New Estimate for the Number of Species on Planet Earth

It was the Swedish physician and naturalist Carolus Linnaeus (1707 – 1778), who formed the basis for the classification of organisms in his Systema naturae, first published in 1735.  He grouped organisms with shared features and characteristics into different sets, creating an interrelated hierarchy of life, sometimes referred to as a “tree of life” as a result of its branching structure.  The basic unit in this classification is the species.  A species is defined as a group of organisms whose members have the potential to breed together and produce fertile offspring. Species are grouped into genera, genera into sub-families, families and so on.

Families are grouped into orders, for example our species Homo sapiens is classified with apes, monkeys into the Primates (order).  Orders themselves are grouped into classes, and these in turn are grouped into phyla.  Members of different phyla differ in fundamental features such as body organisation and methods of locomotion and respiration.  Phyla are finally grouped into the largest commonly used classification a kingdom, such as the kingdom plantae (plants) and animalia (animals).

Scientists have argued ever since the time of Linnaeus as to just how many species there are on our planet.  Back in 1975, I recall reading a paper that estimated (conservatively as it turned out) that there were around one million species of insect on the Earth, other estimates have been made over the years using a variety of techniques and the total number of individual species estimated has varied widely, with as many as 100 million species being cited in some literature.

Not knowing how many species inhabit the Earth is one of the most fundamental questions in science.  Efforts to sample the world’s biodiversity to date have been limited and thus have precluded direct quantification of global species richness, and because indirect estimates rely on assumptions that have proven highly controversial.  However, a group of international scientists have used the basis of the classification system first devised by Linnaeus to propose a new calculation for the number of species.  This team calculates that we humans are amongst 8.7 million other species (plus or minus 1.3 million).

In a paper published in the online scientific journal PLoS Biology (Public Library of Science), the team state that the higher taxonomic classification of species (i.e. the assignment of species to phylum, class, order, family, and genus) follows a consistent and predictable pattern from which the total number of species in a taxonomic group can be estimated. Their work was validated against well-known taxa, and when applied to all domains of life, it predicted 8.7 million species in total with a margin for error of +/- 1.3 million eukaryotic species globally (with a nucleus in the cell).  Seventy percent of our planet’s surface may be covered by sea water, but according to these new calculations there are only 2.2 million species present in the sea.

The researchers including scientists from the Department of Biology, Dalhousie University, Halifax, Nova Scotia, Canada, the Department of Geography, University of Hawaii, Honolulu, Hawaii, and the United Nations Environment Programme World Conservation Monitoring Centre, Cambridge, United Kingdom, in conjunction with Microsoft research propose that some 86% of the species on Earth and 91% in the ocean, still await description.

Co-author Boris Worm of Dalhousie University stated that the recently-updated Red List issued by the International Union for the Conservation of Nature assessed 59,508 species, of which 19,625 are classified as threatened. This means the IUCN Red List, the most sophisticated ongoing study of its kind, monitors less than 1% of world species.

Our Planet’s Complex Biodiversity

Picture Credit: Everything Dinosaur

It is important that scientists have an understanding of the biodiversity of planet Earth as without this, it would be very difficult to establish the current rate of extinction.  Many scientists believe that species are dying out at an alarming rate.  In what is known as the sixth mass extinction event, it has been estimated that as many as three species an hour are becoming extinct – many of these organisms are unknown to science.

As co-author of the study, Boris Worm of Dalhousie University points out:

“If we didn’t know – even by an order of magnitude (1m? 10m? 100m?) – the number of people in a nation, how would we plan for the future?  At its most basic, if we don’t know what we’ve got, we can’t protect it, and we can’t even be sure what we’re losing.”

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