Even amid unthinkable destruction and loss of life, nature has found a way to fill the roles that make ecosystems continue on.

Two prior mass extinctions have shown that even as species disappear at a rate of 90 percent or more, almost all ecological variety is preserved, according to the new findings published this month in the Proceedings of the National Academy of Sciences.

But the catch is: the current disappearance of biodiversity – what some have termed the sixth global mass extinction – shows to be a bit different in their analysis, they add.

“In three major declines in taxonomic diversity – spatially from square to poles today and temporally in the Permian-Triassic and Cretaceous-Paleogene extinctions – only the first one shows a concomitant drop in the number of functional groups, whereas virtually all functional categories survived the extinction events,” they write.

Their analysis is based on the observations of marine bivalves, including shellfish like oysters and clams.

More than 90 percent of all species died out in the two mass extinctions, 250 million years ago and again 65 million years ago. But roles remained filled through the massive die-offs; bivalves survived to filter phytoplankton, and others eating small crustaceans and filling other parts of the food chain.

“The landscape of the world completely and suddenly changed, making it all the more surprising that all functional types survived,” said Stewart Edie, a University of Chicago graduate student, who was the first author. “Even the functional groups with only one or two species somehow make it through.”

But the die-off currently happening is unfolding in a different way. First, the number of species drops gradually from tropics to poles, increasing from a rate of 80 percent to 95 percent. But also the functional diversity rate has declined between 50 and 60 percent, they add.

“We see that functional diversity drops way down from tropics to poles: it parallels species loss in a way that’s totally different from the big extinctions,” said David Jablonski, one of the authors, of the University of Chicago. “That’s wild – really fascinating and unexpected and strange.

“Given that we’re working on a mass extinction right now, which flavor will it be?” added Jablonski. “Will we have a tropic-to-poles type, where we lose half our functional groups and so ecosystems will be massively altered?”

Scientists have attempted to pinpoint the “tipping point” of the forecasted sixth mass extinction. Some other disciplines have looked at the overall biomass loss, even within species that continue to exist.