Are the 'sluggish' more likely to survive?

A new study looks to mollusks, both ancient and modern, to find which species are more likely to survive, and which are more likely to become extinct — and why.

Researchers based at the University of Kansas in Lawrence analyzed data on the evolution of mollusks — including bivalves, such as shellfish, and gastropods, such as snails — in the Atlantic Ocean from the Neogene until the present day.

Their findings — newly published in the journal Proceedings of the Royal Society B — suggest that various species' different metabolic rates impact which are likely to face extinction, and which are likely to be around for a long time.

The team studied the evolution over 5 million years of 299 species of mollusks, focusing on their metabolic rates — more specifically, how much energy the various animals needed to function on a daily basis.

"We wondered," states lead study author Luke Strotz, speaking of the team's premise for the new study, "'Could you look at the probability of extinction of a species based on energy uptake by an organism?'"

"We found," he adds, "a difference for mollusk species that have gone extinct over the past 5 million years and ones that are still around today."

"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."

Luke Strotz

'Survival of the laziest?'

The researchers revealed that species with higher metabolic rates were much more likely to face extinction sooner, though this depended on some other factors, too.

This led the researchers to suggest that the idea of "survival of the fittest" may be questionable; instead, they argue, we may be looking at an instance of "survival of the sluggish."

"Maybe in the long-term," says study co-author Bruce Lieberman, "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."

"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,'" he recommends.

Why is this important? The scientists say that understanding what makes a species more or less resilient may be key to predicting how various forms of life will — or won't — adapt to environmental threats such as climate change.

"In a sense," Strotz points out, "we're looking at a potential predictor of extinction probability. At the species level, metabolic rate isn't the be-all, end-all of extinction — there are a lot of factors at play."

"But," he goes on to say, "these results say that the metabolic rate of an organism is a component of extinction likelihood. With a higher metabolic rate, a species is more likely to go extinct. So, it's another tool in the toolbox."

Exceptions and surprises

Strotz and colleagues also note that higher metabolic rates are linked to a higher risk of extinction, especially when the species lives in a small habitat, restricted to a limited geographical area.

Conversely, however, when that species is spread over a larger geographical area, it is more likely to survive despite its metabolism.

"We find the broadly distributed species don't show the same relationship between extinction and metabolic rate as species with a narrow distribution," explains Strotz.

"Range size," he continues, "is an important component of extinction likelihood, and narrowly distributed species seem far more likely to go extinct," adding, "If you're narrowly distributed and have a high metabolic rate, your probability of extinction is very high at that point."

Also interesting is that, according to the team's analysis, despite how metabolic rates may change and vary between species, the cumulative metabolic rates of larger species communities tend to remain unchanged over time.

"There seems to be stasis in communities at the energetic level," states Strotz. "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."

To the researchers, this came as a surprise. "[Y]ou'd expect the community level metabolic rate to change as time goes by," Strotz observes.

"Instead, the mean energy uptake remains the same over millions of years for these bivalves and gastropods, despite numerous extinctions," he says.

Are the new findings 'generalizable?'

The scientists also explain that the main reason they decided to zoom in on mollusks, rather than animals belonging to other phyla, or groups of organisms, was because so much information is currently available regarding the evolution of mollusk species.

"You need very large data sets with a lot of species and occurrences," notes Strotz, in order to be able to determine the relevance of a factor such as metabolic rate to the likelihood of extinction.

"Many of these bivalves and gastropod species are still alive, so a lot of the data we needed to do this work can come from what we know about living bivalve and gastropod physiology," he notes.

Particularly, he says, there are abundant data about mollusks living in the Western Atlantic region — hence the team's focus on that area.

In the future, the researchers would like to establish whether the same associations apply to other kinds of animals, too. First, they aim to explore whether other marine animals' survival likelihood is also influenced by metabolism.

Eventually, they aim to extend the question to land-living species, too — both invertebrates (like mollusks) and vertebrates.

As Strotz goes on to explain, "Some of the next steps are to expand [the research] out to other clades [groups of organisms], to see if the result is consistent with some things we know about other groups."

He adds, "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?"