How Many Species Are There on Earth and in the Ocean?


Camilo Mora, Derek P. Tittensor, Sina Adl, Alastair G. B. Simpson, Boris Worm
PLOS | Biology August 23, 2011

The article contents:

Abstract
Author Summary
Introduction
Results
Discussion
Materials and Methods
Supporting Information
Acknowledgments
Author Contributions
References

Here I transcript only the introduction and discussion.
For the rest, go to the direction in the web, above.


Introduction

Robert May recently noted 
that if aliens visited our planet, 
one of their first questions would be, 
“How many distinct life forms
—species—does your planet have?” 

He also pointed out that we would be 
“embarrassed” by the uncertainty in our answer. 

This narrative illustrates 
the fundamental nature of knowing 
how many species there are on Earth, 
and our limited progress 
with this research topic thus far. 

Unfortunately, limited sampling 
of the world's biodiversity to date 
has prevented a direct quantification 
of the number of species on Earth, 
while indirect estimates remain uncertain 
due to the use of controversial approaches 
(see detailed review of available methods, 
estimates, and limitations in Table 1). 

Globally, our best approximation 
to the total number of species 
is based on the opinion of taxonomic experts, 
whose estimates range between 
3 and 100 million species; 
although these estimations 
likely represent the outer bounds 
of the total number of species, 
expert-opinion approaches 
have been questioned 
due to their limited empirical basis 
and subjectivity (Table 1). 

Other studies have used 
macroecological patterns 
and biodiversity ratios 
in novel ways to improve estimates 
of the total number of species (Table 1), 
but several of the underlying assumptions 
in these approaches have been the topic 
of sometimes heated controversy, (Table 1); 
moreover their overall predictions 
concern only specific groups, 
such as insects, deep sea invertebrates, 
large organisms, animals, fungi, or plants. 

With the exception 
of a few extensively studied taxa 
(e.g., birds, fishes), 
we are still remarkably uncertain 
as to how many species exist, 
highlighting a significant gap 
in our basic knowledge of life on Earth. 

Here we present a quantitative method 
to estimate the global number of species 
in all domains of life. 

We report that the number of higher taxa, 
which is much more completely known 
than the total number of species, 
is strongly correlated to taxonomic rank 
and that such a pattern allows the extrapolation 
of the global number of species 
for any kingdom of life (Figures 1 and 2).

Discussion

Knowing the total number of species 
has been a question of great interest 
motivated in part by our collective curiosity 
about the diversity of life on Earth 
and in part by the need to provide 
a reference point for current 
and future losses of biodiversity. 

Unfortunately, incomplete 
sampling of the world's biodiversity 
combined with a lack of robust extrapolation 
approaches has yielded highly 
uncertain and controversial estimates 
of how many species there are on Earth. 

In this paper, 
we describe a new approach 
whose validation against 
existing inventories 
and explicit statistical nature 
adds greater robustness 
to the estimation of the number 
of species of given taxa. 

In general, the approach 
was reasonably robust to various caveats, 
and we hope that future improvements in data quality 
will further diminish problems 
with synonyms and incompleteness of data, 
and lead to even better 
(and likely higher) estimates 
of global species richness.

Our current estimate of ~8.7 million species 
narrows the range of 3 to 100 million species 
suggested by taxonomic experts 
and it suggests that after 250 years 
of taxonomic classification 
only a small fraction of species on Earth (~14%) 
and in the ocean (~9%) have been indexed 
in a central database (Table 2). 

Closing this knowledge gap may still take a lot longer. 

Considering current rates of description 
of eukaryote species in the last 20 years 
(i.e., 6,200 species per year; ±811 SD; Figure 3F–3J), 
the average number of new species 
described per taxonomist's career (i.e., 24.8 species) 
and the estimated average cost to describe animal species 
(i.e., US$48,500 per species) and assuming 
that these values remain constant 
and are general among taxonomic groups, 
describing Earth's remaining species 
may take as long as 1,200 years 
and would require 303,000 taxonomists 
at an approximated cost of US$364 billion. 

With extinction rates 
now exceeding natural background rates 
by a factor of 100 to 1,000, our results 
also suggest that this slow advance 
in the description of species 
will lead to species becoming extinct 
before we know they even existed. 

High rates of biodiversity loss 
provide an urgent incentive 
to increase our knowledge 
of Earth's remaining species.

Previous studies have indicated 
that current catalogues of species 
are biased towards conspicuous species 
with large geographical ranges, 
body sizes, and abundances. 

This suggests that the bulk of species 
that remain to be discovered 
are likely to be small-ranged 
and perhaps concentrated in hotspots 
and less explored areas 
such as the deep sea and soil; 
although their small body-size 
and cryptic nature suggest 
that many could be found 
literally in our own “backyards” 
(after Hawksworth and Rossman). 

Though remarkable efforts 
and progress have been made, 
a further closing of this knowledge gap 
will require a renewed interest 
in exploration and taxonomy 
by both researchers and funding agencies, 
and a continuing effort to catalogue 
existing biodiversity data 
in publicly available databases.

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