2023-11-28 14:58:03
When geoscientists talk about a “faunal cut,” they don’t mean gardening or an appointment at the dog groomer, but rather the disappearance of very many groups of organisms in a short period of time, i.e. a mass extinction. For as long as animals and plants have existed, for just over half a billion years, there have been five such events of particular severity: 444, 359, 252, 201 and 66 million years ago. The latter was the one where most of the dinosaurs jumped over the blade. And in view of the current loss of species, some now see a sixth major faunal change coming. Paleontologists have already criticized this classification because the comparability of extinction rates determined by fossil statistics with current ones is not unproblematic. But the latter are certainly well above the long-term geological average.
Plants took over the land and ruined the seas
As is well known, an asteroid is largely to blame for the sudden end of the dinosaurs and, of course, humans are to blame for the current events: a species that has spread across the entire habitable landmass of the planet and is now burdening it with the by-products of its evolutionary success.
However, this is not the first time that one form of life has ruined the livelihoods of others on a large scale. In the first two of the five faunal sections, new organisms were at least involved – namely those that are now considered the epitome of healthy nature: plants.
The first terrestrial plants had just appeared in the Ordovician period, which ended 444 million years ago, and although only mossy and without roots, they increased the weathering of the rocks – and thus the binding of carbon dioxide. Its greenhouse effect was now missing in the atmosphere, and so it became colder. Glaciers expanded and trapped water, sea levels fell, and the shallow seas above the continental shelves dried up. For what was then almost exclusively marine life, these were what the rainforests are today: diversity hotspots that are now deserted. The Ordovician ended with the first mass extinction.
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In the Silurian and Devonian periods that followed, biodiversity rose to new heights again. Among other things, vascular plants now appeared. They have roots and therefore the ability to grow tall and produce seeds, which enables them to thrive in drier soil. Forests spread, the earth became green. But again the glory on land had unpleasant consequences in the sea: roots further increased erosion, and the forests sucked carbon dioxide out of the air. The world once again became an icehouse, and the marginal seas became deserts. On top of that, plant-induced weathering now washed large amounts of minerals into the oceans. There they acted like fertilizer, boosting the growth of algae on the water surface, which deprived deeper layers and their inhabitants of oxygen. At the end of the Devonian the second faunal transition occurred.
Bild: Illustration Charlotte Wagner
Now, at least at the end of the Ordovician – and in contrast to today – there were other factors at play, in particular the extremely unfortunate position of the continents at that time. And the processes behind the mass extinction at the end of the Devonian are poorly understood. But the fact is: danger to a flourishing nature doesn’t just come from outside. On a small and large scale, and even on the scale of an entire biosphere, success always carries with it the seeds of downfall.
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