Dominant species spread across the globe are just as vulnerable during a mass extinction event as more fragile ones confined to a single locale, according to a study published Tuesday.
As Earth enters the sixth such concentrated annihilation of life over the last half-billion years, this could be bad news for humans, the researchers say.
The last major wipeout occurred 66 million years ago when a giant asteroid put a relatively quick end to the age of dinosaurs after their spectacular 150 million-year run.
By comparison, humans have been around for about one tenth of one percent of that time.
Outside of these moments of planetary upheaval -- each of which decimated 50 to 95 percent of life forms -- species tend to disappear at a steady "background" rate that has varied remarkably little.
During the previous big five extinction, however, that rate increased by at least 100-fold.
And that's about where we are today.
"Rates of extinction amongst modern animal groups are as high, if not higher, than those we see in the fossil records during times of mass extinction," comments Alexander Dunhill, a professor at the University of Leeds, and lead author of the study.
Most mass die-offs were associated with climate change, itself triggered by some cataclysmic event -- a massive, continental-scale rupturing of volcanoes in the case of the Triassic-Jurassic juncture 200 million years ago.
"Organisms are unable to adapt quick enough to rapidly changing conditions and thus become extinct," Dunhill said.
Looking at the fossil record of land-living animals around the Triassic-Jurassic event -- in which 80 percent of species ceased to exist -- Dunhill and colleague Matthew Wills asked whether geographically far-flung creatures fared better.
The answer, published in the peer-reviewed journal Nature Communication, was "no".
"Wider geographical range conferred greater resilience ... throughout most of the Triassic and Jurassic," the study concludes.
"However, this insurance weakened towards the end of the Triassic, and was imperceptible during the mass extinction itself."
Thus, the age of giant amphibious reptiles and crocodile-like creatures gave way to the dinosaurs, which in turn yielded to small mammals and birds when their time was up.
Will the same "rule" apply to species alive today, including our own?
There are disquieting parallels, Dunhill said.
The volcanic effusion 200 million years ago spewed vast amounts of CO2 and other greenhouse gases into the atmosphere, causing rapid -- and deadly -- global warming.
"In effect, we are creating the same conditions today via human activity, only on a more rapid timescale," he said.
"Adding to that, human-caused habitat destruction and general exploitation of the natural environment is also a major driving force of extinction today."
Humans may be more resourceful and resilient than any creatures to have slithered, walked or swum across the planet.
"You could say that we have altered our habitat so much that we may well be exempt from such evolutionary process," Dunhill said.
"But most of the world's population is still heavily dependent on the natural world for food, water and energy," he added.
"Massive and rapid upheavals in the natural environment will certainly impact humans in a negative way."