How many species of life forms are there on the planet, anyway? We don't know, but it seems there are a lot more than we thought, according to a paper published in Nature this week.
Take the elephant. You probably think you know what an elephant is, and think there are two types - African and Asian. Not so. There used to be about 350 types of elephants, but even today, the "African elephant" consists of two species – a bush elephant and a savannah elephant. And let's not even get into the pygmy.
The bush elephant, Loxodonta Africana, is smaller than its savannah-ranging cousin, Loxodonta cyclotis. Zoological purists growl that the two can apparently interbreed so they're a pair of sub-species rather than distinct absolute species. But the point is, species or sub, they're not the same thing.
The fact is that while humans are decimating the other big life forms on the planet, we have no idea how many species Earth actually supports.
Estimates range from 2-and-something million existing species to 100 million. The argument over the elephants goes to illustrate that even if you can see the animal looming large, you may be wrong about what it is. And when it comes to obscure little life forms that nobody much cares about, the surprise can be huge: Scientists thought there was only one species of Australian gecko. There turned out to be ten of them.
Tales from the cryptic species
Most scientists assume the number of species is at the lower end of this range. One study that ignores the bacteria, which are a horror to count, estimates the figure to be 8.7 million species.
But if cryptic species are counted, suggests a different paper published in Nature this week – the figure could be enormously higher.
A species is a distinct evolutionary lineage with a unique gene pool. It cannot produce viable offspring with another species, though it theoretically can with a subspecies. One dog breed can mate with another – a pit bull can mate with a husky; neither can mate with a jackal or pig. At least not effectively.
A cryptic species is one that looks very, very much like another species. But it isn't.
The ten species of Australian geckos look so much alike that people thought they were one species, until the DNA revealed otherwise. They possess distinct gene pools.
Genetic analysis indicates they haven't mixed at all for 10 million years. In fact, these look-alike geckos are farther apart than humans and chimps, say the scientists.
In short, the vast advances in genetic analysis have revealed that there are a lot more species out there than we thought. Cryptic species have even been found in some of the most screamingly obvious animals around, such as beaked whales and scalloped hammerhead sharks – yes, there is more than one species of scalloped hammerhead.
The prevalence of cryptic species hiding in plain sight is good news for biodiversity.
But it's bad news for conservation. Take a rare animal like the elephant. If the thinking had been that there was, say, one medium-sized population in the world, now it is clear there are two small ones, that are more at risk than had been assumed.
A fly springs a surprise
Most of the known species are invertebrates, such as insects, spiders and crustaceans. As they stand out less than elephants, or even geckos, they are less known. (Most new animal discoveries are also, naturally, of such small fry – species that had been sitting right there, but hadn't been noticed before. Or like in the case of the unknown animals found in an isolated cave in Ramle, Israel – they'd been isolated from the world for millions of years.)
With so few taxonomists and so many invertebrates, only obviously different groups are identified as distinct species. This sorting is usually based on visual inspection, with no genetic analysis.
These first-pass species are known as morphospecies and they make up the bulk of known biodiversity.
Yet now, when scientists armed with the latest genetic techniques study invertebrate morphospecies using genetic analysis, 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.
What does it matter if there is one African elephant, or two, or even more?
First of all, it matters to the elephant. Lumping all African elephants into a single species could lead to terrible conservation decisions.
Secondly, it can matter to us all - knowing whether a mosquito is one species or several is crucial. Cryptic mosquito species can differ in behavior, habitat and ability to transmit diseases.
And lastly, for the anthropocentric among us, animals can hide wonderful traits in their genomes that are worth studying and could even save lives in the future. If we kill off the animal before even recognizing its species, let alone its unique characteristics, we lose that precious information forever. The efforts to identify and count the species on Earth are therefore much more than an obscure academic exercise.
With writing by Ruth Schuster
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