Pruning the tree of life

 kw: book reviews, nonfiction, natural history, extinction, memoirs

On the inside back cover of Endings: Encounters with Extinction, the author blurb states that Neil Griffin gave up "the glitz and glamour of tropical fieldwork for the financial stability of freelance writing." OK, blurb writer, drag your tongue out of your cheek; it must be getting pretty sore by now.

Fortunately, Neil Griffin is a fine writer, and this lyrical elegy of a book is thoroughly enjoyable in spite of its funerary topic. The current era is being called the Anthropocene by many: a geologic era named for the species that has grown to be a geologic force in its own right, Homo sapiens, us. Yet the first third of the book leads us through a lyrical survey of the Big Five geological extinction periods.

I considered what to use to illustrate this post. A session of "negotiation" with NanoBanana 2.1 under the OpenArt umbrella resulted in this image I call "Pruning the Tree of Life." Note the bucket of clipped twigs at the gardener's feet.

It is often stated, including in this book, that "99% of all species that have ever existed have gone extinct." Having spent a few years specializing in paleontology, including a number of interesting experiences clambering up and down cliffs that cut through ancient reefs, I consider this estimate to be rather low.

"Ninety-nine percent is low?" you may ask, incredulously. Let me explain. Firstly, the statement refers to metazoan life, meaning creatures (animals, plants, & fungi) big enough to see. There are about 1.5 million metazoan species so far described, and between 10 million and 100 million waiting to be described. Admittedly, most of those are going to be little critters the size of a pinhead or smaller. Microscopic entities, protozoa (or protists if you're picky) and bacteria plus archaea and also viruses, may include billions of species. Biologists don't yet have the tools to pin these estimates down further. So I'll just analyze metazoan life briefly:

• First, the Big Five major extinction events:
• End-Ordovician, two periods at 445 mya (million years ago) and 443 mya. About 85% of marine species were lost, and there were hardly any terrestrial species yet.
• Late/end-Devonian, several events between 375 mya and 355 mya. Between 75% and 80% of all species were lost.
• End-Permian (the Great Dying), 252 mya (lasting about a million years). 90%-95% of marine species were lost, and 70% of terrestrial species: about 90% overall.
• End-Triassic, 201 mya. 70%-80% total species were lost.
• End-Cretaceous (the dinosaur killer), 66 mya. 75% total species were lost.

The number of species existing just before these events differed a lot from event to event. We can only combine survival ratios, using averages for the ranges. So, for example, if 75-80% were lost, we can average that to 77.5% and subtract from 100% for a survival rate of 27.5%, or 0.275. So we multiply:

0.15 x 0.275 x 0.1 x 0.25 x 0.25 = 0.000257812 which is 0.0258% survival, indicating 99.974% of species were driven to extinction, just in these five periods.

• Then, the background extinction rate in pre-human times:
• Bookends: 
• Older estimate: one extinction per year per million species.
• Newer estimate, including phylogenetic modeling: one extinction per year per ten million species.
• Species duration:
• Marine invertebrates: About 7 million years
• Terrestrial invertebrates: About 12 million years

The optimistic view is the newer background estimate, so if there are 10 million species currently in existence, one extinction yearly is expected from nonhuman factors. However, just since the big event 66 million years ago, and shortly (less than a few million years) after it the number of species was also about 10 million, there have been more than 60 million extinctions over that time. It also implies that about 60 million new species have evolved to keep the total number stable. A raw estimate of post-Cretaceous extinction, then, is based on a one-in-six survival (16.67%), or ~83%. Considering earlier periods to be very generally similar in this regard, based on their varying lengths, overall survival of species not wiped out in the major events, is about 1.5%, or 98.5% total extinction from background alone. I am not sure how this combines with the other figure, but a bonehead analysis yields 

0.000258 x 0.015 = 0.00000387, for an overall survival of 0.000387% or extinction of 99.9996%

Well, as geeky as I am, that's enough. Let's get back to what the book is about. 

The author's experiences illustrate that animals in particular, and even more particularly, vertebrates, are going extinct at alarming rates. He hopes, and hope is all he has at this point, that this will not continue until the only place most people will ever see most animals is in a zoo or a museum diorama. The cover art for the book is a portion of the painting "The Earthly Paradise" by Breughel, with certain animals whited out. I show a portion here.

In the last scene in the book he relates going with a team of biologists to search for a wild jaguar, as part of a conservation project in Belize. A month of searching turned up nothing more than a couple of footprints and a few piles of jaguar dung. One day he hiked alone in the forest, just to look around, felt he was being watched, and looked behind himself to see a jaguar crossing the path some distance back. It glanced at him, boredly, and slipped into the forest.

This has been more of a riff on the thoughts triggered by my reading Endings than a real review. The best books help us know ourselves better.