The Earth’s Biodiversity Could Be Much Greater Than We Thought

How many species of frog are in the picture? Genetics often says ‘more than we thought’. Michael Lee (Flinders University & South Australian Museum), CC BY-NC

Cryptic species are often revealed only by laborious studies that integrate fieldwork, ecology and genetics.

Our DNA studies on what appeared to be widespread single species of Australian gecko revealed that each consists of up to ten cryptic species. Each is restricted to a small region, never interbreeding with adjacent regions at any time over the last 10 million years.

Despite looking very alike, these cryptic gecko species are much more genetically distinct from each other than, say, humans and chimps. So they are definitely proper species, despite being very similar in appearance (sometimes almost indistinguishable).

Cryptic species have recently been found in some of the biggest and well-studied marine creatures such as beaked whales and hammerhead sharks.

On land, scientists have only just realised that African elephants are probably not a single species but two cryptic species: a bush (savannah) elephant and a forest elephant.

The African bush elephant, Loxodonta africana. Michael Lee (Flinders University and South Australian Museum) The African forest elephant, Loxodonta cyclotis. Richard Ruggiero/USFWS

Most of life consists of small invertebrates, especially arthropods – such as insects, spiders and crustaceans – which are much more poorly known than elephants and sharks.

With so few taxonomists and so many invertebrates, only very obviously different groups are picked out as separate species. This sorting is usually based on visual inspection alone, with no genetic analysis. These first-pass species are known as morphospecies and they make up the bulk of known biodiversity.

When scientists take a closer look at invertebrate morphospecies using DNA methods, they usually find multiple species. These might look rather similar, but never interbreed and haven’t done so for millions of years.

For example, what was once thought to be a single species of malaria-carrying mosquito turned out to be at least seven different species. A major agricultural pest (the tobacco whitefly) was revealed to be 31 cryptic species.

Biodiversity bites back! The malaria-carrying mosquito, Anopheles gambiae, turned out to be at least seven distinct species. CDC/James Gathany

Looking at every known species in such genetic detail will be an immense task, even given the promise of techniques such as rapid DNA barcoding. But when we do so, cryptic species should prove to be the rule, rather than the exception, across the majority of life.

Millions More Species

Most of the 2 million known species are morphospecies. The prediction that there are 8.7 million species on Earth, and other similar estimates, are extrapolations from this 2 million figure (or lower earlier figures such as 1.2 million).

Yet, a torrent of new genetic evidence indicates that many currently known morphospecies could represent up to ten or more cryptic species, all very similar to one another but nevertheless real species with separate gene pools. Most estimates of global species diversity have not accounted for this.

Hence, the 2 million morphospecies already described could easily turn out to represent perhaps 20 million real species, if we ever get around to analysing their DNA. This tenfold increase would swell estimates of Earth’s total biodiversity by a similar magnitude, e.g. from 8.7 million to 87 million.

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