To date, about 1.5 million species have
been formally described in the scientific literature, most of them insects.
Proportionally, bacteria comprise less than 1% of all described species.
Scientists generally agree that many
more species exist than are formally described, but they disagree about how
many there really are. Some studies have estimated 2 million or fewer, whereas
others suggest as many as 12 million (one recent study even suggested the
planet could be home to a trillion species).
In a new paper published in The
Quarterly Review of Biology (September 2017), researchers from the University
of Arizona have estimated that there are roughly 2 billion living species on
Earth, over a thousand times more than the current number of described species.
In coming up with their estimate, the
researchers took advantage of the fact that many estimates now agree on the projected number of insect species, around 6.8 million. They
incorporated new estimates of species boundaries revealed by DNA sequences,
which suggest there might be six times as many insect species, increasing the
total to 40 million for insect species alone.
They then reviewed all groups of
organisms associated with insects as parasites or symbionts. They
found that each insect species most likely hosts a unique species of mite,
roundworm (nematode), a one-celled fungus called a microsporidian, and a
one-celled organism called an apicomplexan protist (which cause malaria in
humans).
Most
importantly, the researchers estimated that each insect species is likely to
host at least 10 bacterial species found nowhere else. Based on these
estimates, they deduce that there should be around 2 billion species on Earth.
The
authors also suggest that the diagram of which taxonomic groups contain the
most species, or the "Pie of Life," is very different from
traditional estimates. Rather than being dominated by insects, as traditionally
shown, their estimates show a pie dominated by bacteria (70 to 90% of all
species), with insects (and animals in general) having a much smaller slice.
See:
Posted by Dr. Tim Sandle
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