General Archives - Demons Dance Alone

Blood of birds, disintegrating butterflies, two end-Permians, innocent volcanism

So, this blog has a loooooong backlog of stuff I wanted to talk about but I didn’t. Old stuff, like, one year old or more. Alas, I cannot hope to make a complete post on all that, but I don’t want to let it rot. Let’s do a bit of quick catching up:

Bird blood on our hands. In 2019 a couple of papers pointed directly the fingers at humans for the extinction of the great auk and the Carolina parakeet, two once-widespread, iconic species of birds that went extinct in the XIX century. Both papers analyzed paleogenomes (is it right to use the ‘paleo’ prefix when it’s a couple centuries ago?) and found out that both species populations had a vibrant genetic diversity until their numbers fell abruptly to zero. Which means: no, they weren’t already fragile, declining species that we gently pushed off a cliff they would have met anyway. We systematically exterminated two robust, healthy bird species in the space of a few decades or centuries. Not exactly unexpected, but now there’s more proof.

Singapore Coney Island Butterfly - Free photo on Pixabay

Butterflies that we will never know. In Singapore, 46% of the butterfly species disappeared (locally) in a mere 160 years, according to a paper of February 2020. Interestingly enough, the study accounts for extirpations of undetected species, using a model. I’m in no position to comment on the math, but the very idea is intriguing and melancholic: about a hundred of species would have gone extinct before we ever discovered them. Of these, some could have well been endemic species: ghosts, of which now we have nothing else than numbers in a statistical analysis. “14.9% of the species discovered before 1900 also were extirpated before 1900. These high early observed extirpation rates, during a period where many species remained to be discovered, suggest that a high number of species were never detected before they were extirpated”
The Karoo Basin, in South Africa, where the best deposits on terrestrial end-Permian/early-Triassic fauna are preserved.
A tale of two end-Permians. Discerning a single event that happened 252 millions of years ago is incredibly hard; discerning a complex interplay of events even harder. Compared to the relative simplicity of the Chicxulub impact, the Permian extinction is a maddening puzzle, muddled by its remoteness in time. There were always hints of multiple extinction events or at least multiple “hits” that led to the Permian catastrophe, but now a paper of March 2020 seems to imply that the extinction on the sea was different from the extinction on the land. It seems that whatever happened on the continents, leading to the demise of most terrestrial fauna and the temporary dominance of Lystrosaurus, happened 300.000 years before the extinction in the oceans: “Instead of the currently favored paradigm of calamitous and globally synchronous turnover in ecosystems, the reported terrestrial turnover in Gondwana occurred hundreds of thousands of years before the marine one and, therefore, marine and terrestrial responses likely had different extinction mechanisms.“. We have to see if and how it will be confirmed, but if so, it seems that the end-Permian extinction is truly two extinctions, above and below water. It will be extremely interesting to grasp how did one influence the other, and how does it translate to our current situation.

Innocent volcano. In January 2020, Pincelli Hull and coworkers put another nail in the coffin of the volcanic hypothesis for the K/T extinction. The K/T event has the distinction of having two competing or possibly synergic explanations: the well known Chicxulub asteroid impact, and the Deccan traps, a major volcanic event. For decades scientists have fought on what of these events was most important, and even if the impact seemed more and more clearly the culprit, the Deccan enthusiasts didn’t lose their grip. However, if the study is correct, it seems that 1)Deccan outgassing isn’t chronologically correlated to the extinction, but the impact is, and 2)the Deccan volcanism simply wasn’t generating enough gas to trigger an extinction, since similar events didn’t alter the biosphere so much. Another paper a few months later even argued that, if the Deccan volcanism had any effect, it was mitigating the extinction effects.

Darwin of a billion years

Is there any chance of predicting the history of life? On the footsteps of Gould, that would seem a foolish quest. Rewind the tape and replay it, he said, and you will see a different movie every time: infinite possible worlds lost in the tiniest of chances. And yet it is all too tempting to search for general rules, and sometimes we are rewarded. What if we could predict what creatures can survive multiple extinctions, on the scale of half a billion years?

This apparently absurd proposition is what a recent study by Matthew L. Knope, Jonathan L. Payne et al. has shown at the end of this year’s February. They analyze a huge amount of living and fossil marine genera -30,074 and 19,992 respectively, another proof of the power of big data when dealing with the history of life- and try to correlate the taxonomic diversity to the ecological diversity over time. That is, the number of species of a clade versus the ecological strategies exploited by members of that clade.

One could find it obvious that there is some correlation -after all, if you have lots of genera, you can imagine there are more chances they evolved to different niches- but it is not necessarily always so. And this is the main finding of Knope et al.: this correlation evolves over geological time.

The graph on the left shows the (log-log) correlations between the richness of genera on the x axis and the ecological diversity on the y axis, divided by geological period. The “rays” must be interpreted dynamically. The dark lines laying low are the correlation as it were in the Paleozoic, starting from the Silurian and Devonian. Going forward the correlation shifts: turquoise and green lines, corresponding to the Mesozoic, grow steeper, until in the Tertiary -yellow lines- it becomes almost double than the Paleozoic one. In other words, the more you go forward in time, the more ecological diversity correlates with taxonomic diversity.

Now, this is nice but hard to interpret. Now see the graph on the right: the slopes of the lines on the left are plotted against time. You can see the vertical dashed lines separating mass extinctions. It is not always convincing, but there is some hint that mass extinctions did something. After each extinction, ecological diversity becomes more tightly related to taxonomic ones.

What is going on? We are seeing how extinctions shape the composition of life on the scale of the Phanerozoic. It is not that ecologically diverse clades are better at diversifying and evolving: actually what they find is that, at least in the Tertiary, they diversify less than ecologically poor clades. Each ecological crisis instead tends to wipe out clades that were not flexible enough, forced to live within a few ecological modes. Clades that instead could exploit more ecological niches were able to find at least some of their members surviving a crisis. They diversify and repopulate the oceans for the simple reason they are the ones surviving, and they survive because they have the strategies to do so.

The classes that had high taxonomic richness and low ecological differentiation during the Paleozoic, such as rhychonelliform brachiopods and crinoids, consisted largely or entirely of nonmotile suspension feeders that mostly cannot occupy infaunal or pelagic habitat tiers. In contrast, the classes and phyla that are genus-rich in the Cenozoic (e.g., mollusks, arthropods, and vertebrates) are generally motile, feed in a variety of ways, live across many habitat tiers, have more control over gas exchange with the environment, and have weathered mass extinctions well.

Let’s take it in what this means. First, this means that we have some hope of predicting what animals groups would survive extinction better than others. Animals who are more motile, more versatile, that exploit more environments and more strategies will have better changes. Brachiopods were dominant in the Paleozoic, but a glance at their physiology compared to that of bivalves, in hindsight, shows us they didn’t have a chance. They were simply too rigidly adapted to a single mode of life to fare well after a catastrophe such as the end-Permian.

Second: Our seas are not different from the ones of the Paleozoic just because we have different animals. Our ecosystems have been selected. The animals we have today in the sea are quicker, more mobile, smarter at exploiting different niches, have better metabolisms – and they are like that because they are the hardened survivors of multiple ecological crises. As Knope explains:

The oceans we see today are filled with a dizzying array of species in groups like fishes, arthropods, and mollusks, not because they had higher origination rates than groups that are less common, but because they had lower extinction rates over very long intervals of time.

We are seeing natural selection acting not on the scale of the individual or of the species: we are seeing it acting on entire ecosystems on the scale of half a billion years. Knope et al. show that the shadow of Darwin is longer than we imagined, as long as the tape of Gould, shaping not only single species but the entire composition of life along hundreds of millions of years.

Paper: Knope et al., “Ecologically diverse clades dominate the oceans via extinction resistance“, Science 367, 1035–1038 (2020)

Sick of biodiversity?

The new Chinese coronavirus is the current talk of the town. As for every emerging zoonotic disease, the relationship between environment exploitation, biodiversity and spillover casts its shadow. Therefore, just as a side note, I am driven to reread this review that appeared in December on Nature Ecology and Evolution. The main message is relatively technical, and it is that the relationship between biodiversity and disease is not linear, but depends on the geographics scale we’re looking at. But to my simple mind, a few words are useful as a remind of our grim, conflictual relationship with the living world: ecosystems are both richness and danger.

Ecosystems regularly pose a threat of disease to humans and wildlife,

Which does not mean that wiping out forests is a good way of managing zoonosis. On the contrary:

targeting conservation toward protecting ecosystems that are not currently posing a major threat of problematic disease to humans or wildlife might prevent increases in disease

and

preservation of intact, functioning ecosystems and finding sustainable, equitable interventions that discourage human incursions into those ecosystems (for example, for logging and bush-meat hunting), could reduce the risk of transmission of multiple pathogens, even if these interventions are not the single most efficient control method for individual diseases. Thus, they could represent win–win scenarios for conservation and disease control.

There is hope, then. But the whole review is interesting for whoever wants to see how little we know and how many the nuances. Ecosystems bring beauty and disaster. Despite what the term “ecosystem service” brings to mind, they’re not there to serve us. They are features of nature, and should be treated with the circumspection and respect this implies.

The hidden bombardment

Earth is very good at cosmetics: it quickly and efficiently covers its scars. The Chicxulub impact that wiped out the Cretaceous was one of the largest impact events in the last hundred million years in the Solar System, and yet it left no visible trace today, save for an arc of carsic structures known as cenotes in the Yucatan Peninsula. We had to dig deep and hard to find it. So it is no surprise that new impact events are still being found. Nozaki et al. now declare on Scientific Reports to have found 11 million years old ejecta in the sediments of the North Pacific. Ejecta, mind you, not the crater. That is unknown yet.

We managed to link only one mass extinction to asteroid impact. That might be the case. But impacts have been (and will be) a constant in Earth history. While globally catastrophic impacts are rare, what about locally catastrophic ones? They must have had important ecological consequences. A region-devastating impact can still alterate weather patterns and radically steer the direction of life history. A promising but localized lineage can be wiped off, a destroyed area can be re1populated and lead to speciation, and so on. Our past must have been sculpted by many more fatal days than the one at Chicxulub, and we still have almost no idea.