Thanks to five mass extinctions, about 99 percent of all animal species have come and gone from the face of the earth.During many of these events, life took the greatest hit where it probably began: in the ocean, a sphere subdivided into layers where animals have specific ecological functions.
One event—the Permian-Triassic, or End Permian, extinction of 252 million years ago—even wiped out about 96 percent of animal life in the sea. But what distinguished the 4 percent that survived? Evolutionary biologists have zeroed in on two possible buffers against being wiped out. One is that species-rich animal groups might overwhelm extinction forces, with some species slipping past the onslaught. The other possibility is that groups with fewer species gained an edge against catastrophe because they could take on many different roles within their environment.
A study published on February 27 in Science offers some support for a diversity of ecological roles, rather than a large number of species, in resisting extinction. The findings could be relevant for understanding the possible impacts of the current extinction crisis and ongoing climate change, although in ways no one can exactly predict.
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What these authors have “elegantly done,” says William Foster, an evolutionary biologist at University College Dublin, is show that animal groups with high species diversity today, such as mollusks and arthropods, achieved this diversity slowly and did not suffer as much during mass extinction events. These survivors had more diverse ecological functions, compared with groups with high numbers of species, such as brachiopods (think double shells hinged at the rear) and crinoids (fernlike marine animals) that occupied similar ecological niches, Foster says.
“Because of low competition for resources following a mass extinction, the assumption in evolutionary biology has been that once an animal group has evolved to occupy a new ecological niche, it rapidly diversifies,” says Foster, who was not involved in the study. He calls these results a new way to think about the “skeleton crew hypothesis,” an explanation for why niches still have remnant organisms occupying them after a mass extinction.
An analogy for this resistance to destruction is the advice often doled out about financial investing, says study author Matthew Knope, an evolutionary ecologist at the University of Hawaii at Hilo. Investors take something uniform, such as money, and diversify where they invest it so that their bundle can survive a market crash. This “portfolio effect,” he says, offers a strong parallel to the “economy of nature,” where groups with varied ecological roles are less likely to experience fluctuations across time and better able to resist extinction, compared with groups that are more ecologically similar.
To assess the underlying economics of the natural world, Knope and his colleagues looked at data for 30,074 groups of closely related species, or genera, of living marine animals and 19,992 genera of fossil marine animals. The species they used represent all known living marine animals and about 70 percent of the known marine fossil record. The investigators determined the roles for each group in their environment today and across 100 stages that covered a span of about 500 million years.
These ecological functions, or modes of life, are defined by factors such as the animals’ location relative to the ocean floor, their mobility and their methods of food acquisition. After categorizing animals by their specific roles in their environment, the team looked at the diversity of these roles for each broad group of animals and both their origination and extinction rates across the past 500 million years, including mass extinctions. The researchers also examined how species diversity and the variety of these ecological roles related to each other across time, up to the present.
Their results were unexpected, Knope says. The previous belief was that a high rate of new species formation would lead to increases in the diversity of genera and the number of ecological niches that would be occupied. Instead, he says, the animal groups that currently have relatively numerous species were already occupying diverse niches early on, despite a low speciation pace and, as a result, a lower diversity of species. A death blow to one niche would not necessarily have wiped them out because they had other places to fit in.
The upshot is that today, in the Cenozoic era, which began 66 million years ago, the groups with the most genera or species had plenty of time to generate their modern-day high numbers. And they reaped this time because their ancient diversity of ecological function buffered them against extinction.
The focus on extinction, not just the rise of new species, is important, says Thomas H. G. Ezard, an associate professor of evolutionary ecology at the University of Southampton in England, who was not involved in the work. The notion of “dead clades walking” is relevant, for describing the groups that aren’t “extinction-resistant,” he says, because even if they have a lot of different genera, “if they all have the same adaptations at a coarse scale, they are all going to be exposed to the same fate” of being wiped out.
The future that awaits today’s species is an open question in the face of what many biologists see as an ongoing modern mass extinction. Knope cites two previous extinction events as “good surrogates for the current climate crisis.” The aforementioned End Permian mass extinction and the End Triassic one of 201 million years ago, he says, involved rapid climate warming and ocean acidification— both of which threaten species today. Knowing that animal groups with diverse roles in their environments fared better during these events can “give us important insights into what the future planet might look like as the current climate crisis continues to intensify,” Knope says.
Ezard says that whether the “current diversified portfolio” for extinction-resistant groups is sufficient to deal with climate change today remains to be seen.