How many species are there?

The south eastern United States is not exactly unexplored frontier. It's been inhabited for a good long while, and the population density is fairly high. You might expect that we have found and described all the animal species living there.

That's why I'd like you to meet Barbicambarus simmonsi.

(From Taylor and Schuster 2010.)

This is a crayfish species that was discovered in Tennessee a couple of years ago (Taylor and Schuster 2010). It was previously unknown to science, which is surprising for two reasons. First, that south eastern region of the U.S. has had no shortage of biologists looking for crayfish there, because it has some of the highest diversity of crayfish species in the world. Second, this is not a small, inconspicuous crayfish. As crayfish biologist Chris Taylor tells it, this crayfish species is so big that when he asked the locals if there were crayfish around, they said no, "but there were some big lobsters upstream." Lobsters!

I bring up this story because it shows that one of the things we don't know is how many species we share the planet with. This is a serious problem if we want to understand and preserve life. Species are critical to how we organize our thinking about life. Each species has its own distinct combination of features that you don't see in any others. People have tried to estimate how many species there are, but those estimates range wildly. Most are in the millions or tens of millions.

You might just accept that we may never get a handle on how many different kinds of microscopic organisms are out there. But still, the problem doesn't go away if you limit yourself to big organisms: the plants, animals, and fungi, mostly.

Admittedly, some of these problem in figuring out how many species there are, are bookkeeping problems. Taxonomists, who are the professionals who describe new species, often had a very hard time tracking down and compiling all the species names published. If a description of a new species was published in a small regional journal, another researcher could be completely unaware of the work and publish a description of the same species with a different name. Online databases are certainly helping with that, though it's not completely solved by any stretch of the imagination.

But, as the crayfish example shows, a lot of the uncertainty in how many species there are revolves around just how much might remain to be discovered.

When you look for things, there's a typical pattern. It's easy at first, and then it gets harder. Think about stuff in our solar system, for instance. We noticed five planets in the night sky with our naked eyes. Finding the remaining two planets took much longer, because it required the development of telescopes. Objects like dwarf planets, asteroids, and comets are still being discovered. At this point, we probably have not found every object in our solar system, but we can be reasonably sure there's not a new planet out there waiting for us to find. [I can confirm we know there's a whole lot of stuff floating around in our planetary neighbourhood that we haven't found and catalogued yet - Ed]

The point is: we found a lot of stuff quickly, but then discoveries tapered off.

Do we see that slowdown for species discovery? In 2006, a paper by Martin and Davis came out that tried to document the rate of species discovery in crustaceans.

Crustaceans are a big group, here's an example of the rate of species description for the decapods, which is the group that includes crayfish, lobsters, and crabs: the animals that most people are familiar with and think of when they think "crustacean."

Here is how fast crustacean species became known to science:

(From Martin and Davis 2006)

The astonishing thing to me about this graph is that the rate of species discovery shows no signs of slowing down. In fact, you see how their total around the year 2000 is just a hair under 8,000 decapod crustacean species? Less than 10 years later, De Grave and colleagues reckoned that there were almost 15,000 decapod crustacean species.

Regardless of whether the differences in numbers are better bookkeeping, improved estimates, or one awful lot of new species described, this suggests we're not even close to finishing the list of decapod species.

This probably isn't a general trend. For some groups, like birds and mammals, we probably have slowed down on species discovery. (There are a lot of birdwatchers out there!) But this example shows that there is a lot of work ahead before we narrow down how many species there are. We may never know exactly how many species there are, but we can come much closer to the actual number than we are now.

Zen Faulkes is an Associate Professor of biology at The University of Texas-Pan American. He blogs at NeuroDojo and is @DoctorZen on Twitter.

References

De Grave S, Pentcheff ND, Ahyong ST, Chan T-Y, Crandall KA, Dworschak PC, Felder DL, Feldmann RM, Fransen CHJM, Goulding LYD, Lemaitre R, Low MEY, Martin JW, Ng PKL, Schweitzer CE, Tan SH, Tshudy D, Wetzer R. 2009. A classification of living and fossil genera of decapod crustaceans. Raffles Bulletin of Zoology Supplement 21: 1-109.

Martin JW, Davis GE. 2006. Historical trends in crustacean systematics. Crustaceana 79(11): 1347-1368. http://dx.doi.org/10.1163/156854006779277321

Taylor CA, Schuster GA. 2010. Monotypic no more, a description of a new crayfish of the genus Barbicambarus Hobbs, 1969 (Decapoda: Cambaridae) from the Tennessee River drainage using morphology and molecules. Proceedings of the Biological Society of Washington 123: 324-334. http://dx.doi.org/10.2988/10-15.1

Link

Yet more crayfish diversity 

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