How Many
Species? A Study Says 8.7 Million, but Its Tricky
In the foothills of the Andes Mountains lives a bat the
size of a raspberry. In Singapore, theres a nematode
worm that dwells only in the lungs of the changeable lizard.
The bat and the worm have something in common: They are
both new to science. Each of them recently received its
official scientific name: Myotis diminutus for the bat,
Rhabdias singaporensis for the worm.
These are certainly not the last two species that scientists
will ever discover. Each year, researchers report more than
15,000 new species, and their workload shows no sign of
letting up. Ask any taxonomist in a museum, and theyll
tell you they have hundreds of species waiting to be described,
says Camilo Mora, a marine ecologist at the University of
Hawaii.
Scientists have named and cataloged 1.3 million species.
How many more species there are left to discover is a question
that has hovered like a cloud over the heads of taxonomists
for two centuries.
Its astounding that we dont know the
most basic thing about life, said Boris Worm, a marine
biologist at Dalhousie University in Nova Scotia.
On Tuesday, Dr. Worm, Dr. Mora and their colleagues presented
the latest estimate of how many species there are, based
on a new method they have developed. They estimate there
are 8.7 million species on the planet, plus or minus 1.3
million.
The new paper, published in the journal PLoS Biology, is
drawing strong reactions from other experts. In my
opinion this is a very important paper, said Angela
Brandt, a marine biologist at the University of Hamburg
in Germany. But critics say that the method in the new paper
cant work, and that Earths true diversity is
far greater.
In 1833, a British entomologist named John Obadiah Westwood
made the earliest known estimate of global biodiversity
by guessing how many insect species there are. He estimated
how many species of insects lived on each plant species
in England, and then extrapolated that figure across the
whole planet. If we say 400,000, we shall, perhaps,
not be very wide of the truth, he wrote.
Today, scientists know the Westwood figure is far too low.
Theyve already found more than a million insect species,
and their discovery rate shows no signs of slowing down.
In recent decades, scientists have looked for better ways
to determine how many species are left to find. In 1988,
Robert May, an evolutionary biologist at the University
of Oxford, observed that the diversity of land animals increases
as they get smaller. He reasoned that we probably have found
most of the species of big animals, like mammals and birds,
so he used their diversity to calculate the diversity of
smaller animals. He ended up with an estimate 10 to 50 million
species of land animals.
Other estimates have ranged from as few as 3 million to
as many as 100 million. Dr. Mora and his colleagues believed
that all of these estimates were flawed in one way or another.
Most seriously, there was no way to validate the methods
used, to be sure they were reliable.
For the new estimate, the scientists came up with a method
of their own, based on how taxonomists classify species.
Each species belongs to a larger group called a genus, which
belongs to a larger group called a family, and so on. We
humans, for example, belong to the class of mammals, along
with about 5,500 other species.
In 2002, researchers at the University of Rome published
a paper in which they used these higher groups to estimate
the diversity of plants around Italy. At three different
sites, they noted the number of genera, families and so
on. There were fewer higher-level groups than lower ones
at each site, like the layers of a pyramid. The scientists
could estimate how many species there were at each site,
much as its possible to estimate how big the bottom
layer of a pyramid based on the rest of it.
The paper drew little notice at the time, but Dr. Mora
and his colleagues seized on it, hoping to use the method
to estimate all the species on Earth. They charted the discovery
of new classes of animals since 1750. The total number climbed
steeply for the first 150 years and then began to crest
a sign that were getting close to finding all
the classes of animal. They found that the discovery rate
of other high-level groups has also been slowing down. The
scientists built a taxonomic pyramid to estimate the total
number of species in well-studied groups, like mammals and
birds. They consistently made good predictions.
Confident in their method, the scientists then used it
on all major groups of species, coming up with estimates
of 7.7 million species of animals, for example, and 298,000
species of plants. Although the land makes up 29 percent
of the Earths surface, the scientists concluded that
it is home to 86 percent of the worlds species.
I think it is an interesting and imaginative new
approach to the important question of how many species actually
are alive on earth today, said Lord May.
But Terry Erwin, an entomologist at the Smithsonian Institution,
think theres a big flaw in the study. Theres
no reason to assume that the diversity in little-studied
groups will follow the rules of well-studied ones. Theyre
measuring human activity, not biodiversity, he said.
David Pollock, an evolutionary biologist at the University
of Colorado who studies fungi a particularly understudied
group agrees. This appears to be an incredibly
ill-founded approach, he said. There are 43,271 cataloged
species of fungi, based on which Dr. Mora and his colleagues
estimate there are 660,000 species of fungi on Earth. But
other studies on fungus diversity suggest the number may
be as high as 5.1 million species.
The authors of the new study acknowledge that their method
doesnt work well with bacteria. Scientists have only
started to really dig into the biodiversity of microbes,
and so they are finding high-level groups of bacteria at
a brisk pace. Dr. Mora and his colleagues write that their
estimate about 10,000 species should be considered
a lower bound.
Microbiologists, on the other hand, are fairly sure the
diversity of microbes will turn out to dwarf the diversity
of animals. A single spoonful of soil may contain 10,000
different species of bacteria, many of which are new to
science.
Jonathan Eisen, an expert on microbial diversity at the
University of California, Davis, said he found the new paper
disappointing.
This is akin to saying, Dinosaurs roamed the
Earth more than 500 years ago, he said. While
true, what is the point of saying it?