Like unhappy families, all mass extinctions are unhappy in their own way. But their aftermaths are surprisingly similar. It takes millions of years, but life eventually bounces back
Is there a common pattern?
In the 1980s, as the Alvarez hypothesis gained ground, it seemed reasonable to assume that all mass extinctions were caused by impacts. Though there have been numerous “discoveries” of craters and other impact signatures coinciding with the other mass extinctions, none has stood up to scrutiny. It now seems that the KT event was unique – the only mass extinction caused by an impact. In fact, we now think that each mass extinction had its own unique cause.
Another idea that was fashionable in the 1980s was that mass extinctions are periodic. Some palaeontologists claimed to have found patterns in the fossil record showing a mass extinction every 26 million years, and they explained this by suggesting that a “death star”, dubbed Nemesis, periodically swings into our solar system and perturbs the meteorite cloud. But Nemesis has never been found and evidence for this pattern is now widely doubted.
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Common features have emerged, however. For example, it does seem that some species are more vulnerable to extinction than others. Large body size makes animals especially susceptible as it is associated with high food requirements, large feeding range and small population size. Species with specialised diets or limited distribution are also likely to suffer. In contrast, the survivors tend to have large population sizes, live in many habitats in many parts of the world, and have a varied diet.
This is not to say that mass extinctions are highly selective. David Raup at the University of Chicago famously characterised the death of species during mass extinctions as the result of “bad luck rather than bad genes”, meaning that the normal rules of natural selection break down. Their success – or lack of it – in normal times has little bearing on their chances of survival when the meteorite hits or the volcano erupts. This holds lessons for current and future extinctions (see back cover). For example, if humans destroy habitats wholesale then all species are vulnerable, whatever their size, diet or habitat.
Life rebounds
Mass extinctions are devastating, and yet life eventually returns to normal. The rate of recovery depends on many factors, but the most important is the scale of the extinction.
After most mass extinctions life recovers within a few million years, though the end-Permian event was different. It was twice as large as most of the others, and so it is no surprise that the recovery time was greatest.
Recovery also depends on which plants and animals survive. If the mass extinction hit all groups more or less equally, as most seem to, then there is a good chance that one or two species from each major group will survive. These act as an ecological framework, occupying most of the broad niches, and so the basic ecosystem structure survives. New species evolve to fill the gaps and the recovered ecosystem may be quite comparable to the one that existed before the disaster.
A more selective event, on the other hand, might leave broad sectors of ecospace vacant. A variety of the survivors then jockey for position, evolving to fill the vacant niches.
After the KT event it was by no means a foregone conclusion that mammals would take over. Indeed, in North America and Europe, giant flightless birds became the dominant carnivores, some of them famously preying upon ancestral (admittedly terrier-sized) horses. In South America, giant birds and crocodilians vied with each other to become the top carnivores, and mammals only replaced them some 30 million years later.
“In South America, giant birds and crocodilians vied to replace dinosaurs as the top carnivores”
Mass extinctions, then, have a creative side. Marginal groups sometimes get a chance to expand and become dominant. Most famously, mammals benefited from the demise of the dinosaurs. In fact, mammals first evolved in the late Triassic, at the same time as the dinosaurs, but they remained small and probably nocturnal because dinosaurs occupied all the key niches.
The end-Permian mass extinction was even more creative, with a yawning post-extinction eco-space providing opportunities for the survivors. In the sea, molluscs (bivalves and gastropods) took over roles previously occupied by brachiopods. Scleractinian corals rebuilt the reefs, and new kinds of light-scaled fish moved into roles previously occupied by more primitive ones. On land, the key beneficiaries of the extinction might have been the dinosaurs, whose earliest ancestors emerged within 5 million years of the crisis.
Impacts. Volcanoes. What else?
The causes of two of the largest mass extinctions are now reasonably well understood (see pages iv and v). But what of the others? In some cases it is difficult to say. The fossil record clearly shows a huge loss of life but not what caused it. Over the years, a number of possibilities have been put forward, but the cause of two of the big five – the end-Neoproterozoic and end-Triassic – remains uncertain.
CONTINENTAL MOVEMENTS. During the Permian and Triassic, all continents were fused into a supercontinent, Pangaea. At one time, the end-Permian mass extinction was linked to this, based on the suggestion that fusion of continents removes intercontinental seas, each with its own unique fauna, and allows land animals and plants to mix. It now seems, however, that such movements are too slow to lead to massive species loss.
ICE AGES. The late Ordovician mass extinction has been explained as a consequence of a massive ice age, particularly the growth of a huge southern ice cap (see map). As the ice spread, species migrated towards the equator and warm-adapted species may have disappeared. Sea levels fell dramatically, reducing many inland seas and causing widespread extinction.FIG-mg28022301.jpg
ANOXIA. The late Devonian extinction has been linked to a lack of oxygen in the ocean, possibly caused by sudden temperature changes or massive increases in the supply of sediment from the land caused by the rise of terrestrial plants.