level.

Methods of Estimation

Taxonomic nomenclature is thought to have originated in the mid 1700s, when the Swedish botanist, physician and zoologist Carl Linnaeus, first suggested the use of a binomial naming system. In 1758, 9000 species were classified in Latinate form, eradicating the barrier of differing languages between scientists.

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Figure 1 – A graph to show the number of new species binomially named each year since 1750.

Solenopsis saevissima is the Latin name for the “fire ant” – a species of ant which has the ability to inflict a painful sting. Solen, means pipe or channel, opsis, means appearance or sight and saevissima, means “the very cruel”. It was and still is a method of describing what you see. It’s thought that there are 12,000 species of ant on Earth, and over 950,000 species of insect. Mammalia on the other hand, is thought to have just over 5000 species. Scientists have deduced that with a typical 10-fold decrease in length, there is a 100-fold increase in species number (May, R. M, 1992). Estimation based on relative body size is one of the ways in which species number can be predicted. Scientists don’t understand the biochemical reasons behind this size to species ratio and so predictions are heavily based on uncertainty. This general pattern also tends to falter below the one-centimetre mark. Microbes, which account for 5% of all recorded living species, fall into this category. Bacteria, viruses and fungi are much more difficult to distinguish into different species’, as they are rapidly evolving, making their genetic material much more diverse and mutated. However, under a new classification system proposed by Manfred Eigen of the Max Planck Institute for Biophysical Chemistry in Göttingen and Peter Schuster of the University of Vienna, microbes, especially viruses, can be classified under a defined set of RNA sequences called the “quasispecies” (May R. M, 1992). From this, microbial taxonomists have described hundreds of new species every year, and in 2016, validly named Bacteria and Archaea surpassed 13,000. According to a recent study of microbial diversity, the prediction of the true species number has been pushed into the trillions (Locey K. J & Lennon J. T, 2016) meaning a huge proportion of these organisms are yet to be discovered. However other studies contradict this assumption, and follow the prediction first made in the 1990’s that microbial species number is in fact more likely to be in the millions (Amann R & Roselló-Morá R. 2016).

To echo the words of Robert May “the sad truth is, no one knows” just how many species of microbes there are, never mind how many species there are in total. Hence, it is difficult to reach a definite conclusion. Other studies have attempted to analyse the structure of food chains to provide a means of quantifying species number. Gaston approximated there are 10 insect species for each of the 270,000 species of vascular plant, by collecting available evidence on the average number of insect species associated with each plant species in a variety of locations and communities. From this, he was able to make the crude estimation of a total of 3 million insect species across the planet (Gaston K. J. 1991).

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Figure 2 – A graph to show the predicted number of species plotted against the actual number of species recorded. Insecta is supposedly the most undocumented out of all of the classes of organisms – excluding microbes.

Other methods of estimation work by scaling up a proportion of a species in a certain part of the world. For example, in the UK, it’s thought that the vast majority of the Earth’s fungal species have been recorded. David L. Hawksworth of the International Mycological Institute in Kew, England, noted that in Britain and other northern European countries that have a long history of strategic species classification, fungal species counts outnumber vascular plants 6:1. By applying this ratio to the rest of the world, Hawksworth was able to predict a total of 1.6 million species of fungi – 20 times that of the known number. Fungi have helped shape the planet. They have aided in the colonisation of land plants and assisted in the spread and diversification of vascular plants, insects and other organisms. So it seems logical that their species number vastly outnumber that of the vascular land plant. However, this estimate is based on the idea that the distribution of fungi across the globe reflects that in the UK. Some areas, like the lowland tropical rainforests of Borneo have much higher levels of species richness. (PHYS ORG. 2005) Plant species number has been recorded to reach near 10,000 in this part of the world, but in the entirety of the UK just under 3000 species of vascular plant have been recorded. In fact, although tropical forest cover around a sixteenth of the Earth’s surface, they may house as many species as the rest of the world’s regions combined. (May. R. 1992). As such, estimations based on this principle are highly generalised and are likely to be non-reflective of the globe’s true plant species number.

Erwin’s Estimate

One of the most ground breaking and arguably most influential pieces of research in the world of organismal classification, was that of Terry Erwin and his co-workers in 1982. He looked at the abundance of canopy beetles on Luehea seemannii, in the forests of Panama. Erwin suggested that 20% of the beetles he collated were specialised to this specific species of tree. As such, he predicted that each tree in the area may hold at least 160 species of canopy beetle based on his findings using insecticidal fog. Erwin inferred that because 40% of all known insects are beetles, then if this proportion applies in the tropical rainforest, then 400 kinds of canopy insect occupy a specific species of tree. Then he estimated the canopy contained two thirds of the total number of insects found on the species of tree suggesting a total figure of 600 insect species on each type of tropical tree. Following this, Erwin crudely estimated that as the Earth was found to support 50,000 species of tree, that 30 million insect species would be present also (600 x 50,000). Of course, the number would be larger still if it was scaled up to the entire globe. His efforts have intertwined both ecological and taxonomic standpoints by diverging his attention to the way in which the producer supports the consumer. It is probably the best attempt to look at the structure of a particular food support system, evenly the estimates are based heavily on uncertainty (May. R. 1992).

 Will we ever truly know?

According to the most recent study covering all aspects of the Earth’s ecosystem, it is believed there are around 8.7 million species in total (Mora C., Tittensor D. P., Adl S., Simpson G. B. & Worm B. 2011). Over the last 20 years the rate at which newly discovered eukaryotic organisms have been described is around 6200 per year. Considering the cost implication of scientific research, it may take as long as 1200 years to describe all of the Earth’s species. The natural background rate of extinction is about 1 to 5 species per year, but now taxonomists are battling with the effects of a rapidly changing environment and an increase in extinction rates which are now exceeding the natural background rate by a factor of 100 to 1000 (Pimm S. L, Russell J. L, Gittleman , Brooks T. M. 1995). As such, many species may become extinct before they are even discovered. It is a race against time, and makes the process near impossible. For scientists to substantially close the gap between the known and the unknown in a shorter space of time, much more funding needs to be implicated into the area, and this will only result with a surge in education, compassion and interest in conservation of the Earth’s biodiversity. Historically, taxonomists were particularly prejudice towards the class of organisms they studied. It is the reason why the vast majority of mammals have been documented and why we are so far from coming to a definite conclusion about the number of species of microorganisms on the planet. With over 250 years of taxonomic documentation and research, the numbers of newly discovered vertebrate species are rapidly decreasing, and so taxonomists are shifting their attention to invertebrates. Even so, it wasn’t until the 1990’s that some form of a central archive existed. Species 2000 is the public registration of the planet’s pre-existing and newly discovered species, which is accessible worldwide. It aids in the prevention of scientists from different parts of world focusing on organisms that have already been the subject of scientific investigation, and allows for time, an incredibly important factor in the process, to be spent purposefully. As a consequence, reaching an accurate measure of the exact figure of known species is becoming more possible, and this in turn will help in quantifying the entirety of Earth’s wondrous abundance of creatures.

Bibliography

MENNINGHER H. & DUNN R. (2016). Could there be 200 million species on Earth?. [Online] [Accessed 6th February 2017] Available from: http://www.yourwildlife.org/2015/04/could-there-be-200-million-species-on-earth/

WIKIPEDIA. (2017). Solenopsis Sevissima. [Online] [Accessed 6th February 2017] Available from: https://en.wikipedia.org/wiki/Solenopsis_saevissima

PEST WORLD FOR KIDS. (2014). Ants. [Online] [Accessed 6th February 2017] Available from:

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FACT MONSTER. (2013). Estimated Number of Animal and Plant Species on Earth. [Online] [Accessed 6th February 2017] Available from: http://www.factmonster.com/ipka/A0934288.html

PHYS ORG. (2005). Borneo – the most species-rich area in the world!. [Online] [Accessed 15th February 2017] Available from:

https://phys.org/news/2005-05-borneo-species-rich-area-world.html

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http://www.biologicaldiversity.org/programs/biodiversity/elements_of_biodiversity/extinction_crisis/

CHRISTENHUSZ, M.J.M. & BYNG, J.W. (2016). The number of plant species in the world and its annual increase. Biotaxa. 261, No 3. 201-217.

GASTON K. J. (1991). The Magnitude of Global Insect Species Richness. Conservation Biology. 5, No 3. 283-296.

LOCEY K. J & LENNON J. T. (2016). Scaling laws pedict global microbial diversity. PNAS. 113. 5970-5975.

MORA C., TITTENSOR D. P., ADL S., SIMPSON A. G. B. & WORM B. (2011). How many species are there on Earth and in the ocean?. PLoS Biol. 9 (8): e1001127. Doi 10.137/journal. Pbio. 1001127.

PIMM S. L, RUSSELL J. L, GITTLEMAN , BROOKS T. M (1995). The future of biodiversity. Science. 269. 347–350.

ROBERT, M. M. (1992). How many species inhabit the Earth? Scientific American. 267. 42-48.

TAPSELL, L. C.; HEMPHILL, I. & COBIAC, L. et al. (August 2006). Health benefits of herbs and spices: the past, the present, the future. Med. J. Aust. 185 (4 Suppl): S4–24.

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