Saturday, March 1, 2014

A Lot of Species?

I struggle with the identification of foraminifera species.  Of course I do, because I believe the differences among these species are often mind-numbingly subtle, and the total number of fossil species is huge.  (Foraminifera are single-celled, shell-bearing protozoa that date back to the Cambrian.  A number of previous posts on microfossils have featured forams.)

As for the first proposition (subtle differences), the distinction between two species can hinge on something as challenging to discern as whether the opening (the aperture) that appears in each shell’s last chamber is "toothed" or not.  By now, I know to couch any identification I settle on with a disclaimer.  For example, the foraminifera pictured below is from the species Florilus chesapeakensis, . . . unless it’s not.

(This microfossil was found at Randle Cliff Beach on the western shore of the Chesapeake Bay, and is probably about 16 million years old – early Middle Miocene.  The scale bar is in microns - a thousand to a millimeter - so the bar is 0.4 millimeters wide.  This identification is informed by Thomas G. Gibson’s Key Foraminifera from Upper Oligocene to Lower Pleistocene Strata of the Central Atlantic Coastal Plain, in Geology and Paleontology of the Lee Creek Mine, North Carolina, I, edited by Clayton E. Ray, Smithsonian Contributions to Paleobiology, Number 53, 1983, p. 355 – 453.)

Recently, I began to wonder about the second assumption of mine (that the number of foram species is enormous).  For much of the time I've worked with the fossil shells from foraminifera, I've felt my life was being complicated by a never ending host of species.  So, when people are foolish enough to listen, I complain, "Good grief, there are 60,000 known species and most are extinct."  (Don’t take this number as gospel, because, as I found while putting together this post, there is no gospel.)

A couple of weeks ago, upon hearing that, somebody foolishly responded, saying that I was overreacting because 60,000 wasn't so large in the scheme of things, and consider, he continued, that the number of bird species might be an order of magnitude larger than that.  (Don't take that observation about bird species as gospel, it's wrong.)

That got me thinking about what the number of foraminifera species (living and fossil) really might be, and whether I was justified in thinking it was a lot, either on its own or in comparison to other taxonomic groups.

On the one hand, the question of the number of species belonging to any taxonomic group or groups is, I think, a scientifically worthwhile one to explore.  On the other hand, expecting a straightforward answer is the mark of an amateur.

Casting my net broadly (why not?), I began with the body of literature about the total number of living species on Earth, those known and yet-to-be-discovered.  Describing the value of addressing this overarching question, the eminent British zoologist Robert M. May observed that this knowledge
is important for full understanding of the ecological and evolutionary processes which created, and which are struggling to maintain, the diverse biological riches we are heir to.  (Why Worry about How Many Species and Their Loss?, PLoS Biology, Volume 9, Issue 8, August, 2011.)
Methods for counting the Earth's known living species vary, as certainly do those for estimating the number of the unknown living species.  For instance, Camilo Mora and colleagues recently applied a methodology based on consistent patterns they found for the number of species at different taxonomic levels in well documented groups.  Applying those patterns to all groups, they predicted that there are roughly 8.7 million eukaryotic species (those having cells with nuclei) on Earth, give or take 1.3 million.  (About 1.2 million species of the grand total, they estimated, were known.)  Previous estimates of the total number of living species, known and unknown, ranged from between 3 and 100 million.  (Camilo Mora, How Many Species Are There on Earth and in the Ocean?, PLoS Biology, Volume 9, Issue 8, August, 2011.)

For their work, Mora et al., turned to the online Catalogue of Life.  In 2001, the Integrated Taxonomic Information System (a group of government and other organizations in North America), and Species 2000 (a worldwide collaboration of taxonomists) joined forces to create the Catalogue of Life with the goal of listing every valid living species on Earth.  I applied this database to my foraminifera count.  In the Catalogue's 2013 Annual Checklist, the order Foraminiferida is listed as having 1,348 (extant) species.  Damn, I thought, not enough!  (So much for a dispassionate search for truth.)

But, for the moment, I decided to accept that number (and the Catalogue's taxonomic placement of the foraminifera, more on that in a bit) and see how it compared to the other orders in the Catalogue.  I managed (with a some tedious work) to create a list of the number of species in each order included in the Catalogue across all kingdoms.  Of this total of 1,069 orders, only 93 featured more total species than the Foraminiferida.  The mean number of species per order was 1,263; the median was just 56.  So, even this suspiciously small number of foraminifera species is at the upper end of the spectrum of orders in terms of valid living species.  Good.

Even better, my sense of problems with the Catalogue's count of foram species was validated when I learned that, for their estimates of protozoa species in the ocean, Mora et al. turned to the World Register of Marine Species (or WoRMS), a government and privately funded effort led by taxonomic experts, because its protozoa data were much more complete.  The Catalogue and most other taxonomic analyses place the foraminifera in the protozoa kingdom.  WoRMS includes the World Foraminifera Database, which, as of February 27, 2014, listed 6,625 valid modern foraminifera species and 1,731 valid fossil species, for a total of 8,084 species.  (I assume the total is less than the addition of the modern and fossil counts because the two categories are not mutually exclusive.)

That's better, but still not 60,000, and the number of fossil species seemed laughingly low.

Ultimately, I did rediscover the source of that nice round number I'd been using.  It was in a volume I turn to often, Donald Prothero's Bringing Fossils to Life (1998).  He wrote,
Over 3600 described genera and perhaps 60,000 species of foraminifera are currently recognized, making them more diverse than any other group of marine animals or plants.  (p. 190)
Unfortunately, I have to assume he lumped living and fossil species together.  The source of his estimate is probably Stephen J. Culver’s 1993 piece on foraminifera (Chapter 12 in Fossil Prokaryotes and Protists, edited by Jere H. Lipps) which isn't any more precise about what's included or how the number was derived.

Of course, I recognize that any count of species (living or fossil, targeted to a single taxon or all taxa) is burdened by myriad challenges, some of them potentially fatal.  Taxonomies can be revised substantially over time.  What it means for a species to be recognized or valid may not be consistently applied.  A mixing of counts of known species and estimates of as-yet-to-be-discovered species may muddy the water.  And the definition of what constitutes a species is probably one of such an effort’s biggest bugaboos (particularly problematic for efforts like Mora's which cross multiple taxa).  According to Lynn Margulis and Karlene V. Schwartz much of this applies to foraminifera taxonomic work:
Unfortunately, although foram lifecycle stages often correspond to extreme changes in shell morphology, paleontologists assume that each morphotype represents a different species.  Protistologists and geologists have different aims and terminologies; thus, the taxonomy of the foraminiferans is in rather a mess.  The bewildering complexity of their organisms and their life cycles assures both groups of scientists much taxonomic work for a long time. (Five Kingdoms:  An Illustrated Guide to the Phyla of Life on Earth, 2nd edition, 1988, p. 119.)
Even if mess is too harsh a term, the higher level taxonomy of foraminifera is in flux.  Efforts have been made to elevate the foraminifera from an order to a class or to a phylum (see discussion in Short Treatise on Foraminiferology (Essential on [sic] Modern and Fossil Foraminifera), by Jean-Pierre Bellier et al., first published online July 1, 2010).  Either of these changes has been adopted only in some taxonomic treatments of forams.  The rest stick with order.  Most confusing.

So, where does that leave me?  One approach would be go with the 60,000 species estimate, despite its squishiness, and, further, assume that perhaps 6,000 to 7,000 of those are living and the rest fossil.  Another would be to stick with the World Foraminifera Database and its much lower, but possibly more defensible, totals, rounding them for ease to 6,600 modern and 1,700 fossil species.  But when I poked around the Database I was pleased to find that its administrators make it clear that they recognize the number of fossil species is a substantial undercount of the valid names that it should list and which "will take many years to add in."

So, for the moment, I will continue to use the 60,000 figure.  I'd like to have proof one way or the other (though, the higher the better).

One last item - the assertion that the number of bird species dwarfs the foram species count.  At least here I have certainty - it's wrong.  If we accept the Catalogue of Life as our source (Mora et al. accepted it for every taxa except marine protozoa), then the current number of extant species in the Aves class is 9,924.  (Back in 1946, the eminent evolutionary biologist Ernst Mayr offered up a hard count of known species, 8,616, and posited that the "final" total would certainly be within 10 percent of that, if not much closer.  (The Number of Species of Birds, Auk, Volume 63, January, 1946.))  So, counts of known living species of birds and foraminifera are not an order of magnitude apart.

Perhaps, there are many living bird species yet to be discovered.  Well, no, not according to those who study these issues.  Daniel P. Bebber et al. explored how changing rates over time in the discovery of new species within different taxonomic groups could be used to predict how many species remain to be found.  (Predicting Unknown Species Numbers Using Discovery Curves, Proceedings of The Royal Society B, Volume 274, 2007, p. 1655.)  For birds, they asserted, the unknown number should be small given that the count of existing bird species was “more or less complete.”  The best estimate from their methodology was that the actual number of living bird species – those yet to be discovered added to the 9,924 already known – was between 9,994 and 10,061.

What of the fossil record?  For birds, I'm not quite sure what to make of it.  There’s debate over its adequacy and its reach (how far back it goes).  It's generally thought that the hollow bones of birds don't fossilize readily and, as a result, many feel the fossil record is weak and likely to remain so.  (See, for example, English ornithologist Ian Newton's The Speciation and Biogeography of Birds, 2003, p. 16.)  That means that relatively few fossil bird species may be known.  For instance, Neil Brocklehurst and his colleagues looked at avian fossils from the Mesozoic Era and concluded there were 124 valid bird species in 82 genera identified from that era.  (The Completeness of the Fossil Record of Mesozoic Birds:  Implications for Early Avian Evolution, PLoS One, Volume 7, Number 6, June 2012, p. 4.)  I haven't found decent counts of the number of fossil avian species known from the Cenozoic.

I seriously doubt that the number of fossil species of bird could ever match that of foraminifera whose shells readily fossilize and whose traces in the fossil record go back into the Cambrian.  Lots of time and lots of species.

I've come full circle.  Though I may be wrong about it, my impression remains that the number of foraminifera species, living or fossil, is inordinately large.  And all of those species are intent on complicating my life.

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