Researchers analyzed the metabolic rates of hundreds of mollusk species from the Atlantic Ocean -- some extinct, others still alive -- and found a link between low metabolic rate and better survivability. In other words, more sluggish creatures have an edge over fast calorie-burning animals as far as risk of extinction goes. Bruce Lieberman, a professor of ecology and evolutionary biology at the University of Kansas and co-author of the new study, calls this the 'survival of the laziest', playfully teasing the famous Darwinian phrase.
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In any ecosystem, success is relative. Many species become extinct because they hyper-specialized -- too adapted to a specific environment or ecological niche. When the environment changes, their highly specific adaptations become a liability rather than a benefit. That's a big part of the extinction of the dinosaurs. Many birds in New Zealand were highly adapted to an environment with no large predators, which meant that they evolved to become flightless. In came large predators (i.e. humans), and many rapidly became extinct.
Many species go through a boom, then a bust -- but the truth is some species can fend off extinction for longer than others. There are a lot of factors at play, and one of them is their metabolic rate, the amount of energy an organism needs in order to live its daily life.
Researchers at Kansas University investigated whether they could predict the probability of extinction of a species based on energy uptake. They analyzed the metabolic rates of 299 species of mollusks that lived over a period of roughly 5 million years from the mid-Pliocene to the present. The findings suggest that lazier, sluggish creatures have a slight edge over animals with a higher metabolic rate.
"Those that have gone extinct tend to have higher metabolic rates than those that are still living. Those that have lower energy maintenance requirements seem more likely to survive than those organisms with higher metabolic rates," said Luke Strotz, postdoctoral researcher at KU's Biodiversity Institute and Natural History Museum.
"Maybe in the long term the best evolutionary strategy for animals is to be lassitudinous and sluggish—the lower the metabolic rate, the more likely the species you belong to will survive," Lieberman said. "Instead of 'survival of the fittest,' maybe a better metaphor for the history of life is 'survival of the laziest' or at least 'survival of the sluggish.'"
As climate change rapidly alters ecosystems around the world, the new findings will serve to refine our predictions of extinction probability. The results suggest that metabolic rate -- while not a be-all, end-all factor -- is an important component of extinction likelihood.
The link between high metabolic rate and extinction probability was most pronounced for species that live in a small habitat, the authors reported in the Proceedings of the Royal Society B.
"We find the broadly distributed species don't show the same relationship between extinction and metabolic rate as species with a narrow distribution," Strotz said. "Range size is an important component of extinction likelihood, and narrowly distributed species seem far more likely to go extinct. If you're narrowly distributed and have a high metabolic rate, your probability of extinction is very high at that point."
One of the most interesting findings was that the cumulative metabolic rate for communities of species remained stable. Even as populations and individual species appeared or disappeared in an ecosystem, the overall metabolic rate of the community remained unchanged over time, seemingly in an energetic stasis.
"In terms of energy uptake, new species develop—or the abundance of those still around increases—to take up the slack, as other species go extinct. This was a surprise, as you'd expect the community level metabolic rate to change as time goes by. Instead, the mean energy uptake remains the same over millions of years for these bivalves and gastropods, despite numerous extinctions," Strotz said.
The authors used mollusks for their study because there's ample data about living and extinct species, including metabolic and extinction rates. A follow-up study will determine whether the same link or metabolic influence exists for other classes of animals.
"We see these results as generalizable to other groups, at least within the marine realm," Strotz said. "Some of the next steps are to expand it out to other clades, to see if the result is consistent with some things we know about other groups. There is a question as to whether this is just a mollusk phenomenon? There's some justification, given the size of this data set, and the long amount of time it covers, that it's generalizable. But you need to look—can it apply to vertebrates? Can it apply on land?"