Global Warming, Corals, Faunal Turnovers

The Economist recently reported on some research which suggested that increasing atmospheric CO2 concentrations will lead to increasing amounts of CO2 getting dissolved in oceans, eventually dropping their pH by about half a unit by end of the century. Oceans absorb around 30% of annual CO2 emissions playing a big role in preventing a huge buildup of CO2 in the atmosphere. Currently the composition of the ocean is slightly alkaline but it will acidify if we do not reduce emissions soon. This may play havoc among marine organisms which secrete a skeleton of calcium carbonate. In acidic solutions the skeletons are likely to dissolve. Without its protective cover, many groups of marine organisms will perish.

I have spent a large part of my career studying limestones, some of them made up of ancient corals, so this story really caught my interest. Corals and reefs are pretty awesome ecosystems, and having seen many of them at close quarters in the Florida Keys, the news that there could be large scale degradation of these ecosystems is quite disturbing. Corals as a group have been around since the early Paleozoic. In the report, Andrew Knoll of Harvard Univ. points to the end Permian extinction event which is believed to have wiped out around 95% of marine species. He suggests that although the Permian extinction may have many causes, a significant cause was the eruption of the Siberian traps, a massive eruption of basaltic lava. This led an enormous increase in atmospheric CO2 levels, which in turn got absorbed in the oceans making them acidic. Knoll points out that there was a selectivity in which organisms did better than others during this "acid" crisis. Bivalves, gastropods and arthropods which had to ability to buffer their internal fluids from which the skeleton is precipitated resisted extinction, while groups such as the articulate brachiopods and corals which apparently did not have these buffering capabilities were wiped out in large numbers.

Throughout Phanerozoic history, biotas have shown compositional changes in their diversities. Below figure known as spindle diagram shows changes in diversities at the family taxonomic level in Bivalves, Gastropods, Articulate Brachiopods and Anthozoans (corals). C refers to Cambrian, P to end Permian mass extinction and K to end Cretaceous mass extinction.

Adapted from: On the Origin of Phyla

The patterns that Andrew Knoll pointed out are clearly seen. Bivalves and gastropods suffer a dent during the end Permian extinction event but their general history since their origin in early Paleozoic is one of increasing diversity. Brachiopods after an early diversification episode in the Paleozoic suffered large scale extinction during the end Permian and never recovered, some say because of competition from the more successful bivalves. Corals show a great volatility in diversity changes during the Paleozoic. They too suffer a crash in diversity end Permian but unlike Brachiopods recover and proliferate in the Mesozoic and again in the Cenozoic. Evolutionary paleontologists interested in long terms trends in biota compositions want to know why certain groups fail while others do well. The ability to internally buffer the chemistry of fluids may just be one such property of marine skeletal organisms on which biota composition gets sorted during times of environmental stress. That does not mean that buffering evolved to make organisms resistant to extinction. Evolution has no foresight. It cannot anticipate future extinction and make long term plans. Buffering must have evolved as a physiological adaptation that helped some organisms in the immediate term as against having some alternative body fluid chemistry. But it also ensured them long term success.

As it happened the Times of India had an editorial which although did not specifically go into changes in diversities through time gave a general account of ocean degradation including possible acidification. But there was some sloppy reporting as well. Here's what they had to say about ocean acidification

"The most vulnerable marine species are those that form shells, because a rising pH factor — which measures levels of acidity or alkali-nity in water — inhibits shell growth, even melts them."

High school chemistry! Did they mean rising hydrogen ion activity? That would translate to lowering of pH and acidification. But I doubt it. I think they just assumed that higher pH means higher acidity and so lower shell growth. Also shells dissolve in acids , they don't melt. Please get a science graduate to look over this slop before publishing.

But back to corals and the current trends in atmosphere CO2. Scientists believe that if pH does drop by half a unit by end century then most of southern ocean reefs may suffer extensive damage, even local extinction. As the figure shows, in the long term, corals as a group have proved to be quite resistant to extinction, surviving not just the end Permian event but also the end Cretaceous event with remarkably little loss in diversity and also later perturbations such as the Eocene Thermal maximum. The pattern suggests that there has been an overall decrease in coral extinction rates through the Phanerozoic. This trend has developed not by one group of corals gradually evolving adaptations that enabled it to resist extinction but through faunal turnovers, meaning replacement of one group or subtaxon of corals by another. Entire orders such as the Rugose and Tabulate corals which were important contributors to mid-late Paleozoic reefs have gone extinct, while others such as the Scleractinia persisted, proliferating in the Mesozoic and Cenozoic reef communities. This replacement has not been random but likely reflects species sorting on extinction rates. In subtaxons with high extinction rates, speciation could not replace lost species at an adequate rate. So over time subtaxons with lower extinction rates became more common while those with high extinction rates disappeared. Extinction rates is an example of a property a group of related species share. It is not a property that individuals possess and upon which natural selection acts on directly. Rather it is an effect of natural selection favoring features that improve individual reproductive fitness. For example, in some ecologic settings natural selection may favor individuals having the property of widespread larvae dispersal. This property will become characteristic of that species and its descendants. Clades with the property of wide dispersal of larvae resulting in a wide geographic range may be more resistant to extinction as compared with clades without this property. Over geologic time, wide larvae dispersing clades i.e. clades with lower extinction rates will replace clades with high extinction rates. I have used larvae dispersal as an example. I don't know if it applies to corals in particular, but such sorting on turnover rates has most likely taken place within corals. Some species of these low turnover or low volatility coral groups will survive this anticipated crisis and on a geological time scale may even proliferate. But that does not help us today. Evolutionary recovery over geological timescales is no consolation for humans whose priorities overlook the next 50-100 years. Ocean degradation and coral loss especially the reef building types will have a serious impact on marine biodiversity, since reefs shelter and provide a habitat to other diverse groups of organisms. It's hard to contemplate that the Lakshadweep islands and the Maldives may not have functioning living corals in a few decades. The time to save corals would be now.