How Many Species?

Erwin's Calculations
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Methods of Estimating Species Numbers

There are an estimated 13.6 million species on this planet.

  • How do scientist arrive at such estimated figures, given that only 1.75 million species have been described?

Methods for evaluating global species numbers include:

  • predictions from current species description rates;
  • comparisons of known to unknown floras and faunas;
  • extrapolations from samples of biodiversity;
  • estimates inferred from theoretical or empirical patterns.

Solving the question "how many species are there?" should not only lead to an improved understanding of the distribution of biodiversity but should also lead to improved predictions as to its fate [Stork 1993] .

How many species?
     
 

Numbers of Species

The conservative estimate of 13.6 million species currently living on this planet, and 1.75 milliondescribed species, comes from [Hawksworth & Kalin-Arroyo 1995] .

This graph shows how the discrepancy between described and estimated species diversity varies between taxonomic groups.
Numbers of described and estimated species
     
 

Current Description Rates

Patterns in the rates of recording of new species can be projected forward, taxonomic group by taxonomic group, to arrive at an estimate of total species numbers [May 1990] , [Hammond 1992] .

However, there are issues associated with such an approach, for instance:

  • how many of the newly described species are true species or just synonyms? (See: [Alroy 2002] ).
  • is there any bias in the taxonomic groups or regions that taxonomists study? (See: [Gaston & May 1992] ).
  • are there enough taxonomists working to ever describe all the species?

Examining the beetle collection at the British Natural History museum
 

Comparisons of Ratios

A simple method for estimating global species diversity comes from extrapolations of the ratios of known to unknown floras or faunas.

Two examples of this method include:

Unfortunately, for many species their geographical range increases with increasing latitude, and this may limit the accuracy of such simple extrapolations.

 
 

Butterfly : Insect Ratios

[Stork & Gaston 1990] looked at the British insect fauna, which is a well-documented, well-sampled fauna.

The ratio of butterflies to all other insect species was 67 : 22,000.

Given an estimated 15,000 - 20,000 butterflies world-wide, they estimated there to be between 4.9 - 6.6 million insect species.
 

Fungi : Plant Ratios

[Hawksworth 1991] looked at the ratio of fungi to vascular plant species in the British Isles, which is an area intensively studied by mycologists.

He found there were 12,000 described fungal species and 2,100 vascular plant species.

This ratio (12,000 fungi : 2,100) was then extrapolated world-wide.

Given an estimate of 270,000 vascular plants in the world, Hawksworth estimated there to be 1.5 million fungal species.

This figure may be an overestimate [May 1995] or an underestimate [Frohlich & Hyde 1999] .

 
 

Extrapolations from Samples of Biodiversity

One method of estimating global species diversity involves extrapolating from collected samples, particularly of tropical forest arthropods.

This direct method of estimating global species diversity involves:

  • sampling species (or a group of species) in a region that has been relatively understudied;
  • determining what fraction of those species has already been described;
  • and then extrapolating these figures to get a global estimate of the numbers of that species or species group.

The most famous of these studies was done by [Erwin 1982] who estimated there to be 30 million arthropod species world-wide.

[Hodkinson & Casson 1991] followed similar methods in sampling hemiptera from Indonesia, and estimated there to be 1.8 - 2.6 million insects world-wide.

It should be remembered that all such estimates are only as reliable as the assumptions and extrapolations on which they are based, and in many cases these assumptions and extrapolations are fairly weak. Erwin's estimates have come in for much criticism.

 
 

Erwin's Calculations

[Erwin 1982] collected canopy insects, from a seasonal tropical rainforest in Panama, by fogging.

Coupled with assumptions about how many of these insects were host-specific to a particular tree, and knowledge of the number of tree species per unit area, Erwin estimated there to be over 41,000 arthropod species per hectare in his rainforest site.

Further extrapolations by Erwin, based on the known relative richness of different insect orders and assumptions on the comparative richness of the canopy to the tropical floor, gave an estimate of 30 million tropical arthropod species world-wide.

Previous estimates had suggested there were only 2 - 3 million species, so Erwins calculations were quite a revelation. Erwin himself was shocked by the size of his answer, saying "I hope someone will challenge these figures with more data."


Insect fogging
 

Estimates Inferred from Theoretical Principles

Studying the empirical patterns in the structure of communities, food chains and webs reveals rules that may allow an estimation of global species numbers. [Gaston 1992] used this method and estimated there to be about 3 million insect species.

Total species numbers can be estimated from examining the number ofspecies in different body-size categories. [May 1990] used this method to produce an estimate of 10 million species world-wide.

Species turnover rates between habitats can be investigated to see how species accumulate with time and distance. [Grassle & Maciolek 1992] investigated such turnover rates for deep-sea invertebrates, and estimated there to be 10 million benthic species.

 
 

Estimates from Food Webs

Photosynthetic plants form the first link in a food chain. By knowing how many other species each plant can support, it might be possible to estimate total species numbers from complete counts of plant species.

Similar arguments can be applied to estimates derived from the numbers of parasitic or symbiotic species associated with individual host species.

[Gaston 1992] found approximately10 insect species for every plant species. Given reliable estimates of between 270,000 - 350,000 vascular plant species, he estimated there to be about 3 million insect species.

Such methods of species estimations, reviewed by [May 1988] and [May 1990] , are likely to be of limited accuracy.

 


Food web
 

Body Size vs. Species Numbers

[May 1990] compared the number of species in different size categories to produce an estimate of 10 million species world-wide.

This process involved extrapolating for species down to 0.2 mm in size.

[May 1988] noted that for non-microscopic organisms there is an inverse relationship between body size and the numbers of species in each body-size class. For each 10-fold reduction inbody length there are 100 times more species.

Using this relationship, and given the numbers of species and their body size for larger species, it is possible to predict how many less well-studied smaller species there are, and so obtain an estimate the global species numbers.

However, the size versus species number relationship may breakdown for species below 0.1 m, since:

  • very small organisms can disperse great distances in air currents and so are less likely to show allopatric speciation;
  • the biological species concept may not apply to microorganisms.


Number of species in different body size categories
 

Species Turnover Rates

Species turnover rates (or beta diversity) measure differences in species richness between habitats. As new habitats are sampled along a transect, new species will be found.

Knowing such values (in terms of the number of new species found per unit area) it should be possible to estimate total species numbers over a large area.

[Grassle & Maciolek 1992] investigated such turnover rates for deep-sea invertebrates, sampling a 176 km transect of the continental slope off New jersey, Norrth America, and finding one new species for every km2 sampled.

Given that the area of seabed below 1,000 m is approximately 3 * 108 km2, [Grassle & Maciolek 1992] estimated there to be 10 million benthic, macrofaunal species.

 


A sample of deep-sea, benthic invertebrates
 

Why Ask the Question?

It is important to know how many speceis there are because:

  • biodiversity is under threat from manysources, and it is essential to know how many species there are, at what rate they are going extinct, and how their distribution might be changing;
  • biodiversity is a resource, providing food, medicines etc., that needs evaluating if it is to be managed in a sustainable way in the future;
  • biodiversity has entered thepolitical arena following the 1992 Earth Summit in Rio de Janeiro;
  • biodiversity issues, including evaluating global species numbers, should be key areas of scientificresearch.

Solving this question should not only lead to an improved understanding of the distribution of biodiversity but should also lead to improved predictions as to its fate [Stork 1993] .

Unfortunately, with current knowledge, "we cannot say how widespread species are, we do not know the size of the species pool, and we do not know how specific species are to a particular habitat..." [Stork 1997]

   
 

Bibliography

Key references on evaluating species diversity include:

Additional reading includes: