A Field Guide To Surviving A Mass Extinction
Recently, we had a look at the hellish conditions that prevailed after the impact that wiped out the non-avian dinosaurs. The impact sent lots of debris high into the atmosphere, most of which then heated up during re-entry, creating a global heat pulse that ignited fires across the planet. The debris from the impact and fires blocked out sunlight for a period of up to two years, shutting down the photosynthesis that drives most food chains.
It's hard to imagine that much in the way of animal life could possibly survive this, yet clearly it did. And this week, the same researchers are back with what could be considered a field guide to surviving a global catastrophe. In it, they examine aquatic ecosystems and find some common factors that could help organisms survive this sort of event. And, to a certain extent, they find that a mix of pre-adaptations and luck made the difference for many species.
Aquatic ecosystems (at least those distant from the site of impact) should have come out of the initial events relatively unscathed. The heat pulse that set the planet burning was intense, but it was probably only intense enough to evaporate off a half-centimeter from the surface of any body of water. The total heating would probably have affected only a few centimeters of water near the surface.
Yet, despite this mild impact, the oceans were devastated. Nearly half of the species present in marine environments seem to have died, and some major components of marine ecosystems died off. Ammonites vanished, as did large marine reptiles, like mosasaurs, plesiosaurs, and pliosaurs.
Part of the problem in the oceans was that the base of the food chain, photosynthetic plankton, was pulled out from under the ecosystem because of the lack of sunlight. This cascaded up to take out the ammonites and the reptilian predators that fed on them. Even as photosynthesis restarted (six months to two years after the impact) it would take some time for the full ecosystem to re-establish itself, leaving the larger creatures on the top of the food chain at risk of starvation. Big can be better in some ways; large, cold-blooded animals can survive extended periods of starvation. This probably helped large fish species, but there's evidence that creatures like mosasaurs probably had evolved the ability to regulate their temperature.
But the loss of food wasn't the only problem arising from the lack of photosynthesis. One of the byproducts of that process is oxygen, which is released into the surrounding water. This helps keep the area near the surface of the water fully oxygenated. With no photosynthesis, that source was gone, leaving the oceans in an anoxic state. What did survive in the oceans? The community that did the best was the one that resided on the deep ocean floor, which largely depended on a rain of debris that came from the surface community. Presumably, this debris contained a few more dead bodies for a few years and then tailed off for a while, but it was probably a less dramatic shift than the surface communities experienced.
All told, the marine extinction rate was roughly 50 percent.
You might expect that, being smaller, freshwater ecosystems would be even worse off. But the exact opposite turned out to be true; only 20 percent of the freshwater cartilaginous fish families died off, and only 10 percent of the freshwater fish families did. Amphibian families escaped unscathed. The total freshwater extinction rate was less than half that seen in the oceans.
It's not that the freshwater ecosystems didn't have the energetic rug pulled out from under them; they didn't see any more sunlight than the oceans did. But several factors probably kept them going. Most freshwater systems are either composed of moving waters like rivers, or receive direct input from them. This would have helped keep them oxygenated. But it would have also washed in some of the debris that the fires generated on land, providing a potential source of nutrients. In addition, many of the species in freshwater systems normally undergo something similar to hibernation during winter months. Since the lack of sunlight would have produced something like a global winter, many of these species might have just shut down for the worst of things.
The final reason that freshwater ecosystems did well is that, in contrast to the open oceans, lakes, rivers, and streams are very diverse. This would have increased the odds of species finding what are termed "refugia," or areas where the conditions were favorable to survival. These rare refugia provided a base from which species could recolonize the devastated ecosystems. (As someone once quipped, the singular of "refugia" is "extinct.")
Overall, the paper makes an interesting companion piece to the one that described a global firestorm in the wake of the dinosaur-killing impact. The utter devastation described there would make it easy to wonder how something could survive. This paper makes it clear that a combination of luck and lifestyle left some species relatively well equipped to survive the specific events that killed off most of their fellow species.
Journal of Geophysical Research: Biogeosciences, 2013. DOI: 10.1002/jgrg.20086 (About DOIs).